Geology
Earth's surface is slowly, yet constantly changing. Much of the evidence of these changes is deep below the surface, out of our sight. Rouge Park is unique in the Greater Toronto Area because it has outcrops of all the principal geological strata in the region. The geological history of Rouge Park spans over 450 million years!

Outcrops
Outcrops, or exposed sections of geological features, in the Park tell a story of thousands of years of changes in Earth's crust. Several of the outcrops, such as those exposing sediments of the last inter-glacial age 125,000 years ago, are of international significance.

Sedimentary layers deposited 13,000 to 30,000 years ago by a river formed from melting glaciers are visible in some areas of the Park. From Glen Eagles Vista, the buff-brown Iroquois Sands you see near the surface are above the grey, mixed sediment of the Newmarket Till layer in this Provincially Significant outcrop. These sediments were deposited while the climate cooled and warmed over thousands of years. Glaciers melted during warmer periods, releasing sediments, and formed or grew as the climate cooled, carrying sediments in the ice.

Glaciation
Water from glaciers melting 12,000 years ago formed ancestral Lake Ontario, which covered this entire area. A large ice lobe, roughly 20 metres thick, blocked the lake from draining eastward, leaving water levels high as the lake slowly drained south to what is now the Mississippi River. The ice lobe finally retreated, draining the lake to the St Lawrence River and forming the Great Lakes as we see them today.

Glaciation occurred when average annual temperatures were only 2 - 5 °C lower than present. The small increase to our current temperatures caused big changes to the landscape. Increases in average annual temperatures now may seem small, but they could cause major changes to the natural environment in the near future.

Water Power

Moving water is a powerful force of change in our landscape. Rivers carve paths through valleys. Flood waters move sediment and deposit it as water drains and settles. Inland, shorelines change as lake levels rise and drop. The Rouge River has carved its banks over time, exposing sedimentary layers and creating the steep sides of the riverbanks, up to 30 metres high. Underground, water travels through the spaces between soil particles, sometimes taking thousands of years to move only a few kilometres. In the outcrops seen in the Park, the darker areas you see are coarser sediment than the surrounding sand, silt and clay. They appear darker because of moisture from groundwater. Groundwater seeps out to the surface faster in coarse sediment because the water moves more quickly through the larger spaces between the particles.

Hoodos and Gullies Valley side slopes are subject to erosion by water. Erosion takes place mainly from mud flow in the Spring when seepage from groundwater is most active, and when there are heavy rains in the Summer. Minor hills are eroded into the cliff face where seeping groundwater is focused into "sag basins". These sag basins and rills eventually form gullies, which cut into the cliff face, resulting in scalloped slopes, where the gullies are separated by free-standing spires and aretes. They are distinctive landscape feature indicating severe erosion which contributes significant amounts of sediment to the stream, degrading aquatic habitat. Sag Basin: A depression occurring at contact between an aquifer and underlying, less permeable unit which acts to focus seeping groundwater.

Paleozoic Bedrock Features

Quaternary Glacial Features

Fluvial Geomorphology
Fluvial geomorphology is a study of the shape and form, or morphology, of watercourses. In an urbanized area, such as that of the Rouge River system, the movement of water and sediment are important factors in how watercourses change. Climate and geology have a direct effect on sediment and water movement, but also influence fluvial geomorphology indirectly through ground water and surface water flow.

The Rouge River watershed, the geographic area where all lands drain into the Rouge River system, is reacting to rapid urbanization and change. Many sections of the watershed are no longer stable and are experiencing channel enlargement, largely due to surface runoff from the impervious surfaces which are the result of urbanization, and the application of engineered technology in an attempt to manage the increased flows.