Geology & Physiography
The lake is located on paleozoic limestone of the Ordovician period. However, due to deep deposits of glacial sediments, this bedrock only appears exposed along the lakeshore on Georgina Island. This rock, however, also appears along the shore of Lake Couchiching, where sediments are thinner. The outlet of the lake at the north end of Lake Couchiching is controlled by precambrian bedrock which first makes its appearance about mid-way along this lake. As a result, the lake has persisted to the present, with the outlet-flow unable to cut down through the rock, and thereby draining the lake. This is unlike its one-time smaller sister-lake of 'Lake Minesing' to the west, being a remnant of Lake Algonquin, and, later the Nipissing stage of Lake Huron. This lake's outlet was founded on sediments located near Edenvale,which have largely been cut through, draining most of the lake. Today 'Lake Minesing' exists as the Minesing Swamp, but it re-establishes itself for a short period each spring when spring run-off floods the basin.
Isostatic rebound from the retreat of the last glaciers results in a steady rise of Lake Simcoe, particularly at its south end, and is further responsible for the deep organic sediments that have accumulated in its one-time shallow southern arm, known as the Holland Marsh. Here, organic deposits created by vegatation have largely kept pace with the steady rise in water level, and today supports extensive market gardening. All rivers, and most streams, flowing into the lake have wide, deep, navigable mouths as a consequence of the rising waters drowning the river channels.
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