Monday, December 31, 2018

2018 Geology Posts and Photos That "Never Made It"

Cyclonically Frozen in New England; Glorious Spring Has Finally Sprung; Born of Necessity; Volcanic Plumbing in Iceland; Seafloor of a Konservat-Lagerstätten; New England's Most Enigmatic Exposure; "Squantum" Tombolo at Low Tide; Testimony to an Arid Interior; Volcanic Dams of the Inner Gorge.

By the time the end of the year rolls around, there are always a number of posts that were never written. And so, with this final one of the year, in what has become a tradition on my blog for six years running, here’s my end-of-the-year post of those that "never made it" in 2018. Please visit the same for 2012 (here), 2013 (here), 2014 (here), 2015 (here), 2016 (here) and 2017 (here). 

Cyclonically Frozen in New England
Newton, Massachusetts

In January, New England was hammered by one nor'easter after another. According to the National Weather Service it's a "macro-scale, extra-tropical cyclone in the western North Atlantic Ocean." It gets its New England moniker since it tracks "down Maine" from the northeast along the eastern seaboard as hurricane-force winds whip the coast in a counter-clockwise direction from the sea.

The assault continued through March when four hit in ten days back-to-back. Like hurricanes, they had alphabetically friendly names. There was Riley, Quinn, Skylar and Toby. Unfortunately, Riley wasn't so amicable.

It underwent a process of bombogenesis when it dropped 24 millibars of atmospheric pressure over a 24-hour period and intensified to explosive levels with an enormous footprint. With plenty of Arctic air to work with, it blanketed the region with over two feet of snow overnight. Instead of being light and fluffy, it was wet, cardiac-heavy and downed trees and power lines everywhere while flooding the coast with enormous destruction of property. Riley did, however, leave the landscape strikingly pristine and sparklingly blue with the reflected colors of the sky. 

Glorious Spring Has Finally Sprung
Newton, Massachusetts

Sunrise on the Summit of Chestnut Hill

"Enough is enough!" "When will it end?"" Is this a spring thaw or the real thing?" In my nearly fifty years living in New England, I never heard so many complaints. It wasn't until May that winter loosened its icy grip. The elation brought about by warm sunshine, a verdant landscape and happy flowers was palpable. As we know, winter astronomically begins on the Winter Solstice and ends on the Vernal Equinox. It's marked on everyone's calendar, but in the Northeast, the dates are meaningless.

On the Solstice the sun appears to be "standing-still" (in Latin) at its southernmost turning-point before reversing direction with the Northern Hemisphere inclined away from the sun in winter. Galileo knew this but was forced to recant his revolutionary theory in 1633. Equinox, on the other hand, means "equal-night", hence equal-day and equal-illumination (or nearly so in reality). In the Northern Hemisphere it produces spring, and fall in the Southern. Historically, the dates were established by Julius Caesar but changed by Pope Gregory XIII to coincide with Easter and again by astronomers to precede Easter. 

At least on the Equinox in New England, although it still feels like winter, the sun's path on the ecliptic is higher and warmer, which melts the snow quicker, thaws the frozen earth and starts the Maple sap flowing. Winter's end-Spring's beginning is "slush season" and "mud month" up here. The unofficial first day of spring is when winter regalia and snow removal tools are noticeably absent, which is a far more accurate gauge than your calendar.

Born of Necessity
Chestnut Hill, Massachusetts

Sunrise on the Chestnut Hill Reservoir during My Morning Run
Named for the area around the surrounding towns of Boston, Brookline and Newton, the Chestnut Hill Reservoir is a quiet, recreational haven and easy escape from the clamor of the city. It's also known as the location of Boston College and the top of Heartbreak Hill on the Boston Marathon route.

Recognizing the need for a source of perennial fresh water, Puritan settlers in 1630 switched from Charlestown to the Shawmut Peninsula of 'Olde Boston' across the Charles River to take advantage of the Great Spring on Boston Common. As the population and demands of the settlement and growing town increased - 30% in the 1850s - one reservoir after another was added to the delivery system for domestic needs and in the event of a major fire.

In the early 1800s, gravity-fed Jamaica "kettle" Pond delivered water to Boston through wooden pipes. Wellesley's Lake Cochituate Reservoir to the west was added in 1863. By 1870, the Chestnut Hill Reservoir was completed some five miles west of Boston, excavated from marsh and meadowland acquired "by purchase or otherwise" from the Lawrence farm. Its basin covered 37.5 acres with a 180 million gallon capacity and conveyed water through cast-iron pipes. In the 1930s, the Wachusett and Quabbin Reservoirs were finally added 30 and 65 miles to the west with a capacity of 477 billion gallons. The latter was developed by forcing residents from their homes, relocating cemeteries of four 1700s-era towns and flooding 38.6 square miles of countryside.

