A "Worm Rock" from the Middle Holocene; A Little Swamp Geology; Geological Torture in the Grapevines; Ubiquitous and (Almost) Forgotten Brick (and Clay) ; The Many Marbled Monument to George; Finding "Needle Ice" on Little Haystack; A Late Triassic Redbed Foretells the Fragmentation of a Supercontinent; Heroes' Tunnel Dives into an Early Mesozoic Sill; Persistent "Fairy Rings" of Late Summer
What shall I post about next? Is the subject matter worthy of discussion? What shall I say? What shall I omit? It’s the blogger’s never-ending dilemma. By the time the end of the year rolls around, at least for me, there are always a few posts that never got written and a few images that never got uploaded. And so, with this final post of the year – in what has been a tradition on my blog for five years running – here’s my end-of-the year post (although a little belated). Please visit the same for 2012 (here), 2013 (here), 2014 (here) and 2015 (here).
January
In spite of the fact that Southern Florida is almost virtually flat and hasn't experienced any significant form of tectonic deformation since it "emerged from the sea" some 25 million years ago, it has a fascinating geologic history (here). But with a paucity of telltale outcrops, no hills to speak of let alone mountains, no roadcuts or readily accessible quarries, doing geology is a challenge.
So, while on vacation with clear blue skies and 1,350 miles of accessible coastline, it seemed logical to head to the beach and see what the tide had brought in. To my surprise, I discovered a fossil remnant of a unique, marine bivalve colony that is responsible for the geomorphology of southwest Florida's carbonate-producing coastline and offshore islands.
Southeast of Marco Island along the southwest coast of Florida is the archipelago of Ten Thousand Islands. It’s a maze of oyster shoals, mangrove trees and brackish tidal channels that are a few miles wide and up to 20 miles long. The islands are actually part of an interesting stratal sequence deposited some 7,000 to 3,000 years ago.
Following the Pleistocene ice age, Holocene transgressions began to flood the Florida shelf. A basal peat layer formed below sea level that overlies eroded Pliocene and Pleistocene limestones. Quartz and shelly sands followed as the sea level rose. Overlying the sands of the inner islands are Holocene-age oyster reef beds that are overlain by modern peat and support the region’s mangroves. The right conditions of rising seas, climate and sedimentation converged over this interval to promote reef development.
The outer islands, in addition to oyster beds, have an up to 10 foot-layer of the worm-like mollusk Petalochonchus varians. The marine gastropod is of the Vermetidae family but doesn’t resemble the coiled shell of the average sea snail and is frequently interpreted as a marine annelid, tube worm, and vice versa. The "worm snails” grow cemented together in complex, anastomosing colonies locally known as “worm rocks.” From about 3,000 years ago, they formed a small barrier reef system until recently, when they began to experience an inexplicable global decline. On occasion, dislodged erratics wash ashore and await discovery by unsuspecting geo-beachcomers.
The foundation of the Florida Everglades and Big Cypress Preserve is essentially a limestone-based, former ocean bottom. The Everglades fills the 4,000 sq mi expanse between Miami on the Atlantic Coast and Naples on the Gulf Coast and resides in a shallow geological basin or paleo-trough confined by the topographically low (a maximum of eight feet above sea level) and narrow Atlantic Coastal Ridge on the east.
Water lazily flows to the south and southwest at a barely perceptible rate since the landscape is almost virtually flat - only two inches per mile! As a result, it is subject to extraordinary extremes of wet and dry weather. Far more than just a stagnant pool with a high watertable, the “River of Grass” is a wide, slowly moving, freshwater sawgrass prairie or marsh. "Sheet-flow" is the frequently used, descriptive term.
Water outflow from Big Cypress is eastward to the southernmost Everglades but also westward into the mangrove swamps and the Ten Thousand Islands coastal region on the Gulf of Mexico that serves as a buffer between the salty sea and freshwater marsh. For information on the geological evolution of the Florida Platform, please visit my post here.
February
The modern metropolis of Boston was built from a variety of local, regional and imported rock and stone assembled in every architectural style imaginable. Examples used in early construction include Late Proterozoic slates and conglomerates of Cambridge Argillite and Roxbury Conglomerate from the Boston Basin, middle Paleozoic granites from nearby Chelmsford, Quincy, Milford, Rockport and Stony Creek, early Mesozoic Portland brownstones from Connecticut's Hartford Basin and New Jersey's Newark Basin, and early Paleozoic marbles from Lee in Vermont. It’s easy to overlook the role that common brick has played in the growth of the city and New England. It's a ceramic structural material with main ingredients that arose from the weathering of igneous rocks and brought together by Pleistocene glacial depositional processes.
Take a drive through any town of considerable size in New England situated on a river that powered its mills in the nineteenth century. Even where building stone was available, the buildings are all composed of brick for reasons of economy and speed of construction. Things changed for brick at the turn of the twentieth century when the demand for high office buildings and less susceptibility to earthquakes increased, resulting in the use of cast and wrought iron, and later, steel and concrete. As for the mills, they closed in New England when alternatives to water power were developed and textile production became more profitable in southern states where cotton was grown and winters were warmer.
While ascending Little Haystack Mountain on the Franconia Ridge Trail in the western White Mountains of New Hampshire, my son and I came upon a large area of “needle ice” on the trail at about 3,000 feet of elevation early in the morning. It was late October, and the night had brought temperatures below freezing but without any precipitation. It was the first time I experienced such a variant of frozen, ribbon-like water and was fascinated how it forms.
