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450 million year old Whitby Shale
outcrops beside Little Rouge River.

Ancient Bedrock

Bedrock exposures only occur in a few locations along the lower Rouge and Little Rouge Rivers. The local bedrock, the Whitby Formation, is a 450 million year old shale formation. The Whitby shale contains fossils of trilobites and other pre-historic creatures that once lived in a shallow inland sea covering much of east central North America during the Ordovician period.

Deep Glacial Deposits, High Bluffs and Hidden Fault Lines

In most places, the bedrock surface is buried by sediments up to 100 metres in depth deposited during the last glaciation. The Wisconsian glacier receded 12,000 to 13,000 years ago depositing the Halton Till which covers much of the Rouge Park. The limestone and sand-rich Halton Till has stones and large granitic boulders (glacial erratics) carried south from the Canadian Shield.

Two-thirds of the Rouge Park south of Steeles consists of gently undulating tablelands falling from a maximum elevation of 175 metre A.S.L. at Steeles Avenue to 75 metres A.S.L. at Lake Ontario. Streams, floodplains, terraces and slopes of the Rouge River and Little Rouge Creek valleys occupy the remainder of the park.

In some places, the Rouge River has eroded 40 metres below the surrounding tablelands creating impressive bluffs. Large riverside bluffs at the Finch Meander (Finch & Sewells Roads), and Twyn Rivers Drive (beside Rouge River), reveal many different layers of glacial deposits. The Twyn Rivers Drive bluff has a sediment fault line associated with a hidden bedrock fault extending into Lake Ontario in Pickering.

Forty metre bluffs at Finch
Meander reveal complex glacial
history of Rouge.

Layers of Glacial Deposits at
Rouge River and Twyn Rivers Drive.

Internationally Important Don Beds

In a few deep locations, sediments from earlier glaciations have been preserved in bedrock valleys and exposed in riverside bluffs. The exposed Don Beds found along the Rouge Valley are of international importance because they are rich in fossils which enable the reconstruction of past climate, vegetation and fauna data.

Finch Meander ANSI

The Finch Meander is a designated Area of Natural and Scientific Interest (ANSI) because of its geological and ecological importance.


South of Steeles, the surface features include nine drumlins, up to 30 metres high and 300 metres to 1,000 metres long, and a number of flutes, long linear ridges separated by shallow troughs. Tableland tributaries often follow the troughs. At right angles to this trend are occasional parallel series of low ridges that are presumed to be created by seasonal pushes along the glacial front.

Halton Till and Peel Ponds

Small clay or sand filled depressions up to two metres deep in the Halton Till also provide evidence of local ice-dammed lakes, the Peel Ponds, which were formed about 13,000 years ago. These small clay plains are found east of Little Rouge Creek around Steeles Avenue, around the CPR tracks and Reesor Road, the east side of the Rouge River south of Steeles Avenue, and south of the Rouge west of Sewells Road. A small sand plain occurs on a knoll at the Finch Meander.

Lake Iroquois Shoreline

The southern part of the park was flooded between 12,500 and 12,000 years ago by Lake Iroquois, an ice-dammed, ancestral Lake Ontario that had water levels up to 60 metres above present lake levels. The old Lake Iroquois shoreline passes through the Metro Toronto Zoo and the Beare Road landfill. Most of the shoreline has been removed by aggregate extraction or covered by landfill. The shoreline consisted of shingle beaches, sandy bays and a shore terrace cut into the Halton Till.

Swamps, Marshes and Beaches

Former Lake Iroquois embayments are found around the Townline Swamp, the cattail marsh at Finch Avenue and Reesor Road, and the Beare Road woodlot. A five metre high terrace survives between Little Rouge Creek and Beare Road, and a shoreline is also present on the west side of the Core Woods in the Metro Zoo. South of this shore, a Lake Iroquois beach extends for up to two km across the Metro Zoo, the interfluvial tableland south to the hydro line, around the Beare Road landfill, and south of the confluence of the Rouge River and Morningside Creek.

The beach consists of coarse sands and gravels and bouldery lags up to five metres deep over the Halton Till. With the exception of the swamps south of the Beare Road landfill, these beach deposits have largely been extracted for aggregate in the 1950s.

A locally recharged shallow aquifer discharges water along the base of these beach deposits, maintaining swamps on the tablelands south of the Beare Road landfill and, at one time, south of the intersection of Sheppard Avenue and Meadowvale Road. A small remnant of this latter swamp still exists in the Centennial Forest and Swamp ESA (City of Scarborough, 1983).

The information contained in this web page is from the Ecological and Earth Sciences Surveys of the Rouge Park, by Varga, Eyles and Boyce et. al.