Some 20,000 years ago what is now New England was completely covered by an ice sheet that in most places was up to two miles thick. The glaciers left the area covered with boulders. Most of the rich topsoil was scraped southward to form Long Island, Cape Cod, and the islands of Nantucket and Martha’s Vineyard. About 18,000 years ago the ice began to retreat. Then about 13,700 years ago a dam formed in the bedrock at present-day Rocky Hill, Connecticut, and to the north of that extended Lake Hitchcock. Year after year the lake grew larger and wider. It extended along the current valley of the Connecticut River from Rocky Hill into Vermont, to about where Lyme, New Hampshire, borders Vermont. The lake varied in width. It spanned twenty miles at its widest near the Connecticut-Massachusetts border and north of that it narrowed through the Holyoke Range, a tilted east-west run of old volcanic vents and lava flows that had been overrun by the glaciers. The lake swelled again as far as Turners Falls, Massachusetts. From there it ran long, narrow, and deep into Vermont. On the lake floor were extensive sand dunes, mud flats, and other features. After the Rocky Hill dam gave way perhaps 12,000 years ago, the blockage at the Holyoke Range held for a time. It gave way, probably explosively, about 10,000 years ago.