The Issaquah Alps follow Interstate 90 from the shore of Lake Washington almost to the western face of the Cascade Range. The hills are composed of andesitic volcanic rock lying on top of older tightly folded rocks from the coastal plain of the North Cascade subcontinent that docked with Washington about 50 million years ago as the entire continent of North America moved west across the ocean. The Alps were heavily eroded by glaciers in the last ice age. The Vashon lobe of the ice sculpted Rattlesnake Ledge, steeply carved the east and west sides of Squak Mountain, and deposited a large erratic on Cougar Mountain, Fantastic Erratic.