Physiography / Geology

The topographical features and surficial geology of the Lake Simcoe drainage basin are mainly attributed to the processes of glaciation during the Late Wisconsin Ice Age. A modified version of the Ontario Geological Survey's depiction of Quaternary Geology is presented for the watershed in the map atlas. Within the Lake Simcoe watershed are five major physiographic areas which include the Oak Ridges Moraine, Peterborough Drumlin Fields, upland till plains, Simcoe Lowlands and Oro Moraine. These features are quite apparent in the hillshade depiction of the digital elevation model.

Oak Ridges Moraine

The headwaters of the streams flowing into the south shore of Lake Simcoe are located on the most widely recognized feature in the watershed, the Oak Ridges Moraine. The Oak Ridges Moraine exists between the Trent River and the Niagara Escarpment at elevations between 305 and 395 metres above mean sea level. The peak of the moraine forms the surface water divide separating flow toward Lake Simcoe and Lake Ontario and consists of five major geological units according to recent investigations (e.g., Sharpe et. al., 1999). These units are, from bedrock to surface; Whitby Shale, the lower deposits, Newmarket Till, Oak Ridges Moraine sediments and Halton Till. The regional Newmarket Till is incised in areas forming significant channels that, in some cases, extend into the lower sediments having completely eroded the till layer. Silts, sands and gravels often filled these channels forming important hydrogeologic units (Barnett et. al., 1998).

Peterborough Drumlin Fields and Upland Till Plains

Lake Simcoe is essentially surrounded by till plains that are overlain by sand, silt and clay in areas as a result of being covered by Glacial Lake Algonquin. On the till surface south and east of the lake can be found a series of elliptical hills (drumlins) that are oriented in the direction of glacial advance (northeast to southwest). The drumlinized till plains were completely covered by glaciolacustrine sediments in some areas and, in other areas, formed island or shore features of glacial Lake Algonquin and therefor are presently at or near the ground surface. In the southern and eastern portions of the watershed north of Oak Ridges Moraine, the drumlins form part of a large feature known as the Peterborough Drumlin Field (Chapman and Putnam, 1984). Beneath the glaciolacustrine deposits, where they exist, are similar geological units to those found in the Oak Ridges Moraine including a dense silty till that is likely equivalent to the Newmarket Till. Bedrock transitions from shale to limestone to the north and outcrops are mapped around Beaverton (Barnett and Mate, 1998).

West of the lake glaciolacustrine sediments are less predominant, drumlin features are present but rare, and the till structure differs from other portions of the watershed in that it has a higher stone and sand content than reported elsewhere in the watershed.

Simcoe Lowlands

The till plains described above are incised in the southern portions of the watershed by flat floored valley features that generally correspond to current river systems (Sharpe et. al., 1999). Organic materials, sands and gravels, and some silts and clays can be found in these lowland valleys deposited by glacial and fluvial processes (Barnett et. al., 1999, Barnett and McCrae, 1996). East of the lake the valleys are less well defined than to the south and glaciolacustrine deposits are found between upland tills (Barnett and Mate, 1998).

North of Lake Simcoe the lowland encompasses both the rivers and upland areas in the Mara Flats and extends to the western watershed divide north of Orillia. Like the till plains west of the lake, glaciolacustrine clays and organic materials overlie a stony sand till. Unlike the till plains to the west, however, the overburden package is quite thin with occasional bedrock outcropping and drumlins are abundant (Finamore and Bajc, 1981). The topographic low that denotes the lowland areas around Lake Simcoe extends southwest from Kempenfelt Bay in Barrie into the Nottawasaga Basin. Abandoned shore bluffs and glaciolacustrine sands and clays are mapped in this extension of the Simcoe Lowlands (Barnett, 1997) which contains glaciolacustrine sands and silts.

Oro Moraine

The Oro Moraine is a feature which lies on the edge of the Lake Simcoe watershed oriented from east-northeast to west-southwest. This orientation is not perpendicular to the direction of glacial advance (northeast to southwest), and has made difficult the explanation of its origin (Chapman and Putnam, 1984). The surficial geology of the area is mapped by Barnett (1997) as glaciofluvial material.

Only a small portion of the Oro Moraine lies within the Lake Simcoe watershed. Rivers with headwaters on the moraine flow in all directions and, including the Hawkstone River which discharges to Lake Simcoe, are reported to receive significant inputs of groundwater discharge (Azimuth, 2000).