Appearing as thin, curved, filamentous and striated combs, the needle ice grows from moist, water-penetrable soil generally before melting in the warmth of the sun. Unlike frost or rime, which obtains moisture from the air, the water source for needle ice is contained within the soil. When the air temperature drops well below freezing, water in the soil may become “super-cooled” well below freezing. The cold water is drawn upward through the soil via capillary action and is rapidly frozen into ice crystals near the surface, while being “fed” as additional water seeps out from the soil and freezes. While “growing”, needle ice may lift small soil particles. Along with cyclical freezing and thawing, frozen water in its many forms contributes to soil creep and even the erosion of mountains.
As mentioned, the early stages of continental extension across the Pangaean landscape promoted the formation of a number of rift basins on the eastern margin of North America and continents yet to form bordering the Atlantic Ocean, yet to open. Within central Connecticut, the Hartford Basin began to receive prolific Late Triassic through Early Jurassic sediments from the surrounding highlands on the east and west.
As the lithosphere beneath the basin stretched and thinned, the Central Atlantic Igneous Province that is thought to have initiated fragmentation of the supercontinent (or was a passive component as some advocate), fed basalts via a swarm of dikes into the Hartford Basin. Intrusive dikes and sills and extrusive flood basalts flooded the landscape and became emplaced between the basin's four sedimentary sequences in three events around 200 Ma over the course of 600,000 years - the Talcott, Holyoke and Hampden Basalts.
The traprock ridge of West Rock (and nearby East Rock, the Sleeping Giant and Barndoor Hills) is the product of the Fairhaven-Higganum feeder dike that supplied the several basalt flows that blanketed the sedimentary successions that filled the Hartford Basin. Due to tilting and erosion, the West Rock intrusive sill appears as if its an extrusive lava flow tilted eastward, such as the Talcott Basalt that flowed at that same time. West Rock's prominence on the landscape, as others in the Hartford Basin, is the result of topographic inversion subsequent to the erosion of the intervening, erosion-susceptible, sedimentary successions that filled the basin.
What process fed the basin's largely tholeiitic basalt, extrusive and intrusive flows, sills and radial dike swarms? Some workers believe the flows weren't just confined to the basins but may have extended across much of eastern North America in association with the Central Atlantic Magmatic Province or CAMP. Its presence is concurrent with the early breakup of Pangaea and spans four million years from 208-202 Ma. Its original extent is estimated to cover more than 7,000,000,000 square miles, and its remnants span the continents that share the Atlantic domain that were once contiguous across Pangaea before its disassembly.
Categorized as a large igneous province (LIP) - the largest on the planet - the CAMP's emplacement is debated as to the underlying mechanism that drives the process and causes continental breakup. Models range from changing plate boundary forces to mantle dynamics. In regard to the latter, was a deep mantle plume an active agent in Pangaea's fragmentation process or merely a passive, buoyant asthenospheric upwelling in response to lithospheric thinning caused by tectonic extension?
Possibly it was a hybrid of both whereby peripheral subduction causes plate extension, and the subsequent arrival of a plume weakens the fabric of the continent causing it to split open and form a new ocean. Regardless of the mechanism, the large igneous province poured greenhouse gases into the atmospheric and is suspiciously viewed as the cause of a devastating tetratodal extinction at the Triassic-Jurassic time boundary.
Having just completed a late summer post on fungi (here), I was quite surprised to discover a cluster of fairy rings on a suburban Connecticut lawn and in mid-December, well after the region's first frost. Their annually, concentrically-enlargening growth is the result of successive generations of some 60 mushrooms of Basidiomycetes fungi that germinate as mushrooms when conditions are right.
Fungi, being saprotrophic, feed on decaying organic matter such as typically found in forests (called "tethered" and are related to mycorrhizal symbiotic associations with trees) and lawns (referred to as "free", since they are not associated with other organisms). The fairy ring is detectable by a circle of mushrooms as well as a necrotic zone of dead grass or, counterintuitively, a ring of thriving, dark green grass, as seen above. In the latter circumstance, the below-ground mycelium, which is somewhat analogous to the roots of vascular plants, absorbs nutrients via the secretion of enzymes from the tips of hyphae, the thread-like, microscopic filaments that comprise the mycelium.
The mycelium gradually moves radially from the center of the expanding ring, when nutrients (generally nitrogen and iron) become sufficiently depleted. When the center dies, the ring become obvious outside the necrotic zone. Surprisingly, some fungi produce chemicals called gibberellins that act like hormones, which favorably affect plants causing rapid luxuriant growth.
Fairy rings are the subject of folklore, myth and the supernatural, especially in Western Europe. In France, they are referred to as “sorcerers’ rings” and in Germany “witches’ rings.” Some believe that anyone stepping into an empty fairy ring will die young. Those that violate the perimeter become invisible to those outside and may be unable leave the circle. The fairies force intruders to dance till exhausted, dead, or in the throes of madness. One of the largest fairy rings ever found is near the city of Belfort in northeastern France. It measures some 2,000 feet in diameter and, based on the rate of growth and expansion, is estimated to be 700 years old.
What shall I post about next? Is the subject matter worthy of discussion? What shall I say? What shall I omit? It’s the blogger’s never-ending dilemma. By the time the end of the year rolls around, at least for me, there are always a few posts that never got written and a few images that never got uploaded. And so, with this final post of the year – in what has been a tradition on my blog for five years running – here’s my end-of-the year post (although a little belated). Please visit the same for 2012 (here), 2013 (here), 2014 (here) and 2015 (here).