At one time, Frederick Law Olmsted, the famous landscape designer of New York's Central Park, envisioned adding the Chestnut Hill Reservoir to the Emerald Necklace, his elegant system of Boston's interconnecting municipal parks and waterways.

These days, the reservoir is offline but on stand-by to maintain water pressure and as a back-up for water emergencies. It's surrounded by majestic old trees and rocky outcrops of Precambrian-age Roxbury Conglomerate. Replete with hilly woodland, stonewalls, walkways, hiking trails and a 1.56 mile-long loop for jogging, strolling and contemplation, it's a place to fish and observe water and birds of prey, turtles, muskrats, rabbits, squirrels and even fox at sunrise...all within city limits!

Volcanic Plumbing in Iceland
East Fjords

Julia Share on an Exhumed Dike in East Iceland

Following a vertical path of least resistance by cross-cutting strata, dikes are relatively shallow and narrow geologic bodies in contrast to sills that are deeper and broader horizontal sheets that dissect between strata. Both intrusions transport magma away from a central volcano, which is supplied by a large deep-seated reservoir.

Dikes may feed surface eruptions and are extremely common in Iceland. Most remain buried and solidified beneath the surface, only to be exhumed over time by erosion as demonstrated by my daughter Julia. 

Dikes, sills and batholiths (deeply-buried magma reservoirs) are testimony to the complexity of volcanic systems that participated in the formation of Iceland, the world's largest volcanic island and one of the youngest at 24 million years. The intrusions are either Tertiary (Miocene-Pliocene), Pleistocene or Holocene in age. This dike is one of many along the Ring Road that encircles Iceland near fjord Hamarsfjörður on the East Coast. It's part of a once-active swarm that fed the Tertiary Basalt Formation, the oldest in Iceland that spans the interval from 16 to 3.3 million years ago. 

Seafloor of a Konservat-Lagerstätten
The Walcott-Rust Quarry
Central New York State

Richly Fossiliferous and Diverse Turbidite from a Middle Ordovician Taconic Foredeep
The abundance of invertebrate remnants in this "hash", implies its preservation in a high energy system. Can you identify various fragments of disarticulated trilobites especially the cephalon? How about a bryozoan attached to a crinoid pluricolumnal? Beyond it's function as an attachment apparatus, might the bryozoan be a symbiont? An expanded post on the quarry is forthcoming in 2019.

On private land, concealed in the woods and surrounded by farm and pastureland of Upstate New York lies a most unique and paleontologically important site. The Walcott-Rust Quarry is a Konservat-Lagerstätten for its exceptional preservation and diversity of fossilized lifeforms. Under the auspices of Dan Cooper of Ohio, a premier fossil excavator and preparer, I was privileged to visit the quarry in the summer, walk in Walcott's footsteps and enjoy a laborious rockhammer workout and fascinating day of discovery.

The quarry was initially worked for about six years in the early 1870s by discoverer and farmer-owner William Rust and 20 year-old, unknown and self-educated paleontologist Charles Doolittle Walcott, who in 1909 discovered the half-a-billion year-old UNESCO Burgess Shale in the Canadian Rockies of British Columbia. Back then, the resistant strata was exposed along the bed and banks of lazy "Gray's Brook" but rediscovered after being "lost" for nearly 100 years.

The "Hole", excavated adjacent to the original streamside quarry, consists of multiple beds of the Rust Formation. It's a hard, fine-grained, shallowing-upwards, micritic (muddy) limestone sequence that entombs a diverse benthic fauna from 457 to 454 million years ago. It includes brachiopods, gastropods, pelycopods, crinoids, bryozoans, cystoids, cephalopods and graptolites. Various trilobites, the quarry's prized arthropods, uniquely have two rows of lateral appendages and antennae that were preserved by obrution (rapid and anoxic burial). How did the deposit form? 

The latest Proterozoic megacontinent of Laurentia (the cratonic core of North America) tectonically-morphed into supercontinent Pangaea throughout most of the Paleozoic. During the Taconic orogeny, the second of four-mountain-building collisions, a foreland basin downwarpped cratonward and filled with marine waters of the Iapetus Sea. Layer after layer of foreland shales, sandstones and limestones along with their resident ecosystems are beautifully displayed in roadcuts along the New York State Thruway. The quarry resides on an unstable slope of the foreland's foredeep and preserves a unique look at a tiny section of the Middle Ordovician seafloor.

New England's Most Enigmatic Exposure
Squantum Peninsula
South Shore of Boston, Massachusetts

Galli and Thompson's Outcrop A of the Western Headland
Located on Squantum Head peninsula that projects into Boston Harbor, the small outcrop is a "heterogeneous sequence of interbedded diamictite (lower stratum), mudstone and sandstone (base of the outcrop).

