Island Life: Geological History

Geological History 

We have not been able to improve on the excellent book by Oldale (1992) that describes the geologic origin and features of Martha’s Vineyard, so we will unapologetically borrow extensively from that work below. Any quotations in the following paragraphs, unless otherwise noted, are from that work, only the page number being given.

The story of the island begins with the bedrock. That bedrock is ultimately part of the tectonic plate that makes up northeastern North America. “The bedrock consists of different kinds of consolidated sedimentary, igneous, and metamorphic rocks of Precambrian to Mesozoic age (roughly 600 million to 66 million years old)” [p. 19]. Relatively little is known about the specific bedrock under Martha’s Vineyard because it has been sampled by bore holes in only a few places, but what is known shows that “the bedrock beneath the Cape and Islands is not significantly different from that elsewhere in southern New England” [p. 23]. It is also known that it ranges in depth from about 400 feet to over 1,000 feet below the island’s surface, sloping generally from north to south.

At a point more than 200 million years ago, a period of mountain-building occurred that created a major mountain range in what is now eastern North America. Over the following 75 million years or so, these mountains eroded away. “Rivers and streams carried sediment eroded from the uplands to the margin of the continent, where it was deposited to form a thick sedimentary wedge that underlies the Coastal Plain, the alluvial surface adjacent to the sea…The continental shelf is the part of the Coastal Plain seaward of the shore; however, this distinction is somewhat arbitrary because the shoreline moves back and forth across the continental margin as the sea level rises and falls…One result of these repeated transgressions and regressions of the sea is an incomplete geologic record because strata laid down during transgressions [are] later, completely or partly, removed by erosion during regressions. Strata that incompletely represent the geologic history of the Coastal Plain…are exposed in the cliff at Gay Head on Martha’s Vineyard” [p. 26]  In addition to erosion, the repeated glaciations during the Pleistocene on that part of the original coastal plain that now constitutes the island also greatly altered the content and distribution of the sedimentary strata. 

The Upper Cretaceous and Tertiary coastal plain strata exposed at Gay Head “…are not in place. They owe their position above sea level to the continental glacier that thrust blocks of strata southward and upward as it advanced . . .We know that these beds have been displaced because . . . they lie far above their level in the [borehole that was drilled on the island] and because they are faulted and folded; in some places older layers occur above younger layers. However, even though the strata have been pushed from their original position, they provide the best opportunity to study and understand the Late Cretaceous, Tertiary and early Pleistocene history of the Cape and Islands” [pp.27-29]. Most of the strata at Gay Head are late “. . . Cretaceous in age, the last geologic period in the age of the dinosaurs. The record preserved by these strata represent events that occurred between roughly 80 million and 100 million years ago” when this area was typified by “. . . streams, lakes and swamps or marshes on the coastal plain. The fossil marine shells show that at times the coastal plain strata formed in lagoons and embayments along the seashore” [p. 29]. A rather large number of plant species have also been identified from these strata.

Tertiary strata at Gay Head represent only a small portion of the overall, but one bed of green sand dating from the Miocene epoch contains one of the most interesting assemblages of fossils found in New England. “Fossil pollen and spores in the greensand indicate that about 5 million years ago the Cape and Islands region had a subtropical climate and was much closer to the equator than now . . . Pollen and spores from the youngest Tertiary strata (Pliocene) in the Gay Head Cliff indicate a cool temperate climate that might be a precursor of global cooling and the initiation of glaciation during the Pleistocene . . . [D]uring much of the Tertiary period the Cape and Islands region was submerged beneath the sea and…open continental shelf conditions prevailed. However, from time to time, the sea regressed and the region was exposed to erosion, which resulted in gaps in the Tertiary geologic record.” [pp. 30-32]. Such a gap occurred in the Oligocene epoch, a period of roughly 20 million years in between the Eocene and the Miocene epochs, for which no strata are found at Gay Head.  

This brings us to much more recent geologic time, the very late Pliocene, the Pleistocene and the Holocene, or Recent, epochs, from about 3 million years ago to the present day. “The Pleistocene was not a time of perpetual cold global climate, snow, and ice. Instead, it was a period when, from time to time, the climate cooled and glaciers began to expand” [p. 33]. Ultimately, the glaciers expanded until they joined one another and formed vast continental ice sheets that spread south from the polar region to the north. “Alternately there were periods when global climate was warm, even warmer than today. During these times the glaciers receded, and in some places, the continental ice sheets melted away altogether” [p. 33]. Each of at least four glacial advances during the Pleistocene left a distinctive mark on the landscape. “However, as each glacial advance erased the evidence of the previous advance and retreat, by incorporating older glacial and interglacial deposits into its own deposits, a complete record of the number of glaciations and interglaciations in the Cape and Islands region [has not been] preserved” [p. 34].