January
A “Worm Rock” from the Middle Holocene
Naples Beach, Southwest Coastal Florida
So, while on vacation with clear blue skies and 1,350 miles of accessible coastline, it seemed logical to head to the beach and see what the tide had brought in. To my surprise, I discovered a fossil remnant of a unique, marine bivalve colony that is responsible for the geomorphology of southwest Florida's carbonate-producing coastline and offshore islands.
| Image may be NSFW. Clik here to view.  |
| How am I going to do any geology here? |
Southeast of Marco Island along the southwest coast of Florida is the archipelago of Ten Thousand Islands. It’s a maze of oyster shoals, mangrove trees and brackish tidal channels that are a few miles wide and up to 20 miles long. The islands are actually part of an interesting stratal sequence deposited some 7,000 to 3,000 years ago.
Following the Pleistocene ice age, Holocene transgressions began to flood the Florida shelf. A basal peat layer formed below sea level that overlies eroded Pliocene and Pleistocene limestones. Quartz and shelly sands followed as the sea level rose. Overlying the sands of the inner islands are Holocene-age oyster reef beds that are overlain by modern peat and support the region’s mangroves. The right conditions of rising seas, climate and sedimentation converged over this interval to promote reef development.
| Image may be NSFW. Clik here to view.  |
| Extant Vermetid Marine Gastropod Serpulorbis squamigerus Rather than having regularly coiled shells, the elongated tubular shells of vermetid worm snails grow on hard surfaces either solitary or cemented together. The adult or apertural end portion of the shell is free and directed upward. From Wikipedia |
The outer islands, in addition to oyster beds, have an up to 10 foot-layer of the worm-like mollusk Petalochonchus varians. The marine gastropod is of the Vermetidae family but doesn’t resemble the coiled shell of the average sea snail and is frequently interpreted as a marine annelid, tube worm, and vice versa. The "worm snails” grow cemented together in complex, anastomosing colonies locally known as “worm rocks.” From about 3,000 years ago, they formed a small barrier reef system until recently, when they began to experience an inexplicable global decline. On occasion, dislodged erratics wash ashore and await discovery by unsuspecting geo-beachcomers.
January
A Little Swamp Geology
Big Cypress National Preserve, Southern Florida
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| An Alligator-infested Open Strand in Big Cypress Preserve Surrounded by Cypress and Deciduous Hardwood Trees |
Water lazily flows to the south and southwest at a barely perceptible rate since the landscape is almost virtually flat - only two inches per mile! As a result, it is subject to extraordinary extremes of wet and dry weather. Far more than just a stagnant pool with a high watertable, the “River of Grass” is a wide, slowly moving, freshwater sawgrass prairie or marsh. "Sheet-flow" is the frequently used, descriptive term.
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| Southern Florida's hyrologic ecosystem includes the Everglades, Big Cypress and Ten Thousand islands |
The Everglades system is no longer a single hydrologic unit. For purposes of flood control, agricultural irrigation, habitable real estate and fresh water, its natural flow has been re-engineered into a "water management system" that has been compartmentalized, fragmented and sub-sectioned with a network of canals, flood gates, levees and highways that criss-cross and subdivide it. It's a nutrient-poor ecosystem supplied by rainfall and plant decay, which over eons has created a stratum of peat in depressed areas. But now, it's in a state of nutrient overload from fertilizer that has drifted downstream from the agriculture district below Okeechobee and from both animal and human waste. The result is a stressed, altered and unsustainable ecosystem without natural flow.
West and southwest of the Everglades and confined by the limestone Immokalee Rise lies the state's other major wetland, the 1,200 sq mi swamp of Big Cypress. Eastern America's "last great wilderness" is named for its area rather than the size of its flood-adapted, deciduous trees. It's actually an extension of the Everglades hydrologic system. The two are integrally-related and similarly nutrient-poor, but their character, biology, ecology and geology are surprisingly quite different.
West and southwest of the Everglades and confined by the limestone Immokalee Rise lies the state's other major wetland, the 1,200 sq mi swamp of Big Cypress. Eastern America's "last great wilderness" is named for its area rather than the size of its flood-adapted, deciduous trees. It's actually an extension of the Everglades hydrologic system. The two are integrally-related and similarly nutrient-poor, but their character, biology, ecology and geology are surprisingly quite different.
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| Over half of the Big Cypress Preserve is a cypress swamp, but its includes open stands of small cypress trees that grow seasonally among seasonally-flooded grasslands known as cypress prairie. Another quarter is comprised of various forms of treeless wet prairies and marshes, while some 15% supports pine forests, less than 4% is elevated enough to support upland hardwood forests and around 1% extends into the mangrove zone along Florida's southwest coast. Exotic invasive plants (such the Australian tree melaleuca was introduced in the early 1900's as an ornamental and lumbar source) live within the preserve that often place native species in peril by competition. The same holds true for native sawgrass in the Everglades and fauna such as the Burmese python. Can you spot the alligator lying in wait? |
In common, they occupy one of the lowest, youngest and most geologically stable platforms in North America. And like the Everglades, due to its low topography, Big Cypress has repeatedly been submerged and exposed by the sea within the last 50,000 years. But, the bedrock beneath Big Cypress - thousands of feet of carbonate strata - is harder and less porous, which is reflected in the growth conditions of the vegetation, although, like the Everglades, it can store a great deal of water.
