FIELD TRIP STOP 4 - North Cascade Highway (State Route 20)

Diablo Dam

Damming a canyon of stretched granite

Sills (vertical white ribbons parallel rock foliation) and dikes (even whiter crosscutting ribbons) in outcrop on road to Diablo Dam, east side of dam.

Road cuts approaching the dam, as well as and cuts along the old railroad bed to the top of the railroad lift on the north abutment of the dam, reveal criss-crossing patterns of white ribbons cutting the darker rocks. The first good look at these rocks is on the right (east) side of the road as it winds along the cliffs at the southern abutment. Unfortunately, this is not a good place to stop. The ribbons are really dikes and sills. A dike, is a sheet of igneous rocks which filled cracks cutting across existing bedding or metamorphic foliation dikes. If the rock cracked along the same plane as the bedding or foliation, the invading sheet of igneous rock is called a sill. The broad rock slabs of the Diablo Dam spillway also show the dikes and sills.

Flattened rock with foliation versus stretched rock with lineation. Combinations of flattening and stretching are more common and more confusing

To see an example of rock lineation in the blasted outcrop at the north abutment of the dam, find a conveniently-sized piece of orthogneiss, recognized by its black and white minerals and streaky look. The granitic rocks here are lineated; that is, they have been stretched in one direction only, elongating the minerals into parallel lines when seen from the sides. To be sure the rock is lineated and not foliated, the observer has to see the structure from three sides, which is not always as easily done with an outcrop as it is with a loose block. Much of the igneous rock that became lineated orthogneiss intruded about 45 million years ago, 15 to 45 million years later than the foliated orthogneisses within the Skagit Gneiss Complex, such as the one described at Newhalem.

Diablo railroad lift, viewed from station at bottom.

To see the railroad lift, walk west along the spur road to an overlook above the town of Diablo. The lift that ascends the cliff to reach this spot was first used to lift men and materials during the construction of Diablo Dam, which was completed in 1929. Then, beginning in 1937, supplies for construction of Ross Dam were brought by train to the foot of Diablo Dam. From there, the laden railroad cars were brought up on the lift to the upper level, loaded on barges, and taken up Diablo Lake to the new dam site. A tour of the dams offered by Seattle City Light includes a ride on the railroad lift. Check in Newhalem or Diablo, or call Seattle City Light in Seattle, for information.