A Beginner’s Geologic Exploration of Martha’s Vineyard
Martha’s Vineyard, a small island off the coast of eastern Massachusetts, became a growing vacation spot in the early 1900’s. Although connected in formation with Staten Island, Long Island, Nantucket, and Cape Cod, Martha’s Vineyard has unique features which add to her distinctive characteristics. Martha’s Vineyard Island is considered new land, only 12,000 – 15,000 years old. It was inhabited in the late Pleistocene era 10,000 years ago by hunters and gatherers. More recently, at the turn of the 17th century, an Algonquian tribe was the main occupant and guardian of the island. Europeans arrived thereafter.
Martha’s Vineyard is a continental island meaning it was created either by a rise in sea level (burying most of the land except for the coastal highlands underwater), or by the sea breaking through a peninsula and cutting the land off from the mainland.
The origins of the actual land can be traced from the last ice age and glacial movement 10,000 years ago. Ice sculpts the land and removes material from one place while depositing it elsewhere. These deposited materials that were pushed or carried by the ice are known as Moraines. Lateral Moraines are deposits left by the edges of moving glaciers. Ground Moraines are deposits that dropped out of the ice as the movement of the glacier stopped. Finally, Terminal Moraines are large deposits of this soil, rock, and gravel that mark where the leading edge of the glacier stopped moving forward. These deposits can also be known as till.
The Holocene Epoch (less than 10,000 years ago) is considered the post-glacial period. The world started to warm. As temperatures increased the glaciers started to move and melt. An ice sheet completely covered North America as far as Missouri. As glaciers move back and forth they picked up the continental land and left sand, gravel, other ground up rocks, and boulders in its path. The melting glaciers released this land as they moved. Staten Island, Long Island, Nantucket, Cape Cod, and Martha’s Vineyard are terminal moraines that mark the farthest point of expansion of glaciers to the southeast. As the glaciers melted, sea level rose and parts of the continental shelf became separated as seen in the figure below.
Martha’s Vineyard Island soil is sandy. The sedimentary rock that forms the island is logically dated to the Holocene and Pleistocene Epochs around the time of the last glacier movement 10,000 years ago. In the figure below one can see Cape Cod, Nantucket, and Martha’s Vineyard are sediments from the Pleistocene and the Holocene Epochs. On the mainland of Massachusetts there is a wide range of other sedimentary, igneous, and metamorphic rock. It makes sense that the islands formed by the Terminal Moraines are the youngest in geology.
Although the above figure states that Martha’s Vineyard Island is sedimentary rock from the Pleistocene and Holocene Epochs, there is a unique piece of the island which exhibits a much earlier formation. Gay Head or “Aquinnah” is the westernmost end of the island. It is attached to the rest of the island by a barrier bar and is famous for the Gay Head cliffs.
The one mile of exposed cliffs is a pre-glacial sedimentary formation that rises 150 feet above sea level. The cliffs show a cross section of strata from the Cretaceous through the Pleistocene Ages. This documentation of the geologic phases on the continental shelf dating from 100 million years ago is one of the Island’s biggest tourist attractions. The cliffs in the past have been sources for fossils as well as clay used for pottery. Recently there are restrictions placed on access to the cliffs due to erosion. The natural erosion from the ocean and weather takes about two feet off of the cliffs each year. One can easily compare the black and white photograph taken in the late 1800’s to the more recent photographs taken in 1996 to illustrate this point.
Martha’s Vineyard shares a history with Cape Cop, Nantucket, Long Island, and Staten Island. Being a part of the Terminal Moraine formed around 12,000 years ago at the end of the last ice age; Martha’s Vineyard is considered to be new land. Yet differing from the other areas it contains cliffs exhibiting sediment formation from 100 million years ago. Terminal Moraines and the Gay Head Cliffs in addition to other geologic occurrences demonstrate the ability of our changing Earth to record history and keep track of events that happened long ago.