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| The Swamp Lily or String Lilly (Crinum americanum) is actually an Amaryllis. |
Reduced drainage into the underlying rock during the dry season in the Everglades promotes periodic soil fires, which are actually beneficial and no longer extinguished (and even prescribed), since it releases nutrients back to the soil and promotes biological diversity. It is lethal to cypress with the exception of their resistant above-ground portions. Big Cypress bedrock is more protective of the vegetation. Water is from direct rainfall without significant contributing flow from the north in contrast to the Everglades that is fed from the vast and shallow headwaters of Lake Okeechobee.
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| Big Cypress's Majestic, Swamp-loving Wading Bird, the Great Blue Heron (Ardea herodias) |
Water outflow from Big Cypress is eastward to the southernmost Everglades but also westward into the mangrove swamps and the Ten Thousand Islands coastal region on the Gulf of Mexico that serves as a buffer between the salty sea and freshwater marsh. For information on the geological evolution of the Florida Platform, please visit my post here.
February
Geological Torture in the Grapevines
Lost Valley and Titus Canyon, Death Valley National Park
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| Looking NNW into Lost Valley From Red Pass Towards Leadfield and Titus Canyon Enter the following coordinates into an online mapping program such as Google Earth, and it will take you there: 36°49' 44.12"N, 117°02'03.21"W. |
Geological torture and Death Valley are synonymous. A good example is at "Lost Valley" or "Canyon" below Red Pass at 5,250 feet in the Grapevine Mountains on Death Valley’s northeast side. The landscape has seen it all - twisting, gnarling, folding, faulting, extension and compression. The "Bloody Pass" lies midway on a spectacular, 26-mile, single lane, high clearance drive westward from Amargosa Valley on the east to Death Valley that finishes with a climactic drive through narrow Titus Canyon. The excursion is a complicated exercise in structural and stratigraphic geo-gymnastics.
The strata of Lost Valley spans time frames from Cambrian to Recent and represents continental clastic sedimentary rocks deposited on the miogeosyncline of western Laurentia (the rifted passive margin of the supercontinent of Rodinia), ash flow tuffs (the products of volcanic eruptions related to the middle Miocene multi-calderic southwest Nevada volcanic field) and lavas (Miocene to Pliocene in age). Multi-colored conglomerates and sandstones in the walls belong to the Eocene to Oligocene Titus Canyon Formation. Alluvial fan and lacustrine deposits, megabreccias and conglomerates were deposited within a fault-controlled basin and provide evidence for Early Oligocene extension before the formation of Death Valley. Banded grays are mostly limestones of the Cambrian Bonanza King Formation, while beyond are volcanics.
The strata of Lost Valley spans time frames from Cambrian to Recent and represents continental clastic sedimentary rocks deposited on the miogeosyncline of western Laurentia (the rifted passive margin of the supercontinent of Rodinia), ash flow tuffs (the products of volcanic eruptions related to the middle Miocene multi-calderic southwest Nevada volcanic field) and lavas (Miocene to Pliocene in age). Multi-colored conglomerates and sandstones in the walls belong to the Eocene to Oligocene Titus Canyon Formation. Alluvial fan and lacustrine deposits, megabreccias and conglomerates were deposited within a fault-controlled basin and provide evidence for Early Oligocene extension before the formation of Death Valley. Banded grays are mostly limestones of the Cambrian Bonanza King Formation, while beyond are volcanics.
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| The Upper Narrows of Titus Canyon Geologist, author and guide Wayne Ranney (here) takes in the solitude and shade of the wider, upper narrows of Titus Canyon. Water (and the rocks and boulders that it carries) funneled down from the watershed of Lost Valley are responsible for the erosive-genesis of this otherwise bone-dry canyon. It's a commentary on the tremendous carving capacity of stream cutting in association with tributary erosion and mass wasting. But in what time frame? Geologists estimate that a 1-inch rainfall over the region's 35 square-mile watershed can excavate a few hundred thousand cubic feet of material in merely 50,000 years to create the massive fans that radiate outward from the canyon's mouth at Death Valley, not far from this point. Of course, these are arid times, and don't take into account the wetter post-Pleistocene climate that came before. |
Haven't seen enough torture? Beyond Red Pass the road drops some 1,100 feet. Beyond the region of the notorious Leadfield mining district and ghost town, it almost abruptly enters the narrows (the reason the excursion is one-way) of 8.8 mile-long, rugged and cliff-walled Titus Canyon, named after a mining engineer that mysteriously perished in these parts back in 1905. A low-angle, normal fault dominates the structural framework of the canyon, which places younger strata above the fault in association with older rocks below the fault. On close inspection, the bedding appears to be horizontal and undeformed, but don't be fooled. A "big picture" stratigraphic and structural analysis will indicate that it is over-turned!
One distinctive canyon wall is a massive mosaic of erosion-polished, brecciated (angular, flat-sided) dark carbonate fragments of the Middle Cambrian to early Late Cambrian Bonanza King Formation. It was deposited offshore - as was the familiar and geo-equivalent Middle Cambrian Muav Limestone of the Grand Canyon to the east - on the passive margin of Laurentia subsequent to the fragmentation of the supercontinent of Rodinia. How did the "mosaic" form? Analyze it a minute before you answer.
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| Counterintuitively, a drive down Titus Canyon takes you topographically downward into progressively younger rocks. It's because the strata has been inverted within a folded anticline. |
One distinctive canyon wall is a massive mosaic of erosion-polished, brecciated (angular, flat-sided) dark carbonate fragments of the Middle Cambrian to early Late Cambrian Bonanza King Formation. It was deposited offshore - as was the familiar and geo-equivalent Middle Cambrian Muav Limestone of the Grand Canyon to the east - on the passive margin of Laurentia subsequent to the fragmentation of the supercontinent of Rodinia. How did the "mosaic" form? Analyze it a minute before you answer.
