The glacier builds up from a great deposit of snow winter after winter,until it can be over a mile deep, with minimal summer melting. When it becomes over a hundred feet deep, the ice at the bottom cannot stay rigid any longer. As if it were wax in a warm room it begins to spread out, becoming less deep. As it spreada out, fissures develop into crevasses. In summer melting puts water into crevasses, and there may be outwash from the southern margin. The Finger Lakes in New York were created by massive melting behind a dam-like formation. Out West not far below the Border, a tremedous lake formed, until it was truly huge. When it finally overflowed the obstruction, it caused enormous erosion - a miniature Grand Canyon. Get stales to tell you about it.