Geologists have been attempting to unravel the formative history of the Boston Basin for over 100 years, and the Squantum "tillite" is central to the enigma. It has arguably initiated more discussion than any other geological locality in New England. I've visited it a number of times, most recently on a field trip with Ken Galli, Ph.D, Department of Earth and Environmental Science of Boston College, who enthusiastically expounded upon its attributes, enigmatics and theorized geo-genetics. 

The Boston Basin is a large fault-bound, topographic depression surrounded by vastly eroded highlands of the Dedham-Lynn-Mattapan-Brighton volcanic complex. It extends a distance out to sea and includes Boston and surrounding towns, roughly everything within Route 128 for those familiar with the region. It's filled with rocks of the Boston Bay Group that was deposited as the basin rapidly subsided in a subduction/magmatic arc system. The group includes mudstones and sandstones of the >570 Ma Cambridge Argillite and the underlying <595 Ma Roxbury Conglomerate Formation. The latter is a thick, tripartite stacked-package of Squantum, Brookline and Dorchester Members of multi-sized clasts of metavolcanic and metasedimentary rocks embedded in a coarse sandstone matrix.

After rifting from the northern margin of Gondwana ~630 Ma, the basin was delivered to eastern Massachusetts onboard the elongate Avalonia island arc during the Devonian-age Acadian orogeny. According to Galli and Richard Bailey, Ph.d of Boston's Northeastern University, the environmental setting at the time of deposition of the Bay Group was a subsiding, intermontane basin surrounded by volcanic highlands that bordered the sea. 

For the longest time, the Squantum exposure was interpreted as a mixed sequence, matrix-supported sediment - a diamictite - and glacially-derived - a tillite - since major global glaciations existed during the Late Proterozoic era. But the authors' current thinking is that it's a mass debris flow - a debrite - delivered by streams and rivers and gravitation to the coast and into the sea, perhaps indirectly influenced by regional alpine glaciation within the highlands but not directly of glacial origin as was once thought.

A full post is forthcoming as Part II of "Part I: The Geology of Back Bay" seen here.

"Squantum-Thompson" Tombolo
Between Squantum Peninsula and Thompson Island
South Shore of Boston, Massachusetts 

"Thompson-Squantum" Tombolo al Low Tide
That's the skyline of Boston's Back Bay and Boston Harbor in the distance from the south.

Derived from the Latin word tumulus or "mound", the ephemeral landform connects an island to the mainland, in this case, glacial drumlin Thompson to Squantum Peninsula on which we stand. It's also a spit (transported coastline) or bar (submerged shoal) that formed by deposition on the lee side (sheltered downwind or downcurrent side) of the island as wave energy and longshore drift are reduced.

As waves sweep sediment from both sides of the island and re-deposit it, the tombolo conforms to the shape of the wave pattern and current. With finer sand on top, coarser below and cobbles at the base, it morphologically fluctuates contingent on sea level, dominant wave pattern, larger longshore sediment supply and of course, storms.

We're assumedly near the eastern extent of the Boston Basin, which is submerged on the continental shelf beneath Wisconsinan glaciomarine blue clays. It records over a half billion years of geological evolution from Late Proterozoic supercontinent Rodinia to Quaternary continental glaciers that bulldozed the region. The drumlin field of Boston's Harbor Islands, that resulted from at least two different age drifts, and their sediments form the ever-changing spits of tombolos. By the way, that's the Squantum Member in the foreground.  

Testimony to an Arid Interior
Zion National Park
Southwestern Utah

Spectacular Wall of Navajo Sandstone in Zion National Park of Utah
The Navajo assumes many forms: immense cliffs, ridge-shaped cuestas, rounded domes and broad bluffs. They're due to the rock's porosity, permeability, fracture susceptibility and resistance to erosion. Largely Middle Jurassic in age, the erg or sand-sea is famous for its dark streaks of desert varnish, massive conchoidal fractures, large-scale cross-bedded paleo-dunes, thin lenses of limestones, iron concretions and muted colors.

Every geologist has a favorite rock formation. For me, it's a toss up between the Late Triassic Chinle Formation and the Middle Jurassic Navajo Sandstone of the Southwest. Visually impressive and highly recognizable, the Navajo constitutes the White Cliffs of the Grand Staircase (here), the majestic domes of Capitol Reef (here) and the towering sheer walls of Zion National Park in Utah where it's nearly 700 meters thick. The sand sea or erg tells a dramatic story of the interior paleo-climate of supercontinent Pangaea. 

