PURGATORY CHASM
by Caitie Sams and Ali Quirk
Massachusetts Geology and History
The New England area has been shaped by mountain building events called orogenies and glacial retreat and advance cycles that shaped the landscape and composition of rocks in Massachusetts. The mountain building events that shaped Massachusetts geology have occurred for a billion years, whereas more recent changes to the landscape have occurred due to two specific glacial advances and retreats. All of these events have a significant effect on Massachusetts geology.
Grenville Orogeny
Taconic Orogeny
This was a 350 million year event that occurred 1.1 - 1.2 billion years ago when many small continents collided and created the super continent Rodinia. The collision created huge mountains on many of the continents during this time, one of the mountains that were formed was when Laurentia (Proto-North America) and Amazonia (Present day South America) collided (Butler). They created mountains that later separated due to rifting and formed the beginnings of the Atlantic Ocean. The Laurentia plate drifted up to the equator and a majority was left under water which caused a lot of limestone rocks to form (Butler).
450 million years ago, the Taconic plate advanced towards a convergent boundary and turned into an island arc. This was called the Taconic Island Arc, which eventually thrust up and deformed the continental shelf, slope and rise of Laurentia (Butler). Also, it deformed the Grenville Mountains and covered most of them with the incoming rock. It created the western part of New England and rocks to the west of the Connecticut River valley show evidence this collision. The Taconic Mountains were formed as a result of this collision (Skehan).
Acadian Orogeny
410 million years ago, the Avalonian advanced towards the convergent boundary and formed the Avalonian island arc. This arc pushed up towards the Laurentia plate and bulldozed seabed material into the plate. As a result, existing mountains were pushed up and extremely tall mountains were formed (Skehan). The result of this orogeny was extreme compression, resulting in metamorphic rock.This event is given credit for creating the Appalachian Mountains (Butler).
Alleghanian Orogeny
This event occurred between 354 to 250 million years ago during the Pennsylvanian and Permian time in southern New England. The Meguma micro continent collided with the eastern shore of Laurentia and the ancestral African continent (Gondwana) collided with southern New England. It was a complex event that involved compression and over thrusting (Butler).
The collision of Avalonia formed the Appalachian mountains and the bedrock of Purgatory Chasm.
These mountain building events show that Massachusetts geology and bedrock was formed and altered by small continental plates joining with the Laurentia plate. This is the theory of suspect terranes; when pieces and fragments of the crust break crash into other larger tectonic plates are build upon the present plate. The result is a variety of rock types and bedrock all joined together. Massachusetts is just this. Different types of bedrock piled together and massive, majestic topography.
Glacial Advances and Retreats
There have been four glacial advances and retreats over the past million years, but only two were major glacial cycles. There have been four glacial advances and retreats over the past million years, but only two were major glacial cycles. Glaciers are thick slabs of ice that move on land. They form from compaction, melting and refreezing, and finally, regellation. The more compaction the glacier goes through, the heavier the glacier becomes. A glacier moves on land because of shifts in its budget. The budget of a glacier is the balance of accumulation and ablation of ice. This depends on the temperature of the surrounding area. If it is hot, the glacier will go through more ablation, causing it to retreat until the zones of accumulation and ablation are equal. If the temperature drops, there will be more accumulation. This will cause the glacier to advance until the zones are once again equal to each other.
The most recent glacier to advance and retreat over Massachusetts was the Laurentide Ice Sheet. It formed 75,000 years ago and advanced southward in three lobes covering the Massachusetts coast and bay. The ice sheet advanced until 21,000 years ago when it reached its glacial maximum around Martha’s vineyard and Nantucket (Martin).
The ice sheet started its retreat 18,000 years ago and each of the lobes retreated at different speeds, resulting in the formation of proglacial lakes ahead of the ice and countless other depositional features throughout Massachusetts. It scraped off rock material from the underlying bedrock, erased most of the past ice sheets and caused the land to sink along the coast in an isostatic sea level change because of the weight of the ice sheet on top of it. It retreated up until Baffin Island, Canada 5,000 years ago (Martin).
Glaciers have not passed over Massachusetts since, but the remnants of the Laurentide glacier can be seen all over Massachusetts as erosional and depositional features of glaciers.
