The unique landscape of the 1000 Islands and Frontenac region can be attributed to past geological activity along with recent ice ages.
Let’s begin by exploring the early formation of the bedrock in the region. The first deposits of sedimentary rock were mostly sandstone, limestone and shale; this occurred around 950-1000 million years ago. Now geology buffs know that sedimentary rock deposits are often caused by large bodies of water, and that is exactly the case here. During this time period, the area was covered by an ancient marine sea. Many thousands of years later and after increasing pressure from above these sedimentary rocks turned into metamorphic rock and around this time massive plutons (Large rock formations) of igneous rock were developing. These changes led to huge mountains of granites and gneisses, but over time the elements wore down this mountain range to almost nothing. This is why we see an abundance of beautiful Precambrian rock throughout the biosphere.
Millions of years after the initial formation of the Frontenac region a global cooling event happened which caused the most recent ice age event. During this period almost all of Canada was covered by massive glaciers with some areas being under 2-3 km of ice. The melting of these glaciers occurred between 10-12,000 years ago and severely changed the landscape. If you have ever seen large scratches on exposed rock running north-south, this may have been caused by the receding glaciers. Overall the glaciers dragged much of the topsoil away and once they had fully receded the meltwater covered the area once again in water. Once the weight of the glaciers had been taken off the land and the water drained away we began to see what we know today as the 1000 islands archipelago.
If you are interested in some more in-depth content regarding the geology of the region, be sure to check out this link: http://www.oliverkilian.com/ecology/thousand-islands/island-insights/geology/rocks.html