Geology

The Oak Ridges Moraine was formed 12,000 years ago during the last glaciation period. Today, it serves as the headwaters of several river systems in the Greater Toronto Area, including the Rouge Watershed.

The Oak Ridges Moraine (ORM) is an important geological landform in southern Ontario. It covers about 1,900 km2 (about 1/3 the size of Prince Edward Island), and is characterized by rolling hills and river valleys. The moraine is about 100 metres higher than the surrounding landscape (approximately 340 metres above sea level) and runs 160 km east to west. Part of the Oak Ridges Moraine lies within the northern area of Rouge National Urban Park.

The story of the Oak Ridges Moraine begins 13,000 years ago during the last glaciation event, when the massive Laurentide Ice Sheet covered most of Canada. Around this time the ice sheet began to retreat, forming two lobes with a distinct crack between them. This crack was the future ridgeline of the ORM. Flood channels beneath the melting ice began transporting sediments into the crack. As the ice continued to retreat, approximately 12,000 years ago, a large accumulation of sediment was deposited along a 160 km-long ridge, forming the ORM.

Nicknamed southern Ontario’s rain barrel, the ORM collects and transports precipitation to various watersheds in southern Ontario, including the Rouge Watershed. The complex sediment composition and the elevated topography of the ORM are important features that make this landscape an excellent source of groundwater. The sandy, permeable soils of the moraine can store and filter large amounts of groundwater, while the high elevation allows the water to discharge into streams and watersheds that drain into Lake Ontario and Lake Simcoe.

Rouge National Urban Park protects an important corridor between Lake Ontario and the Oak Ridges Moraine. This corridor is beneficial for wildlife and also ensures that farmers and residents of the Rouge Valley will continue to have a quality freshwater supply.

The Rouge Watershed drains a large area in the eastern end of the Greater Toronto Area. A significant portion of the watershed is protected within Rouge National Urban Park.

A watershed is an area of land that catches precipitation and drains into a body of water such as a lake or an ocean. Watersheds are composed of a network of streams called tributaries that eventually connect to a main river. Watersheds support numerous plant and animal communities and also provide our drinking water and water for agriculture. Natural areas in a watershed, including wetlands and streamside vegetation, help to filter out contaminants and sediment and improve water quality.

The Rouge Watershed covers an area of 336 km2 and stretches from the Oak Ridges Moraine in the north to the shores of Lake Ontario in the south. Land within the Rouge Watershed is a mix of rural, urban, natural and agricultural areas. Rouge National Urban Park protects a significant portion of the watershed and provides a vital link between the Oak Ridges Moraine and Lake Ontario. There are two main rivers in the Rouge Watershed – the Rouge River and the Little Rouge River – along with many smaller tributaries. The Rouge and Little Rouge rivers both originate at their headwaters in the hills of the Oak Ridges Moraine and flow south before converging just north of Highway 401 in the City of Toronto.

A healthy watershed provides many important ecosystem goods and services, such as supplying safe drinking water, providing natural areas for recreation, supporting healthy streams and wetlands, providing habitat for plants and wildlife, reducing flooding, and supporting agriculture. However, watersheds are also sensitive to pollution and changing land use. For example, urban areas have a lot of paved surfaces, which prevent water from seeping into the ground. This increases the chance of damaging floods and alters how water flows through the watershed. Runoff from roads can also carry contaminants such as road salt, which affect the health of downstream areas.

The Rouge Watershed is one of the healthiest watersheds in the Greater Toronto Area, but it is still under stress from increasing urbanization. Parks Canada, the Toronto and Region Conservation Authority and other organizations, community groups, and residents are working hard to protect and manage the Rouge Watershed and ensure that it continues to provide these important services.

Glaciers have played a large role in carving out the landscape of the Rouge Valley as we see it today. The retreating glaciers also left behind large deposits of sand and silt, creating this area’s fertile soils.

About 2.5 million years ago, Earth entered its most recent ice age. Colder temperatures allowed snow and ice to accumulate until nearly all of Canada was covered by the Laurentide Ice Sheet. This massive ice sheet played an important role in shaping the park landscape that we see today.

The Laurentide Ice Sheet repeatedly advanced and retreated over time as the climate cooled and warmed. During warmer periods, melting ice formed rivers that transported large amounts of sand and silt away from the glacier, depositing new layers of sediment over the underlying bedrock. These sediment deposits accumulated over thousands of years, forming layers more than 100 metres thick in certain areas. Today, glacial sediments are exposed in some areas of the park, such as the bluffs visible from Glen Eagles Vista. These bluffs consist of two layers: the Newmarket Till, which was deposited starting 25,000 years ago and the Iroquois Deposits, which were laid down 12,500 years ago.

About 20,000 years ago, the Laurentide Ice Sheet reached its maximum size, with ice covering Toronto believed to be approximately one kilometre thick. The ice sheet in the Rouge began to retreat 14,000 years ago, and by about 13,000 years ago the ice had disappeared from the Lake Ontario basin. The retreating ice left behind mounds and ridges of till (glacial sediment) known as drumlins and flutes. These glacial landforms are still evident on the landscape today.

Glacial meltwater also accumulated in the lake basin, forming Lake Iroquois, a larger predecessor of Lake Ontario. Lake Iroquois was 60 metres higher than Lake Ontario and its shoreline and was located 4-15 km further inland, passing through areas that are now the Toronto Zoo, and the former Beare Road landfill site. Lake Iroquois drained through the Hudson Valley to the southeast, near present-day upstate New York, as the St. Lawrence Valley was still blocked by ice at that time.

About 12,000 years ago, the ice in the St. Lawrence Valley broke up, allowing water from Lake Iroquois to flow out through the St. Lawrence River. The water level of Lake Iroquois steadily dropped, eventually equalizing about 20 metres below modern-day levels, forming a smaller lake known as Lake Admiralty. By about 8,000 years ago the ice sheet had retreated even further and the land around the eastern end of the lake began to rise up as it recovered from the weight of the ice (a process known as isostatic rebound). This uplifting reduced the flow of water out of Lake Admiralty, causing lake levels to rise until modern-day Lake Ontario was formed.

The landscape of the Rouge Valley that we see today is a product of thousands of years of natural events. The beautiful hills, ridges and valleys throughout the park would not have existed without the power of glaciers carving through the lands of southern Ontario, while the fertile soils of the Rouge would not have been present without the massive deposits of sands and silts left behind by the retreat of the Laurentide Ice Sheet.

The bedrock in this area is 450 million years old. Known as the Whitby Formation, this shale rock was formed when a shallow inland sea covered much of North America.

About 450 million years ago, a large network of rivers flowed across what is now Southern Ontario into a massive inland sea that covered much of current-day North America. The clay-rich sediments that settled to the bottom of the sea compacted over time to form shale rock. This rock is now known as the Whitby Shale formation and provides the underlying bedrock throughout Rouge National Urban Park. The inland sea was home to a variety of prehistoric creatures, including some of the very first multi-cellular animals found on the planet such as corals, brachiopods, sponges, clams and trilobites, some of which are preserved as fossils. Within the park, the Whitby Shale is exposed in a few places along the Rouge River and Little Rouge River where stream channels have eroded the overlying sediments.