Carved by Ice and Water
How did Grenadier Pond come to be? The answer dates back to the last ice age. Around 20,000 years ago this location was covered by an ice sheet 1 km thick. Over the millennia it slowly melted, forming the huge Lake Iroquois - 95 m deeper than its successor, Lake Ontario. The steep slope at Davenport Road, 8 km north of here, marks its original shoreline (see letter A on the map).
As the ice age ended, the waterways readjusted, causing the shoreline to recede by 5 to 10 km (see B). Later the continent's landmass rebounded and the shoreline expanded again, eventually forming Lake Ontario as we know it today (see C). Water flowing from the north carved through deposits of sand and gravel left by the retreating glacier, deepening old rivers and creating new watercourses such as Wendigo Creek and Spring Creek.
About 2,000 years ago, a sandbar closed the mouth of Wendigo Creek, and the trapped water became Grenadier Pond. Over time, the pond occasionally broke through to reconnect with the lake as water levels fluctuated with the seasons. When Toronto expanded in the 1800s, railway beds and roadways built along the lakeshore created a permanent barrier, and the pond's outflow was redirected through a weir west to the Humber River.
Plaque via Alan L. Brown's site Toronto Plaques. Full page here.