Why is this lake long and narrow?
Lakes are bodies of water deep enough to contain water for long periods of time. Looking at a map of Ontario, it is easy to see that there are many lakes across the province, especially in the north. In fact, there are about 250,000 lakes in Ontario, covering about 15 per cent of the province. This is about one third of all the freshwater on the surface of the Earth. These lakes were filled with water in the last 10,000 years.
Lakes of different sizes and shapes were created by a number of natural geologic processes that started millions to billions of years ago and continue today.
How does geology control where lakes form?
The rock that makes up the Hudson Bay and James Bay lowlands consists mostly of limestone and dolomite. These rocks dissolve rather easily in water. The rock to the south and west of the lowlands however, is very old, hard rock that we call the Precambrian Shield. Most of the shield rock was created one to three billion years ago. Over the millions and millions of years, the shield rocks were cut by geological faults. A fault is a zone in the earth along which blocks of rock slide past each other. Geologists note that an ancient fault often occurs down the centre of, or along the edge of, these long narrow lakes. The fault can run for many kilometres across the surface of the land.
How did a fault prepare the land for long, narrow lakes?
The force of the geological faulting broke the hard Precambrian Shield rock into tiny pieces and crushed some of the rock into clay. This fault rock is much softer than the unfaulted Precambrian Shield rock.
Long after the faults were active, during the last great ice age, an ice sheet covered all of Ontario. The ice sheet was up to two kilometres thick. This ice sheet, called a glacier, did not just sit on the surface of the earth. The glacier moved very slowly across the land. The grinding weight of the glacier on the land acted like sand paper and it carved many large and small depressions into the surface of the land. When the glacier rode over an especially weak zone of rock, such as a narrow fault zone, the glacier scraped and dug out the soft broken fault rock, but left the hard, unfaulted shield rock.
The result was a long, narrow surface trench in the land. When the glacier melted about 10,000 years ago, the freshwater collected in the trench to create a lake. So, many long, narrow lakes formed along ancient fault trenches from water that melted from the glaciers.
What other kind of lakes are there?
Glaciers also dug bowls into the land and filled the bowls with water when they melted. The Great Lakes in Southern Ontario formed this way. Most of the water in the Great Lakes is the original water that came from the melting glacier 10,000 years ago!
Kettle lakes formed on top of deposits left behind by the last glaciers. When the glacier melted, blocks of ice (some as large as apartment buildings) broke off and sat in the sands and gravels. When this ice block melted, a round, shallow lake formed, called a kettle lake.
Lakes are also created when a river channel winds across the land, like a snake, eroding deposits left by the glacier. This type of river can turn on itself, like a bow tie, and cut off a piece of the river channel. This isolated piece of river becomes an oxbow lake.
There are also rare lakes that form where a large piece of rock from outer space, called a meteorite, crashed into our planet and impacted the earth. Usually these lakes caused by this impact are round or banana-shaped. Lakes created this way in Northern Ontario include Wanapitei Lake near Sudbury, lakes in the Brent Crater east of Mattawa, and Skeleton Lake in the Muskokas.
There are many ways that geology influences the size and shape of a lake. Understanding how a lake forms helps explain lake water quality, depth, and colour. These lake-forming processes create different habitats for fish, plants, animals, and even people.
So, the next time you look at a lake, think about the complex geological history that shaped the land, created the landscape, and created the lake — all part of the “Ontario beneath our feet.”
About the author:
Andy Fyon is the director of MNDM’s Ontario Geological Survey. For more information about the geology of Ontario: http://www.mndm.gov.on.ca/en/mines-and-minerals/geology.
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