Glacial Origins
Narragansett Bay is a ria that consists of a series of flooded river valleys formed of dropped crustal blocks in a horst and graben system that is slowly subsiding between a still-shifting fault system; however, the estuary system is vast compared to the present flow of the four small rivers that enter the bay, in the northeast, the Taunton River and in the northwest, the Providence and Seekonk Rivers, along with the Pawtuxet River from the west. The present shape of Narragansett Bay is instead the result of the most recent glaciation of New England, under the edges of the Laurentide ice sheet at the Last Glacial Maximum, about 18,000 B.P. Sea level was lowered so much that the continental shelf was exposed, under its weight of ice, and the glacier calved into the Atlantic at its foredge south of Block Island. Glaciers flowing through a geologically old sedimentary basin carved channels through the younger sediments and exposed much older bedrock. North-to-south cuts gouged by the ice can be seen clearly on the map: they form the West Passage that separates Conanicut Island from the western mainland and the East Passage that now separates Conanicut Island from Aquidneck Island.
As the ice stalled, then retreated, the region became ice-free by about 14,000 B.P. A complicated sequence of marine ingression and isostatic rebound flooded and emptied the landscape. A fresh water proglacial lake called by geologists Lake Narragansett formed about 15,500 B.P., impounded behind terminal moraines: the lake lasted about 500 years, leaving the powerful flow of a post-glacial river running down its north-south axis. Then salt water filled the valley, as rising sea levels permanently flooded the area.
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