Waterfall Geology

Geology can explain why some waterfalls are vertical and others are gentle. It can also tell us why some waterfalls have deep gorges but others are almost gorge-less. And it can tell us why waterfalls are common in some areas but absent in others.

We don't need to be amateur geologists to appreciate waterfalls. This page only scratches the surface of the importance of geology to waterfalls. But it should help you to understand waterfall classification.

Tilted metamorphic rock at Wilson Falls, cut by an igneous dike in the foreground.

Why Waterfalls Only in Some Places?

The bedock surface on the left continues under the overburden on the right.

There are two main ingredients for a natural waterfall: exposed bedrock and flowing water. Bedrock is found beneath every place in Ontario, but in many places it is hidden. No bedrock? No watefalls!

Geoscientists use the term overburden to describe the loose earth material that covers bedrock. This includes the soil we see at surface, but also layesr of sand, silt and clay hidden beneath the surface.

Most overburden was brought here by glaciers as recently as about 10,000 years ago. These mile-high sheets of ice scoured lands to the north and then dumped the overburden as a blanket over the bedrock.

In many areas of southern Ontario the overburden can be tens of metres thick. It often completely smothers the bedrock, resulting in large areas with no waterfalls.

Waterfalls in the south generally only appear where a river has eroded deep enough through the overburden to reach rock. They also occur where a prominent rock ridge, like the Niagara Escarpment, protrudes through the overburden.

In most (but not all) of northern Ontario, the overburden is only a couple of metres thick. This means that bedrock is more commonly exposed, and explains why waterfalls are more widely distributed in this area.

Thick overburden beside the Ausable River at Port Franks. Bedrock is still a long ways below the surface here!

Sedimentary Bedrock of Southern Ontario

Sedimentary rock is always layered. Each layer records a different geologic environment in the past.

Most of southern and eastern Ontario is underlain by sedimentary bedrock. This includes rocks like limestone, sandstone and shale, which are usually dull grey in appearance and always appear in layers.

These rocks formed about 400 million years ago, when Ontario was covered by shallow seas. Clay settling to the bottom of sea was compacted and cemented into shale. Sand at a beach was compacted and cemented into sandstone. Limestone formed by the precipitation of calcium carbonate from sea water.

Each time the geologic environment changed, a different type of rock layer was left behind. The alternating layers of rock that are visible at many southern waterfalls are a record of these changes.

Crystalline Bedrock of Northern Ontario

Most of northern and central Ontario is underlain by crystalline bedrock. This includes rocks like granite, gneiss and quartzite. While these rocks can appear dull grey at the ground surface, they can be very colourful at a road cut.

Crystalline rock can be igneous, which means that magma cooled and solidifies either at the surface (at a volcano) or underground. Metamorphic rocks form when igneous rocks (or sedimentary rocks) are baked and squeezed many kilometres below the earth's surface.

Unlike sedimentary rocks, igneous rocks rarely appear layered. While metamorphic rocks can appear layered, the layers can be oriented in any direction. This differentiates it from sedimentary rock, which in Ontario, is only present in horizontal layers.

Igneous rock is often appears chaotic, and is very rarely layered.

River Erosion

All creeks and rivers erode the landscape and carry material away. The process is just way slower at waterfalls.

Rocks and overburden are both eroded by the elements. At a waterfall, running water is an obvious force that causes erosion. But freeze-thaw, wet-dry, and gravity also play a role.

Flowing water can dislodge sand, silt, clay and gravel, both from the riverbed and from the river's banks. As these materials are slowly carried away downstream, the river's valley slowly expands, and the surrounding landscape changes.

The river's job gets a lot harder when it encounters exposed bedrock. Since rock is so much harder, it forms a barrier to stream erosion.

Over time, even the rock succumbs to the power of erosion. Stronger rocks are more resistant to erosion than weaker rocks, and can be left protruding above everything else around them.

Rocks are weakest at natural cracks that geoscientists call "fractures". In sedimentary rocks, fractures usually parallel and perpendicular to each other. In crystalline rocks, the fractures are usually (but not always) much more random.

The differing rates of erosion and differing orientation of fractures combine together to form the different types of waterfalls defined by Waterfalls of Ontario.

Fractures within rock masses are weak spots for erosion. Some layers are stronger than others, and last longer.

Waterfalls of Ontario Project

This project has been online since 1999, in print since 2003, and on social since 2011. (See archives: 2003, 2012, 2018). It was the first to inventory and map Ontario's waterfalls for recreational purposes. With your continued help, it grows. Learn more...
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This page last updated on May 26, 2024. Earlier versions can be examined on Archive.org, dating back to 2003.