Originally thought to have been a marine deposit, it's considered to have been one of the largest eolian terrestrial formations in the geologic record, comparable to the Sahara Desert. Located on the highland of the Colorado Plateau in most of Utah and parts of Nevada, Arizona and Colorado, its muted colors are due to thin coatings of mineral oxides, iron in particular. Acquired after deposition by water flowing through the mass, they cemented the Navajo's sand grains and lithified its paleo-dunes.

As Pangaea tectonically aggregated global landmasses, it increasingly left the vast interior of the supercontinent exposed at hot equatorial latitudes. Proximity to the Tethys Ocean (my post here) acted as a source of moisture that maximized summer heating when the planet's axis was tilted toward the sun with the reverse occurring during summer. It is thought that the resulting mega-monsoonal circulation (seasonal wind reversal) hyper-dried and mega-heated the interior on the leeward side of the Central Pangaean Range that formed as the supercontinent assembled.

Sand grains from the weathering mountains may have been delivered to the west in four phases by a transcontinental river system long-gone, while the northwest winter monsoonal and dry easterly trade winds concentrated the erg within a flexural basin that formed at sea level. It eventually uplifted en masse with the Colorado Plateau to its present locale, while mass wasting, erosion and time did the rest.

A large taphone (singular of taphoni) in Zion's Navajo Sandstone makes a perfect window for my son Will.

Volcanic Dams of the Inner Gorge
Tuweep Overlook
North Rim of the Grand Canyon
Northern Arizona

"What a conflict of water and fire there must have been there! Just imagine a river of molten rock running down a river of melted snow. What a seething and boiling of waters, what clouds of steam rolled into the heavens!"
John Wesley Powell, August 25, 1869

View West from Toroweap Lookout on the North Rim
Remnants of ~518 ka Prospect Dam that spanned the Inner Gorge are preserved in patches of lava flows that cling to the north wall of the South Rim and the large flow that drapes down the North Rim's south wall (arrows). Vulcan's Throne lies just off to the north (right). The gently-undulating Esplanade Platform is well developed on both sides of the Inner Gorge that formed as the Hermit Formation eroded back from the canyon and exposed erosion-resistant Esplanade Sandstone. Prospect Canyon, directly across, is also a product of Toroweap fault but is being re-excavated by erosion.

With the exception of geologists, river-runners and backcountry enthusiasts, most everyone is surprised to discover that there's a ~72 ka volcano called Vulcan's Throne - albeit extinct - perched some 3,000 precipitous vertical-feet above the Colorado River on the North Rim of the Grand Canyon. What's more, lava has cascaded into the Inner Gorge and created a 700 meter-high Prospect Dam that impounded the river upstream past Moab, Utah. The reservoir that formed likely was greater than the combined volumes of Lakes Powell and Mead.  

Even more incredible is that over 13 Pleistocene-age lava dams have done the same or similar with many that catastrophically failed as waters of the Colorado re-excavated the canyon, re-established the former gradient and flowed downriver in a massive torrent. In fact, given the ~2 Ma age of volcanic activity of the ~600 sq mi Uinkaret Volcanic Field on the Uinkaret Plateau, it's likely that even older dams existed within the Grand Canyon.   

Our viewpoint is from Toroweap Overlook situated on the vertiginous edge of the broad Esplanade Platform. It's a flat, east-west, gently undulating expanse of erosion-resistant Esplanade Sandstone on both sides of the Colorado River in the western Grand Canyon that formed at the expense of soft shales of the overlying Hermit Formation. The still-active N-S Toroweap fault slices through the region and gave rise to downdropped Toroweap Valley that filled with over 150 individual lava flows of the Uinkaret Volcanic Field. Early flows went north, whereas, more recent ones spilled into the Inner Gorge and once funneled by the canyon, traveled far downstream.

The volcanic field is a consequence of ongoing extension along the fault and is a manifestation of the western margin of the Grand Canyon that is slowly foundering as the Basin and Range Province is encroaching upon it and pulling it apart. It has implications for future volcanics in the region and the Grand Canyon and Colorado Plateau on a large scale. Time will tell.

Vulcan's Throne and Escalante Sandstone Reflecting Pools from the East
Although the cinder cone is extinct, the region of the Uinkaret Volcanic Field is prime for another eruption probably not too far off on the geological time scale. In the photo, the south slope of the Throne (left) is literally perched on the rim of the Inner Gorge and lava flow drapes into the Inner Gorge's north wall. The north slope (right) extends outward in the direction of Toroweap Valley.

That's all for 2018. 
Thanks for following and contributing to my blog. 
As always, I'm humbled by your comments and most appreciative of your visits. 
Have a Happy and Healthy New Year! Can't wait to see what 2019 will bring.