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Living at Sea Level, Walking Through History

by Brendan O'Neill

At the Gay Head Cliffs, one sees the deforming and dislocating effects that the most recent ice age sheet had on pre-glacial sediments as the glacier advanced to its southern terminus some 20,000 years ago. The pre-glacial deposits in the Gay Head Cliffs consist of ancient continental shelves, river deltas and beach sediments as much as 135 million years old.

The “basement” granitic rocks beneath this part of the Vineyard formed some 200-570 million years ago and are found some 800-1,000 feet below the surface. What is remarkable about the Gay Head Cliffs is the several geological epochs that are clearly evidenced at the surface:

  • Cretaceous outcroppings dating to about 95 million yrs old, consisting of red clays and white clays, similar in age to the white Cliffs of Dover.
  • Black lignite deposits from about 90 million years in age, consisting of old plant material deposited when areas beyond the current shoreline were above water.
  • Miocene Greensands strata containing a diversity of fossils dating to about 10 - 14 million years ago.
  • The Aquinnah Conglomerate consists of more recent glacial debris, dating to about 100,000 yrs old. It is in this fairly thin layer near the top of the Cliffs where ground quartz pebbles (as well as camel and other bones) have been found.

The Gay Head Cliffs present evidence of the effects of global plate tectonics over vast periods of time. It is a geological history of the coastal plain formed as a result of the erosion of the once great mountains of the eastern seaboard thrust up when the continents of Africa and North America collided. The subsequent continental retreat and spreading created the Atlantic Ocean and brought an end to the eastern mountain building epoch.

Martha’s Vineyard was under water during many of these geological periods, but some 23-35 million years ago it was above water, with evidence of erosion processes at work. Layers from this period include fossils of plants, mollusks and other material deposited in ancient streams, embankments and lakes.

The Cretaceous Period is broadly described as between 60 and 144 million years ago. The so-called “thrust beds” at the Cliffs describe a situation where some upper layers are older, mostly Cretaceous material that was “thrusted over” younger layers.

The Greensand beds are much younger, as recent as 5 million years old. They contain fossils of fish, whales, rhinoceros, and mastodon, as well as fossilized coral and “gizzard stones” called gastroliths from seals and other sea mammals. Based on fossilized pollen spores, it appears the climate at this time was subtropical.

But by about 1.5 million years ago, the climate began cooling, becoming temperate: pre-glaciation, pre-Pleistocene. This cooling was episodic, with cooling and glacial formation driven by Earth’s orbital changes and polar rotation changes.

This most recent Ice Age impacting eastern North America is known as the Wisconsin Glacial Stage, part of the vast Laurentide Ice Sheet that spread its way south from the Arctic. That southward advance of glaciers began some 100,000 years ago, coming to a halt in the vicinity of Martha's Vineyard around 21,000 years ago. A warming trend then triggered a slow glacial retreat northward, beginning about 18,000 yrs ago. During that period, the sea level around the Vineyard was about 300 feet below the present level, with a vast plain stretching out to the Continental Shelf.

Two distinct glacial lobes have been described, the Buzzards Bay Lobe to the west, and the Cape Cod Lobe to the east. The glacial advance was uneven, mirroring the shape of the bedrock beneath, depositing “drift” in its wake.

Today, we are experiencing an interglacial stage called the Holocene Epoch, where the next anticipated glacial stage is likely being impacted by human influences including global warming and broader climate change.

Below, excerpts from Woodward and Wigglesworth, 1934: Geography and Geology of the Region Including Cape Cod, the Elizabeth Islands, Nantucket, Martha’s Vineyard, No Mans Land and Block Island

The largest exposures of beds of Cretaceous clay in the Gay Head cliffs may be seen in a section over a mile long. Here may be found the whole series so far as it is known, beds of lignite, upon which lie the beds of clay and white kaolin sand. The section has been gullied considerably and presents much the same form as a cliff in a badland region.

The colors of the beds here are strikingly brilliant. Beds of Cretaceous clay may be seen in a section over a mile long. Upon the Upper Cretaceous clay lie the Miocene greensand and some Pliocene sand, which is in turn overlain by Pleistocene deposits.

The thickness of the Cretaceous beds exposed here is exceedingly difficult to estimate on account of complicated folding and overthrusting, but it probably nowhere exceeds 100 feet.

 

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