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| Dee and the Enigmatic Jigsaw Puzzle Wall of Titus Canyon |
Here's one explanation. The white rock is crystalline calcite that lacks bedding planes and appears to hold Bonanza King fragments in suspension. It's a clue as to how the carbonate matrix formed. In addition, small lenses or tongues of calcite penetrate the Bonanza fragments, which almost fit together like the pieces of a puzzle. There are no obvious faults here. Therefore, the breccia is likely not a product of faulting but progressive fracturing.
The mosaic may have formed under severe stress, deep underground and concurrent with the formation of an anitformal recumbent syncline (large-dimensional folds that are younger at the core and lie on their side). The host rock, the Bonanza King, fractured along with the intrusion of the molten, now flowable calcite that was derived by pressure solution of the host rock. Proof of recumbency is that older formations lie above younger ones. What's your interpretation?
The mosaic may have formed under severe stress, deep underground and concurrent with the formation of an anitformal recumbent syncline (large-dimensional folds that are younger at the core and lie on their side). The host rock, the Bonanza King, fractured along with the intrusion of the molten, now flowable calcite that was derived by pressure solution of the host rock. Proof of recumbency is that older formations lie above younger ones. What's your interpretation?
April
Ubiquitous and (Almost) Forgotten Brick (and Clay)
Financial District of Boston
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| A Collage of Building Materials and Architectural Styles in the Financial District of Boston |
Take a drive through any town of considerable size in New England situated on a river that powered its mills in the nineteenth century. Even where building stone was available, the buildings are all composed of brick for reasons of economy and speed of construction. Things changed for brick at the turn of the twentieth century when the demand for high office buildings and less susceptibility to earthquakes increased, resulting in the use of cast and wrought iron, and later, steel and concrete. As for the mills, they closed in New England when alternatives to water power were developed and textile production became more profitable in southern states where cotton was grown and winters were warmer.
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| Postcard of the Brick-built Mill Town of Manchester, NH, on the Merrimack River |
The first Europeans to arrive in New England needed brick for the chimneys of log cabins, which were built of stone plastered with lime made from the endless supply of crushed clam shells. Much needed New England timber was shipped to England, which required stone as a ballast in the empty holds on the return sail. Legend has it that river cobbles were used that were repurposed as street pavers. But, apparently “ballast” brick was substituted, found throughout New England in Period fireplaces, chimneys, street and sidewalk pavers, and foundations. Before long, as the demand rose, the colony’s growing population sought out local sources of clay for brickmaking, as kilns began to fire up everywhere. In New England, the first brick kiln was erected in the town of Salem, Massachusetts, in 1629.
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| Colonial Brick Buildings, Sidewalk-lined, and Cobble-paved Acorn Street in Boston's Beacon Hill Although cobblestones were noisy under hooves and wagons in the old city, the cobbled streets didn't succumb to the degradative effects of long New England winters and remained mud free. Eventually they fell out of favor to rectangular granite setts in the 1800's and asphalt in the 1900's. |
Clay is an essential ingredient in brick that happens to be extremely plentiful in New England. During the Pleistocene the Laurentide continental ice sheet made multiple advances and retreats over northern North America. Glacial scraping and gouging of the landscape began to end some 20,000 when the climate warmed, and it made a final retreat bringing the northeast into its current interglacial period. Erosional and depositional glacial features littered the landscape in the form of outwash (well-sorted and well drained sand and gravel) and till (an unsorted, non-stratified mix of clay, sand, silt, pebbles cobbles and boulders).
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| Typical Gray to Greenish New England Glaciolacustrine Clay Pit Wikimedia Commons |
Clay was typically excavated from clay pits that formed at the bottom of the many post-glacial lakes that dot the landscape, delivered and sorted by glacial rivers and streams. The glacio-lacustrine deposit has a particle size smaller than 2 µm (which differentiates it from silt) and is defined as a fine-grained rock or soil combined with organic matter and certain minerals that form in the presence of water (commonly hydrous aluminum phyllosilicates and various metallic oxides such as iron and magnesium). Its silicate composition is a result of weathering of the glacially-scoured, granitic bedrock commonly found in upper New England. If clay remains in the soil long enough and is subjected to sufficient pressure, it may become a shale, argillite or metamorphose into slate.
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| The Old South Meeting House of Boston Constructed of Colonial-era Brick from Local Clay Pits A walk on the Freedom Trail in Boston is a veritable geological field trip of the rock and stone that built the city. A good example is "Old South" built in 1729, a historic church and cherished landmark in the heart of the old city. It's the second church that occupied the site, all of which were constructed by Puritans. During the siege of Boston, British troops used the wood of the parsonage for firewood, while the church's brick construction likely saved it from a similar fate as it did during the Great Fire of 1872 that ravaged the city. |
Clay's small particle size and unique crystalline structure confers it with desirable properties of plasticity (due to high water content), and brittleness, hardness and heat- and fire-resistant (upon drying and firing in a kiln at about 2,000° F). Under these conditions, clay is "metamorphosed" and undergoes a permanent physical and chemical change converting it to a ceramic material and, in the case of brick, a colorful (due to metallic oxides such as iron and magnesium), load-bearing material that is also valuable in pottery, chinas, porcelains and tiles.
