Muskoka’s geological evolution — dynamic and evolving
By John Mason | August 29th 2026
The more we can learn about our watershed of rocks and trees and water, the more real it becomes to us.
To begin with, our environment is not just there. It has had a very long and dynamic history. How Muskoka’s terrain got to be where it is today is a fascinating story.
We are fortunate in the Muskoka River Watershed to live on what has been geologically termed the Grenville “Province” a southern portion of the stable Canadian Shield. These rocks are 1.1 to 1.8 billion years old in geological time (age of the earth is 4.6 billion years).
The earth is truly dynamic, alive and ever changing! It is hard to believe the forces of nature that impacted these rocks after they were deposited. Essentially “The Grenville” was mountainous terrain (mountains more than 6,000 metres high) that experienced tremendous heat and pressure to give us the spectacular textures and swirls we see in outcrops or quarried flagstone in Muskoka.
The mountains were then eroded down over millions of years and sheared off by a series of glaciers.
Living organisms were present when rocks of the Grenville Province were being formed. Fossil algal mounds or stromatolites 1.5 billion years old are preserved in limestone rocks in Bancroft, Thunder Bay and Atikokan.
A later article will tell the story of stromatolites and their distant descendants that occur in Muskoka today.
Travelling fast forward in geological time to 25,000 years ago, look at the last ice age and its effect on our neighbourhood. The Laurentide Ice Sheet had advanced from the north and covered much of the Canadian Shield (Central Canada), including Muskoka.
Ice thicknesses were up to three kilometres. The weight, plus movement of the advancing ice, sheared off landforms and compressed the earth’s crust.
Advancement slowed as climate warmed; then the glacier started to retreat, leaving deposits of sand and gravel over the bedrock in Muskoka.
Soil thicknesses remaining were marginal, with only 10-30 cm of soil being the norm. As the ice melted and retreated, huge volumes of meltwater moved through valleys and under the ice, transporting significant amounts of sediments (sand, gravel and boulders).
Some interesting features were created. Eskers were formed from meltwater and sediment flow under the ice or at ice margins (these are often now prized as good aggregate sources for road building and construction). Kettle lakes were formed when ice blocks were trapped in deep sediments during the ice sheet retreat. These blocks then melted, creating lakes.
Also drainage channels were enlarged with the flows and large areas were inundated with glacial lake water. Glacial Lake Algonquin, created 11,000 years ago, was the ancient start of the Great Lakes. At this time, random ice blocks were melting during the retreat, creating wetlands and lake basins.
The dynamic earth had more surprises for us.
With the massive weight of the ice gone, the crust of the earth started to slowly rise, altering pathways of water flow progressively until the current system of Muskoka watersheds was formed. The geological term for this uplift is “isostatic rebound” and it continues to this day.
So, in a nutshell, formation of the Grenville Province, ice erosion, flooding/submergence of land, meltwater drainage and isostatic rebound combined to produce Muskoka’s present landscape.
This article was first published by MuskokaRegion.com.
John Mason
This is article No. 34 in the current series, Nurturing Our Watersheds, from Muskoka Watershed Council. Its author is John Mason, D.Sc., professional geoscientist, recent Muskoka resident, and a director of Muskoka Watershed Council. The series is edited by Peter Sale.