From Hawk Mountain's website: "The River of Rocks was formed during the last Ice Age 10,000 to 12,000 years ago.

The boulders you see here were once part of the ridge escarpment. Though the glaciers stopped 40 to 50 miles north of here, climate changes resulted in cycles of freezing and thawing. Water seeped into small cracks in bedrock, froze and expanded, fracturing the bedrock into boulders that slid slowly, with mud and ice, to the lowest point in the valley.

This boulder field is 30 to 40 feet deep in places; an underground stream prevents sediment from accumulating. The rock itself was formed 300 million years ago."