Rock features along the Seaway Trail reveal the geologic forces that formed the Thousand Islands region. Precambrian gneiss, the roots of an ancient mountain system, formed through heat, pressure, and intrusive action during mountain building. Later, overlying sandstone and limestone or dolostone layers formed as sedimentary deposits in the Potsdam Sea. Glaciers advancing from the north then modified the landscape by gouging valleys, scouring hills, and plucking rocks from the upstream sides of the islands. The Frontenac Arch is a narrow band of ancient rock connecting the Canadian Shield bedrock to the Adirondacks, and the Thousand Islands are exposed rises on the surface of this ancient rock mass, uplifted after younger rocks eroded away and then further shaped by glacial grinding. The rock record also includes an unconformity spanning 600 million years, caused by erosion or non-deposition, and remaining sedimentary rock reflects millions of years of erosion, including several episodes of glacial scouring.