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Home > TWAHRS > Existing Conditions: Physical Processes & Cultural Influrences

Nearshore Geology

  1. Post-glacial Shorelines
  2. Western Lake Ontario Bathymetry
  3. Wave Zone Areas
  4. Toronto Waterfront Substrates and Features
  5. Shoreline Profiles

Post-glacial Shorelines

The modern shoreline of Lake Ontario is situated between two post-glacial abandoned shorelines. The landward abandoned shoreline originally marked the edge of the higher post-glacial Lake Iroquois, resulting in a stranded shoreline bluff and abundant beach material along the present day tablelands. The Lake Iroquois shoreline influences the morphology of modern streams and focusses the mid-reach recharge of ground water sources. However it has a minor effect on current aquatic habitats.

An off-shore abandoned shoreline created by the lower post-glacial Admiralty Lake has a much greater effect on today's shoreline. The former Admiralty Lake shoreline has left a variety of submerged features including a prominent off-shore bluff known as the Toronto Scarp that runs parallel to the Toronto Islands and Scarborough shoreline. Admiralty Lake was also the source of relict sand and gravel deposits that can still be found in deep off-shore waters. The most significant surficial geological features that affect and determine current shoreline conditions are found between the abandoned Admiralty Lake shore and the modern shoreline. Most current and historic habitats were created in this inundated area. For example, historically, the dynamic movement of littoral material established the peninsula and lagoons of Toronto Bay. The bulk of this material was supplied from shoreline erosion of the significant deposits of sands found in the Scarborough Bluffs and re-worked beach deposits made available during rising water levels. In addition, the Toronto Scarp at the shoreline of the former Admiralty Lake is an important area of congregation for salmonid fish.

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Western Lake Ontario Bathymetry

The bathymetry of western Lake Ontario displays a number of features that affect aquatic habitats. Lake Ontario is a deep, cold, oligotrophic (nutrient-poor) lake with relatively steep shorelines, particularly on the northern shore. Shale bedrock is apparent along the shorelines of Niagara Region, Halton Region, Mississauga and Etobicoke. A major depositional zone exists at the Hamilton lakehead. There is an underwater bluff, similar to the Scarborough Bluffs, off the Niagara Region shoreline.

The Toronto area shoreline can be described in five zones:

  1. Etobicoke Shale Outcrop
  2. Humber Bay Depositional Area
  3. Toronto Scarp
  4. Scarborough Sand Plains
  5. Scarborough Boulder-laden Till

1. Etobicoke Shale Outcrop

Along the western sector a thin till layer that originally covered the bedrock has been scoured off by glacial action leaving a prominent area of bedrock substrate that extends from the mouth of Mimico Creek westward to Burlington. This bedrock forms a convex shoreline profile consisting predominantly of broken shale boulders on top of bedrock extending into deep water.

Humber Bay Depositional Area

From Humber Bay east to Ontario Place the substrates are dominated by fine material. Humber Bay and Toronto Bay are depositional areas containing recent silt deposits that predominantly come from the suspended sediment loads associated with the Humber and Don Rivers. The depositional area of Humber Bay is thought to be formed as a result of significant fluting in the underlying bedrock which produces the deep basin-like depression of the Bay.

Toronto Scarp

The Toronto Scarp represents the former shoreline of Admiralty Lake about 5km from the existing Lake Ontario shoreline. It is a prominent underwater bluff comprised of extensive sand deposits. The water depth increases abruptly at the edge of the shelf from about 20m to approximately 60m.

Scarborough Sand Plains

An extensive underwater sand plain occurs from the south shore of the Islands to the Toronto scarp and eastward to Bluffers Park. This material is a very thick deposit of sand that is most likely a glacial relict of flooded beaches and eroded material that originated from an interglacial river deposit of deltaic sands derived from the cathedral section of the Scarborough Bluffs. Within these sand substrates there are small pockets of gravels and cobbles, especially in the nearshore areas just west of Bluffers Park. This section of sand-dominated substrates displays a prominent concave shoreline profile.

Scarborough Boulder-laden Till

From the east side of Bluffers Park to the East Point area there is a transition zone from sand to cobble, gravels and boulders. This coarser material originated from the high boulder content of adjacent tills that were eroded from the shore and re-worked as a boulder pavement. The headland created at East Point is a direct result of the high boulder and cobble content of the till, creating an area resistant to erosion. The boulder pavement provides an excellent example of unconsolidated material forming a convex shoreline profile. The extensive quantity of nearshore gravels is thought to provide a high degree of shoreline protection by attenuating waves and providing a dynamic equilibrium between erosion and accretion.

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Wave Zone Areas

Along the wave zone area bedload sediments from the major rivers have surcharged the shoreline with sand and helped to establish the barrier beaches associated with local coastal wetlands at the mouths of the Rouge and Highland Rivers.

The boulder-laden till also loaded the wave zone areas with a vast quantity of aggregates. Approximately 1 million cubic metres of stone were historically removed by stone-hooking for use in construction activities.

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Toronto Waterfront Substrates and Features

In summary, as shown below, Toronto Waterfront Substrates and Features, the key substrates along the Toronto waterfront are shale bedrock, sand, muds and clay, and boulder, cobble and gravel.

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Shoreline Profiles

The shoreline profiles vary considerably along the waterfront. For example, in the vicinity of the Toronto islands (section 1 and section 2) the Toronto Scarp appears as a precipitous drop that varies from 15 - 60 metres to the deep lake, with the relatively shallow waters of Toronto Bay being sheltered by the islands. In section 3, there is a gradual slope into the Lake from the base of the Leslie Street Spit, followed by a deep bluff formed by the Toronto Scarp. In section 4, the effects of the Toronto Scarp have almost disappeared, and in section 5 there is the gradually sloping convex shoreline of the Scarborough boulder till.

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