The last glaciation period, known as the Wisconsinan, is the one that was the primary formative influence for Martha’s Vineyard. There are some features on the island which have been interpreted by geologists as suggesting that the previous glaciation, known as the Illinoian, which lasted from about 350,000 to 125,000 years ago, may have also reached our area. The Lagoon, Lake Tashmoo, and the valley through which North Road runs today are thought by some workers to be valleys of rivers that flowed here shortly after the Illinoian glacial period. However, the Wisconsinan did most of the work. The Laurentide ice sheet that began to grow less than 100,000 years ago began to spread across New England about 25,000 years ago and reached the farthest point of its advance about 21,000 years ago. As the ice sheet grew, the sea level fell as water was removed from the oceans all over the planet. “During the maximum growth of the ice sheet, sea level was about 300 feet below present sea level, and the coastal plain south of the Cape and Islands extended to the present edge of the continental shelf, where old shorelines can be seen on the sea floor” [pp. 38-39]. As the global climate began to warm, the ice’s retreat began as melting at its southern edge began to exceed its rate of advance. It appears that the retreat of the ice from our region was fairly rapid once it started, taking only a few thousand years at most [p. 94]. By 6,000 years ago the only remnant of the ice sheet was confined to northern Canada, and the sea had risen again, filling in the lowlands where Vineyard and Nantucket Sounds are today and isolating Martha’s Vineyard from the mainland and from Nantucket. At that point, however, the shores of Martha’s Vineyard were probably a half mile to several miles farther seaward from their present locations, depending on the part of the island involved [p. 100].

“At the time of the maximum advance, the Laurentide ice sheet probably had a steep profile as it thickened rapidly northward. When the ice front was at the location of the offshore islands, the ice thickness over Cape Cod was greater than 1,500 feet. In northern New England the ice was more than a mile thick . . .” [p. 41]. The front of the ice sheet was not straight but lobed. One lobe lay in Nantucket Sound and Cape Cod Bay (the Cape Cod Bay lobe) and another in Vineyard Sound and Buzzards Bay (the Buzzards Bay lobe); their positions were influenced by broad shallow depressions in the bedrock surface beneath the ice. 

It should be emphasized here that the Martha’s Vineyard we know today only began about 6,000 years ago. As far as we can tell, before that what we know now as the island was just part of the broad coastal plain along the east coast of what is now North America. Also, the climate and physical characteristics of this area were radically different from today when the fossil deposits now at Gay Head were laid down in the Cretaceous period 75-100 million years ago and again in the Miocene epoch about 5 to 24 million years ago. (See the chapter on the Fossil Record for more detail.) 

Martha’s Vineyard was created primarily by the deposition of debris carried by the Wisconsinan glacier at the intersection of the Cape Cod Bay and Buzzards Bay lobes. This debris, or drift, consisted of everything from huge boulders to microscopic clay particles carried on, inside and under the ice sheet. When the ice front stood in one place for a significant period of time by advancing just as fast as it melted, the drift accumulated there in the form of an end moraine in which the types of debris were all mixed together. In addition, the weight of the advancing ice sheet passing over coastal plain strata, older glacial deposits, or its own glacial deposits forced this material forward and upward beyond the ice front, not unlike a bulldozer, adding to the moraine. Though there have been other suggestions, current thinking is that there is only a single end moraine on Martha’s Vineyard which runs from West Chop along the north coast of the island to Gay Head, Squibnocket Point and out to Nomans Land Island. The rest of the present-day island consists of outwash plain, most from the end moraine and some, particularly in the eastern part of the island, brought by meltwater from the Cape Cod Bay lobe during early stages of its retreat [p.85]. Rivers and streams flowing from the glacial ice front carried some rocks into the upper edge of the outwash plain, but most remained in the end moraine. This explains why the highest hills and stone walls are in parts of the towns of Aquinnah, Chilmark and West Tisbury, especially, and none in most of Oak Bluffs and Edgartown.   

As the ice continued to melt, streams and rivers of water flowing from the ice front carried material from the end moraine south into an outwash plain. Depending on the volume and speed of the rivers, these streams and rivers were able to transport rocks, pebbles, sand and finally only fine sediment further and further from the ice face as the speed of the water gradually declined. Most of the melt-water streams on the island are thought to have flowed across the coastal plain eventually to enter the sea near the outer edge of the continental shelf, then about 75 miles away. The “bottoms” that run south from the morainal hills of West Tisbury and Tisbury to the coves on the north sides of the coastal barrier beach ponds on the island’s south shore may be the old valleys of the melt-water rivers and streams [pp. 67-72]. An alternative suggestion is that they were formed by a process known as “spring sapping” in which springs of ground water migrate up a sloping surface [pp. 72-73; Uchupi  & Oldale, 1994]. Occasionally, a large piece of ice was left behind in the retreat of the ice front which was then partially or wholly buried by material carried by the melt-water streams. When the ice block melted, it left behind a steep-sided pond called a “kettle hole.” A few major and several minor ponds on Martha’s Vineyard had their origin in this manner. Examples are Seth’s and Old House Ponds in West Tisbury, Fresh Pond and Dodger’s Hole in Oak Bluffs, Lily and Jernegan’s Ponds in Edgartown, and Harlock’s Pond in Chilmark, to name just a few.    