Clay is one of the oldest building materials on Earth, used by the Persians, Assyrians, Egyptians, Greeks and Romans, sun-dried in its most primitive form. The Byzantines devised a technique for exposing brick and giving it decorative expression especially when arranged in various patterns. At the beginning of the nineteenth century, mechanical brickmaking processes were employed that replaced ancient hand-fashioning methods. Between one-half and two-thirds of the world's population, in both traditional societies as well as developed countries, still live or work in buildings made with clay baked into brick.
May
The Many-Marbled Monument to George
District of Columbia
I snapped this iconic photo of the George Washington Monument from my seat on the National Mall, while waiting for my daughter’s college commencement exercises to begin. It's the world’s tallest stone structure and tallest obelisk at 554 feet and 7 11/32 inches. I couldn't help noticing that the color and texture of its stones differed markedly and wondered about its construction history. A little research confirmed that its 36,000 stones weren’t excavated from the same quarry, which explains the difference in the stones at the bottom one-third. Accounts indicate they matched initially, but differences in the composition of the carbonates have allowed weathering to accentuate the two since construction was initiated in 1848.
Apparently, funding and concerns over the region’s swampy foundation - the original site designated by Pierre L'Enfant was moved - delayed construction for 25 years after the base had been initiated. When construction was about to resume in 1876, the builders discovered that the foundations were inadequate and the monument was sinking and tilting. To stabilize and straighten the monument, wider sub-foundations were constructed to a depth of nearly 37 feet. When construction resumed, a different marble was used. Actually, a third type of marble was used in the transition zone.
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| First Phase of Construction The Washington Monument was the tallest building in the world upon its completion in 1884. The structure was completed in two phases, one private (1848-1854) and one public (1876-1884). |
The first stone consists of Cockeysville Marble from quarries in the Piedmont province at Cockeysville and Texas, Maryland, just north of Baltimore. It’s a fine-grained, magnesium-rich, clear white stone with a few pale streaks or bands, which give an effect of pale gray. The marble from the Texas quarry is whiter and coarser grained and is nearly pure calcium carbonate. Some specimens of both marbles contain veins and pockets of mica and pyrite, which have stained the marble from exposure to the elements.
In 1879 work began again on the upward projection of the monument, and four courses or rows of white marble from Sheffield, Massachusetts, were laid above the Texas marble. However, because of difficulties with timely delivery and quality control, the contract with the Sheffield quarry was annulled in 1880. The upper part of the monument was finished with Cockeysville marble.
October
Finding "Needle Ice" on Little Haystack
Franconia Ridge of the White Mountains of New Hampshire
While ascending Little Haystack Mountain on the Franconia Ridge Trail in the western White Mountains of New Hampshire, my son and I came upon a large area of “needle ice” on the trail at about 3,000 feet of elevation early in the morning. It was late October, and the night had brought temperatures below freezing but without any precipitation. It was the first time I experienced such a variant of frozen, ribbon-like water and was fascinated how it forms.
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| Franconia Notch State Park of the Western White Mountains of New Hampshire Located in the heart of the White Mountains National Forest, the notch - a New England geological term for a glacially-scoured mountain pass - is a product of the last advance and retreat of the Laurentide continental ice sheet. Eight miles of north-south Interstate 93 slices through it with spectacular cliffs of Cannon Mountain on the west (former site of the "Great Stone Face" of the Old Man of the Mountain) and peaks of Franconia Ridge (Mounts Lafayette, Lincoln and Little Haystack) on the east. The ridge is notorious for its unpredictably inclement and dangerous weather at any time of the year. Its also famous for classic glacial geomorphology. Louis Agassiz, the renowned Swiss naturalist, geologist and Harvard professor, confirmed in 1847 that continental-scale glaciers were responsible for the appearance of the landscape. In fact, the terrain north of the gap (seen above) contains Alpine-like "ancient moraines" that he studied. Aerial photo courtesy of Bill Hemmel. Please visit him at here. |
Appearing as thin, curved, filamentous and striated combs, the needle ice grows from moist, water-penetrable soil generally before melting in the warmth of the sun. Unlike frost or rime, which obtains moisture from the air, the water source for needle ice is contained within the soil. When the air temperature drops well below freezing, water in the soil may become “super-cooled” well below freezing. The cold water is drawn upward through the soil via capillary action and is rapidly frozen into ice crystals near the surface, while being “fed” as additional water seeps out from the soil and freezes. While “growing”, needle ice may lift small soil particles. Along with cyclical freezing and thawing, frozen water in its many forms contributes to soil creep and even the erosion of mountains.
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| My son Will and I on Little Haystack The Franconia Ridge Traverse and the nine-mile, seven-hour Loop are one of the most popular climbs in New England. It's a small segment of the 2,180 mile-long Appalachain Trail that stretches from Georgia to northern Maine. National Geographic promoted the traverse in an article entitled "World's Best Hikes: Twenty Dream Trails", and it's just over a two hour drive from Boston! You remain above treeline for each peak on this second highest range in the White Mountains. But beware, since the weather can change on a dime. On this beautiful day, the wind was intermittently gusting upslope at 40 to 50 mph! The fascinating geology on this part of the Whites, a Jurassic ring-dike, will be the subject of a future post. |
November
A Late Triassic Redbed Foretells the Fragmentation of a Supercontinent
Route 15 Roadcut in South-Central Connecticut
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| The Late Triassic New Haven Arkose of the Hartford Rift Basin of Central Connecticut The lowest beds in the exposure tilt to the southeast, the direction of the Eastern Border Fault, while upper beds appear horizontal, indicative of intermittent tilting of the basin. |
Along the westbound lane of south-central Connecticut's Route 15 is a small but glorious roadcut of the Late Triassic New Haven Arkose. Likely unnoticed by passersby and unappreciated for its regional albeit global significance, the redbed was deposited in the Hartford Basin during embyronic stages of the Late Proterozoic supercontinent of Pangaea's breakup. Rivers and streams delivered sand, silt and rocky debris from neighboring highlands into the rift basin as it subsided and tilted eastward on the landscape. The arkose is the lowest sedimentary succession of four that filled the basin and are intercalated by three lava flows.
