The tour extends beneath the now extinct volcanic cone of Mount Underwood, along a seven-mile section of highway opened in May 1937. Construction here lent itself to the most modern highway engineering, including heavy rock work, riprap, tunnels, grade separation structure, drainage culverts, and a major bridge.

The first construction was an overhead crossing above the railroad tracks. Right of the highway was a combination sawmill and shingle mill unit, which provided employment for many residents of Underwood and the surrounding area. What remains of the sawmill was once part of the Broughton Lumber Mill, which provided employment for the residents of the area from 1923 until 1986, when the logging operation closed down.

Sections of the Broughton Log Flume can be seen on the side of Chemawa Hill, from the boat ramp at Drano Lake. The 9-mile-long, 1,000-foot-drop flume was the last operating log flume in the United States, floating lumber as much as 150,000 board feet per day from Willard, Washington, to the Broughton mill in Hood, Washington. At the mill the logs were processed into finished lumber and shipped east or west on the Spokane, Portland & Seattle Railway.