“The age of the oldest organic deposits on the glacial drift surface indicates that the glacial landscape may have remained essentially unvegetated for several thousand years. Throughout the Wisconsinan, the coastal plain south of the glacial limit had a cold periglacial climate, and vegetation was tundra-like, with low bushes, grasses, and stands of arctic trees. Nevertheless, this arduous environment was the place of refuge for plants and animals that would first colonize the land made available by the retreat of the Laurentide ice sheet . . . The plants and animals that presently characterize southern New England retreated far to the south during the Wisconsinan stage and returned very slowly following glacial retreat. Many tree species did not arrive in southern New England until the early Holocene and some of the trees that are common in the present forests of southern New England, including the Cape and the Islands, arrived during the middle Holocene” [p.148].

“It appears . . . that Indians may have arrived in the Cape and Islands region between 11,000 and 8,000 years ago” [pp.148-149]. By that time, the ice had retreated north of what is now the Canadian border [p. 39], and the sea had begun to rise across part of the continental shelf. “The climate on the exposed shelf was undoubtedly more severe than exists in the Cape and Islands today, but it was certainly temperate enough to support abundant and diverse plant and animal populations. Many of these plants and animals would be familiar today in the northeastern United States and eastern Canada. Forests were made up of spruce, fir and pine trees, with open parklands made up of tundra plants and grasses. Scattered about were abundant swamps and marshes on the gently sloping, emerged shelf. The animal community would have included mammals, birds, reptiles and fish. Large animals probably included deer, bear, wolves, moose, caribou, bison and musk ox…[P]erhaps as recently as 10,000 years ago, mastodons and mammoths roamed the region of the emerged continental shelf” [p.96], which the Indians surely hunted.

“Although the Indians missed the glaciation of the Cape and Islands, they witnessed rapid and dramatic changes in the environment. As the climate moderated, the evergreen forests and tundra that first occupied the Cape and Islands died out. They were replaced by the hardwood forests and other plants of a more temperate climate. Tundra animals such as caribou and musk ox moved north following the migration of their preferred food plants” [pp. 96-97]. In addition, the sea began to rise, rapidly covering the outer edges of the gently sloping continental shelf [p. 98]. “In addition to shaping the Cape and Islands [by wave erosion and longshore drift of sand], the rising sea brought with it an abundant food supply for the human inhabitants” [p. 151]. There is evidence from the discovery of characteristic arrow points that Indians were present on the continental shelf in what is today Martha’s Vineyard as early as about 9,500 years ago (Richardson, 1983, 1985). “Shell heaps or middens occur in many places, indicating the importance of this food source to the Indians. Finfish and marine mammals were also important,…as were the waterfowl that inhabited the bays, salt marshes and ponds. Today, the maritime environment provides the temperate maritime climate that makes the weather pleasant for most of the year and continues to shape the land to perpetuate the attractive landscape favored by residents and visitors alike” [p. 151].              

“The future of the Cape and Islands is good news and bad news. The bad news is that [they] are slowly being eroded away by the waves. Their ultimate future can likely be seen by observing George’s Bank…The good news is that sooner or later every piece of property will be waterfront . . . The shoreline exposed to the open ocean retreats fastest. Maximum average retreat ranges from about 7 to 11 feet per year. [However, a]verage rates of erosion . . . can be somewhat misleading. During about 30 years at Wasque Point at the southeast corner of Martha’s Vineyard, the average erosion rate was about 33 feet a year. During most of that period, the point was protected by a wide foreshore, but one year when the foreshore was absent and the point was exposed to direct wave attack, it retreated 350 feet…The susceptibility of the sandy Cape and Islands to the forces of the sea may be a strength, not a weakness. It could insure that they will exist in one form or another for a long time. The Cape and Islands may sometime be nothing more than islands of beach sand, capped by dunes and surrounded by sandy shoals. Maybe we can take comfort and pride that this fragile but adaptable landscape will survive in one form or another long after the rock-bound coast of New England and its coastal cities may be deeply submerged by the sea” [pp. 171-173]. 

The excellent map by Oldale and Barlow (1986) shows the distribution of the glacial and post-glacial deposits on the island.

Allan Keith; edited by Matt Pelikan, July 19, 2022