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| Simplified Cross-section of the Hartford Basin of Connecticut Flanked by the Western and Eastern Uplands acquired during the Taconic orogeny, the half-graben is floored by a pre-Triassic Paleozoic crystalline basement. The has subsided to the southeast along the Eastern Border fault. From Window into the Jurassic World |
The sedimentary successions of the Hartford Basin formed in a tripartite stratigraphic sequence: the fluvio-arkosic Late Triassic (to possibly Early Jurassic) New Haven Arkose as the basin initiated subsidy and tilting; followed by Early Jurassic, dinosaur track-infused, climate and celestially-controlled, ephemeral lacustrine-playal silt- and mudstones of the Shuttle Meadow, East Berlin and Portland Formations; and a final arkosic cap of the Early Jurassic (upper) basin subsidy-ending Portland Formation.
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| Idealized Cross-section through the Hartford Basin Four Late Triassic through Early Jurassic sedimentary successions are interposed by three lava flows that in the Hartford Basin that progressively tilted across the landscape to the southeast at the Eastern Border fault. Intrabasinal faults and various rider blocks comprise the basin's internal structure. From Window into the Jurassic World |
The north-south trending Hartford Basin constitutes the Central Lowlands of Connecticut and is flanked by the Western and Eastern Uplands. It's a subbasin of the "greater" Connecticut Basin along with the Deerfield Basin of central Massachusetts. The two subbasins possess similar temporal, stratigraphic and formative histories and are members of a large system or rift province of basins called the Newark Supergroup (after the first and most studied rift basin in northern New Jersey) that lie up on down the eastern seaboard landward of North America's passive continental margin.
The basins are either exposed, buried or submerged and formed during the early stages of continental extension that ultimately led to the fragmentation of Pangaea and the opening of the Atlantic Ocean that led to the rifting and drifting of the continents of the modern world.
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| Early Mesozoic Rift Basins of the Circum-Atlantic and Eastern North American Domain (a), Rift Basins landward of the rift zone on North America, Greenland, Great Britain, Iberia, North Africa and South America (not shown); (b), Rift basins that form a serpentine arc landward of the eastern margin of North America. Note the generally parallelism of all the basins on strike with the Appalachian orogen and Mid-Atlantic Ridge. Note also the Connecticut Basin (red arrow) and others such as the Fundy, Pomperaug, Newark, Gettysburg, Culpeper , Taylorsville, Richmond, Danville, etc. Modified from Roy M. Schlische |
The majority of the rift basins are either NNE to NE-trending and are grabens or contain half-graben type of subbasins (lithospheric extensionally-related, subsiding and tilting, fault-bound troughs). Their trend roughly parallels the Appalachian orogen that assembled throughout the Paleozoic as Pangaea assembled. In addition, the rift basins roughly align with the Mid-Atlantic Ridge spreading center that developed between the diverging continents of North America and Europe.
The schisms and sutures are inherited tectonic structures that have dictated not only the assembly and disassembly of the supercontinents of Rodinia and Pangaea but the general, parallel trend of the early Mesoozic rift basins of the Supergroup. You can easily see it on a geologic bedrock map of northeastern North America (lower left). You can experience it on a west-to-east drive anywhere across the Appalachian orogen or regionally within Connecticut. Every mile that you travel to the west takes you back in time, as the geology changes accordingly. It explains why the regional geology and terrain doesn't change much from north to south, since you're travelling along the preserved margins of accreted and rifted terranes.
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| Left: Tapestry of Time and Terrain (USGS); Right: Map showing the record of tectonic inheritance through two complete Wilson cycles in eastern North America: the assembly of Rodinia, opening of the Iapetus Ocean, assembly of Pangaea, and opening of the Atlantic Ocean (William A. Thomas, 2005 GSA Presidential Address). |
Consistent with the time frame of the Late Triassic and Early Jurassic, both quadrupedal and bipedal tracks of various reptiles including crocodylomorphs, theropods and lepidosaurs are found in sedimentary strata of the Connecticut Valley and other early Mesozoic rift basins, although osseous remains are scant. It was not known at the time of discovery and for a time afterward, the tracks and trackways of the Connecticut Basin, the Deerfield Subbasin in particular, were the first evidence of dinosaurs found in North America.
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| A Tridactyl, Bipedal Dinosaur Track in a Rippled Bed of the East Berlin Formation Medium to large bipedal tracks of theropods in the East Berlin Formation of the Hartford Basin are abundant. This one is located near Rocky Hill, Connecticut, the location of Dinosaur State Park (Go there). We are unable to make a positive identification from a track, thus we can only surmise what particular dinosaur left these prints. As a result, tracks are categorized by an ichnotaxonic classification system that includes characteristics such as number and length of pedal digits, axial alignment of the metatarsalphalangeal pad and squamation (degree and arrangement of scales). Best guess? Dilophosaurus or coelophysis. |
Heroes' Tunnel Dives into an Early Mesozoic Sill
Route 15, New Haven in South-Central Connecticut
Everyone knows the location of "the tunnel" on Route 15 in south-central Connecticut, just north of New Haven. More than just a geographic landmark for travelers, Heroes' Tunnel has both geological and American historical significance as well. To reach it from the west or east, the highway drops into a hollow on the landscape, eroded into soft sediments of the Hartford Basin, and then plunges into the 1,200 foot-long tunnel cut into West Rock, an erosion-resistant, north-south trending, basaltic "mountain" ridge.
