When the last of the continental ice sheets blocked this valley, a great lake extended over 200 miles into Montana. Ice about as high as the mountain ridges held back water as deep as 800 to 1000 feet at Missoula, ten to twenty thousand years ago. At times this lake cut through the ice dam, and Lake Missoula discharged tremendous torrents before the ice came back. Finally the climate turned warm and the great ice sheet melted. Lake Pend Oreille is a small remnant of all that glacial action.