The forests surrounding Lake McDonald  are Glacier National Park  at its most Pacific. Western red cedar and western hemlock form the climax forest; Douglas fir, larch, pine, and spruce add diversity.
The oldest rocks on the west side of the park are around the head of Lake McDonald. These slatelike rocks were laid down under seawater. Up the Going-to-the-Sun Road , greenish mudstone of the Appekunny Formation developed in shallower water. Farther east and higher on the slopes, the rocks of the Grinnell Formation were originally mudflats. They turned red when exposure to the air oxidized their ferrous minerals.
Once the road begins climbing, it moves into the Empire Formation, grayish-green rocks deposited underwater, and then to the Helena (Siyeh) limestone, which comes into view near the Loop. The Helena Formation is dotted with stromatolites, cabbage-like fossil traces of blue-green algae.
Lake McDonald Valley has all the earmarks of glacial action; it’s long, straight, and U-shaped. The lake is 10 miles long, one mile wide, and more than 400 feet deep. Dozens of hiking trails take in everything from damp old-growth forests to alpine meadows.