Glacial geology of beautiful Sawtooth Lake, the largest alpine lake in the Sawtooth Mountains, Idaho

Summary

The video is a field explanation of the geology and landscape around Sawtooth Lake in central Idaho. The narrator describes it as a very large alpine lake—possibly the largest in the Sawtooth Mountains—with a depth exceeding 200 feet and an elevation of about 8,500 feet. A cold, wet spring has left snow and ice along its shores.

Main Ideas

• Glacial landscape: Sawtooth Lake occupies a glacial cirque, a bowl-shaped hollow where snow accumulated, compressed into ice, and eventually formed a glacier. As the glacier moved under its own weight, it eroded and deepened the landscape. After the climate warmed and the glacier disappeared, the cirque filled with water.
• U-shaped valleys and erosion: A thick glacier flowed south from the lake, carving and widening the canyon into a U-shaped valley. The narrator estimates that the ice may have been hundreds of feet thick, perhaps approaching 1,000 feet.
• Moraines: Glaciers carried eroded granite toward the eastern edge of the mountains and deposited it near Redfish and Pettit lakes. These accumulations of glacial debris are called moraines.
• Granite and the Idaho Batholith: The area is underlain mainly by granite, with some granodiorite. These rocks formed underground during the Cretaceous Period, when a tectonic plate was subducting beneath North America. Magma rose and fed volcanoes, while much of it cooled underground to form the large bodies of granite that shape the mountains.
• Later intrusions and dikes: Dikes formed when magma pushed through cracks in older granite. The narrator points out dark dikes, likely basaltic or andesitic, and mentions a later intrusion, about 50 million years old, associated with pinker, potassium-feldspar-rich granite.
• River headwaters: The Sawtooth Mountains are the source region for the Salmon, Payette, and Boise river systems. These rivers ultimately flow into the Snake River.

The video closes with reflections on the area’s alpine scenery and a plan to examine another dike more closely in a later video.

Speaker

• Shawn Willsey — sole narrator and geology presenter