The Thousand Islands are the roots of an ancient Precambrian mountain range worn down by successive glaciations to granite bedrock. Over the centuries, the bare, gnarled hills were covered with rich forests and carpets of wildflowers, and where the St. Lawrence flows eastward, these hilltops became islands. The Thousand Islands link the natural communities of the Algonquin highlands with the Adirondack Mountains to the south. During the 19th century, the land was cleared for agriculture, although farming on the thin soils produced marginal yields. Today, the forests have returned, and the landscape resembles the one that existed two centuries ago.