The highway pierces the main divide by way of Chinook Pass some 20 miles south of the old immigrant crossing through Naches. The pass offers the first good opportunity to see glacier-ribbed Mount Rainier. From the summit of the pass, a tumbled rampart of mountain peaks is visible to the west. To the east is the deep valley of the American River; northward is an orderly staircase of cliff-sided peaks, dropping away from the summit; and to the south are jagged hills, their snowy slopes stubbled with sparse green forests.

The pass is named for the warm wind that comes from the southwest, melting depths of snow in a few hours. The Native Americans had so named the wind because it came from the direction of the Chinook tribe, who lived near the mouth of Columbia River.

The sharp broken peaks around the pass, vestiges of old lava flows, were tilted to an acute angle by intrusions from below. Only rock pyramids resting on a solid foundation escaped the abrasion of glaciers. Red rock, andesite, and gray granite occur in patches along the roadside.