December 20, 2002
This beautiful photo, taken by Lawrence Dodge, shows a “lateral moraine” that was deposited by a valley glacier during the last ice age. The moraine is the curved ridge along the left side of Mission Reservoir, about 30 miles (48 km) north of Missoula, Montana. The forested ridge is made up of rock material that the glacier removed from the mountains in the upper part of the drainage basin, high above the lake.
Rock material that has been transported and deposited by glaciers is called "till". As glacial ice formed near the top of the ridge, rocks would stick to its bottom and sides. Then as the ice flowed toward the valley floor, these rocks scoured away even more of the mountain’s surface. The glacier flowed to the location marked by the lake, and as it melted, it dropped the rocks that had been transported from the upper portion of the basin. For thousands of years, snowfall continued to replace the ice as it flowed away from the mountain tops. This conveyor belt took much of the mountain with it, forming the snake-like deposit. A considerable amount of the till deposited at the end of the glacier was washed away by the melting glacier. Thus, the “terminal, or end moraine” is missing. Since the end of the last ice age (10,000 years ago), soil has developed on the moraine and trees have taken root.