If your eyes are able to depart from the road long enough (something all geologists are highly skilled at), you would see massive, postpile-like pillars of dark-colored, fine-grained, orthogonal columnar basalt called diabase - locally called trap rock (after the Swedish word "trappa" for stairway) - standing guard at both portals.
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| The cliffs of West Rock seen from the southwest. The rusty brown color is due to iron contained within the basalt. |
As mentioned, the early stages of continental extension across the Pangaean landscape promoted the formation of a number of rift basins on the eastern margin of North America and continents yet to form bordering the Atlantic Ocean, yet to open. Within central Connecticut, the Hartford Basin began to receive prolific Late Triassic through Early Jurassic sediments from the surrounding highlands on the east and west.
As the lithosphere beneath the basin stretched and thinned, the Central Atlantic Igneous Province that is thought to have initiated fragmentation of the supercontinent (or was a passive component as some advocate), fed basalts via a swarm of dikes into the Hartford Basin. Intrusive dikes and sills and extrusive flood basalts flooded the landscape and became emplaced between the basin's four sedimentary sequences in three events around 200 Ma over the course of 600,000 years - the Talcott, Holyoke and Hampden Basalts.
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| Almost adjacent to the New Haven exposure to the west, this roadcut exposes the Talcott Basalt and overlies the arkose. |
The traprock ridge of West Rock (and nearby East Rock, the Sleeping Giant and Barndoor Hills) is the product of the Fairhaven-Higganum feeder dike that supplied the several basalt flows that blanketed the sedimentary successions that filled the Hartford Basin. Due to tilting and erosion, the West Rock intrusive sill appears as if its an extrusive lava flow tilted eastward, such as the Talcott Basalt that flowed at that same time. West Rock's prominence on the landscape, as others in the Hartford Basin, is the result of topographic inversion subsequent to the erosion of the intervening, erosion-susceptible, sedimentary successions that filled the basin.
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| 1849 Painting of West Rock by Frederic Edwin Church Wikipedia Commons, Public Domain |
What process fed the basin's largely tholeiitic basalt, extrusive and intrusive flows, sills and radial dike swarms? Some workers believe the flows weren't just confined to the basins but may have extended across much of eastern North America in association with the Central Atlantic Magmatic Province or CAMP. Its presence is concurrent with the early breakup of Pangaea and spans four million years from 208-202 Ma. Its original extent is estimated to cover more than 7,000,000,000 square miles, and its remnants span the continents that share the Atlantic domain that were once contiguous across Pangaea before its disassembly.
Categorized as a large igneous province (LIP) - the largest on the planet - the CAMP's emplacement is debated as to the underlying mechanism that drives the process and causes continental breakup. Models range from changing plate boundary forces to mantle dynamics. In regard to the latter, was a deep mantle plume an active agent in Pangaea's fragmentation process or merely a passive, buoyant asthenospheric upwelling in response to lithospheric thinning caused by tectonic extension?
Possibly it was a hybrid of both whereby peripheral subduction causes plate extension, and the subsequent arrival of a plume weakens the fabric of the continent causing it to split open and form a new ocean. Regardless of the mechanism, the large igneous province poured greenhouse gases into the atmospheric and is suspiciously viewed as the cause of a devastating tetratodal extinction at the Triassic-Jurassic time boundary.
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| The Central Atlantic Magmatic Province on the Continents of the Atlantic Domain The Connecticut Basin and its Deerfield (D) and Hartford subbasins (H) are labelled in association with extrusive and intrusive volcanics. From Whiteside et al |
December
Persistent "Fairy Rings" of Late Summer
Rocky Hill, Connecticut
Fungi, being saprotrophic, feed on decaying organic matter such as typically found in forests (called "tethered" and are related to mycorrhizal symbiotic associations with trees) and lawns (referred to as "free", since they are not associated with other organisms). The fairy ring is detectable by a circle of mushrooms as well as a necrotic zone of dead grass or, counterintuitively, a ring of thriving, dark green grass, as seen above. In the latter circumstance, the below-ground mycelium, which is somewhat analogous to the roots of vascular plants, absorbs nutrients via the secretion of enzymes from the tips of hyphae, the thread-like, microscopic filaments that comprise the mycelium.
The mycelium gradually moves radially from the center of the expanding ring, when nutrients (generally nitrogen and iron) become sufficiently depleted. When the center dies, the ring become obvious outside the necrotic zone. Surprisingly, some fungi produce chemicals called gibberellins that act like hormones, which favorably affect plants causing rapid luxuriant growth.
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| Modified from victoriaweb.com |
Fairy rings are the subject of folklore, myth and the supernatural, especially in Western Europe. In France, they are referred to as “sorcerers’ rings” and in Germany “witches’ rings.” Some believe that anyone stepping into an empty fairy ring will die young. Those that violate the perimeter become invisible to those outside and may be unable leave the circle. The fairies force intruders to dance till exhausted, dead, or in the throes of madness. One of the largest fairy rings ever found is near the city of Belfort in northeastern France. It measures some 2,000 feet in diameter and, based on the rate of growth and expansion, is estimated to be 700 years old.
December
Cenozoic Sunrise over a Widening Atlantic Ocean
In another place. In another time.
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