Interstate 90 drops through the Thorp Drift, which marks the oldest and furthest reaching known glacial moraine in the Kittitas Valley. Changes in the types of vegetation become more evident in this area. The changes are the result of a drop in elevation of about 1,400 feet from the summit of Snoqualmie Pass to Thorp, and a significant drop in precipitation of about 107 inches average a year at the summit of Snoqualmie Pass, to 42.94 inches average a year at Thorp.