 During the last two to three million years, a series of ice sheets flowed and receded across northern North America, eroding mountains and creating U-shaped valleys.The glacier that covered Mount Desert Range, part of which is now called Mount Desert Island, was thick and moved at the rate of a few yards (meters) per year.The mountain range was heavily eroded by the glacier which rounded off mountaintops, carved saddles, deepened valleys, and created the fjard known as Somes Sound, nearly dividing the current island in half.