Boston geology

Alerts In Effect Dismiss. Dismiss View all alerts. Geologic Formations. Aerial view of the drumlins of Boston Harbor. Photo by Ken Mallory Boston Harbor is part of the Boston Basin, a topographic lowland underlain by sedimentary layers deposited at the end of the Precambrian time. Where bedrock is exposed Calf Island, the Brewsters, and small islands near Hingham , it is a shaly to slaty formation called Cambridge Argillite which was deposited on the muddy floor of an ocean dating back some million years.

The remains of the Grenville Mountains are now in Western Massachusetts. What becomes Boston at a later date will eventually collide to this ancient formation. Around million years ago, there was a subduction zone along the edge of the ancient continent of Gondwana.

Note that this continent was in the far Southern hemisphere, whereas early North America where the Grenville Mountains formed was near the Equator. As in all subduction zones, there was a great deal of volcanic activity, and this led to the formation of the volcanic Avalon mountain chain. Rifting occured along the edge of Gondwana, and within the Avalon area major faults and rift valleys formed. Included in this area is what we now know as the Boston Rift Basin.

Around million years ago, Avalon and other microcontinents separated from Gondwana and drift North-West. At this point, Avalon was an "island arc", much like Japan, Indonesia or the Phillipines is today.

Map of the Earth million years ago, as Avalon was splitting from Gondwana. Avalon is the island arc containing New England and Nova Scotia on the map. The bedrock of the Greater Boston area consists of a variety of rock formations.

Under most of downtown Boston is the Cambridge Formation, called Cambridge Argillite also called Cambridge Slate or Cambridge Mudstone, depending on which geologist you are talking with. The argillite is a slightly metamorphosed, reasonably weak, layered sedimentary rock. Slightly uphill from that, around the city, especially south and west is the Roxbury Conglomerate. Launch in Google Maps.

Description of the Topographical Model of Metropolitan Boston , "The relief showing the topography included within the Metropolitan District of Boston, exhibited at Paris, , is a solid piece of plaster, thirty feet in circumference, weighing about a ton.

The scale is approximately five inches to the mile, 1 : , and a vertical multiplication of six, the area included being nearly five hundred square miles, with a diameter of twenty-five miles Curtis, Sculpture - Internet Archive. Blue Hills Reservation; Massachusetts , E. Additions, Corrections, Comments?

Please email contact bostonbasinhills. Hills of the Boston Basin. Website under construction. Boston-Shawmut Peninsula. Deer Island. East Boston. The result was startling differences from the continent we know today….

A large part of Maine was then submerged; on the other hand, Nova Scotia home of the Debert site and the Grand Banks were islands.

By about 11, years ago, the ice sheet had continued to retreat and was just north of the St. Lawrence River in Canada. According to an article in the American Antiquity journal, northern Maine at the time was a frozen tundra while Massachusetts was mostly forested:.

After the ice sheet retreated it left behind many geological features such as drumlins hills carved by glaciers , kettle holes hollows dug by glacial ice , glacial erratics non-native rocks carried by glaciers over great distances as well as valleys and mountain passes known as notches. The melting ice also released large amounts of water, creating many glacial lakes and kettle holes ponds across the state. Boston Harbor in Boston, Massachusetts a depression carved by glaciers and later filled in by rising seas.

As the glaciers melted and the land rebounded from the weight of the heavy ice, the sea levels rose and the glacial drumlins in Boston harbor were submerged and became islands, according to an article on the National Parks Service website:. Geologists believe the islands illustrate two separate periods of glacial action.

Many of the islands have more than one drumlin. For almost 70, years, present-day Cape and Islands were connected to the mainland, comprising during this time the interior highlands of the Continental Shelf.

Meanwhile, the rising sea levels flooded the coastal regions and the shoreline slowly receded to its present location, possibly submerging many early coastal archaeological sites in the process. The large ice age animals that lived in the region eventually either moved north with the retreating glaciers or died out.

By about 10, years ago, these animals became extinct possibly due to either human overhunting, climate change or both.

Around that time, different animals began to arrive in the area, such as caribou, moose, bear, elk and white tail deer, which the Paleoindians also hunted for food and clothing. As the ice sheet continued to retreat and more land became habitable, the Paleoindians continued to explore and move about the region, establishing a handful of known Paleoindians sites across the state and many more across the Northeast.

During the Archaic period, starting around 9, years ago, the earth experienced the Holocene Climate Optimum, during which the climate warmed.