In October 1781, when the British army of Lord Cornwallis was bottled up at Yorktown by the Allied Army, and the French fleet was blockading the Chesapeake Bay, as many as twenty-six British vessels were scuttled off Yorktown. This prevented a landing by the French fleet. Cornwallis attempted to evacuate his troops across the York River, but a violent squall prevented their escape. A British flag of truce was flown the next day; and on October 19, 1781, the surrender was signed, effectively ending the Revolutionary War. Investigations of the Yorktown Shipwrecks Maritime Archaeological Site by the Virginia Department of Historic Resources located nine ships dating to the wartime period. In 1982 a cofferdam was erected around one of the best preserved of the Yorktown Wrecks, a British supply ship, the Betsy. The systematic excavation of the Yorktown Wrecks site by DHR archaeologists was a precedent-setting demonstration of underwater archaeological techniques.