At the very base of the strata which comprise Niagara Falls (Figure )are Ordovician and Cambrian aged sedimentary rocks (Tesmer, 1981). These rocks were deposited on the stable continental shelf and craton. The carbonates and sands of the Late Cambrian compose the Theresa Formation. Erosion or nondeposition marks the Early Ordovician. Deposition continued during the Middle Ordovician when the Black River and Trenton shelf carbonates were laid down.
The Taconic Orogeny had a great impact on New York. The rock deposited during the Silurian are the result of erosion and weathering of the Taconic Range. At the base of the Niagara Gorge is strata of the red Queenston Shale (Figure ). Next in the column, the Medina Group includes sandstones, siltstones and silty shales. They are rich in iron oxide. Also included are sedimentary structures such as ripple marks, rill marks, mud cracks, cross-bedding and scour structures. White Whirlpool and Thorold sandstones indicate former beach environments. Interbedded in the sandstones are the Power Glen deltaic muds.
Toward the end of the Silurian the deposition rate of these muds slowed indicating the Taconics had been deeply eroded. Later the Clinton Group (Figure ) contain sediments deposited on a stable platform. Highly vegetated sea floors created hospitable conditions for fossils such as bivalved brachiopods and echinoderms. During this time the Iroquois limestone formed. Above this unit, the Rochester Shale contains many will preserved fossils. Burial of organisms was immediate so the shells did not decay. Fossil types include strophomenid brachiopods, trilobites, ostracodes, and Tentaculites.
Late in the Silurian the environment changed to a saline oceans. The Lockport limestone and dolostone were deposited. Large coral reefs built. Then in the Late Silurian many of the fossil types became extinct. The ocean in western New York became cut off from the ocean. The water became highly saline and many of the organisms died. Evaporties formed the Salina Group. A similar occurrence is happening in the Dead Sea. The Bertie Formation contains abundant eurypterids, the strange arthropod on the cover. Dolostones formed in lagoons as salty conditions changed.
Above the strata, lie glacial deposits. End moraines are apparent at Fort Erie and Niagara Falls. The bedrock is striated parallel to direction of glacial flow and erratics dot the landscape. Lake Tonawanda (Figure ), a glacial lake, spanned 93 km from East of Niagara Falls to Holley, New York. The silts and clays deposited at lake bottom created furtile soil for local farmers. Eleven thousand years ago this lake began the initial cutting of the Niagara Gorge. The initial falls (Figure ) occurred at the plunge pool further to the north near Lewiston and Queenston (Figure ). The river at this point was wider. Moreover the rate of cutting back of the falls has varied since erosional rates at the falls have not been constant. The volumes of water which run through the falls have varied from centimeters a year to meters a year. This rate depends on the amount of available water; after the melting of the glacial lakes a large volume of water caused rapid erosion.
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