Geology of the Dishman Hills Area


(From March 2006 issue of Lights and Shadows from the Dishman Hills  monthly newsletter by Michael Hamilton)

Several topographic features in the northern and eastern parts of the Natural Area have a very interesting geologic story to tell. A series of parallel ravines starting with the main trail south of Camp Caro to Caro Cliff and through Enchanted Ravine, and Deep Ravine along with several more ravines further to the east, all represent faults where the Earth's crust has been fractured and moved.

These ravines run from southeast to northwest and are part of a larger set of faults in the Spokane area. Recent work by Washington State geologists has shed some light on the nature and origin of these faults. The fracture plane of these structures is thought to be near vertical and the motion is the western side to the north. Movement on one of these faults, just north of Gonzaga Univ., was the source of the earthquakes that visited Spokane about five years ago. Where the earth is faulted the fractured rock is more susceptible to erosion; thus faults are often hidden under the bottom of valleys. The 80-mile-long Hangman Creek/Spokane River valley on the west side of the city is our best local example of fault liniment topography. On the eastern side of the Natural Area a younger granite has "oozed" up through these fractures resulting in a fine building stone which was mined from a quarry off 'Dishman Road.

Besides providing beautiful, steep-walled ravines and handy locations for trails, what are these faults all about? They have the same orientation as California's famous San Andreas Fault, and also have the same connections to plate tectonics. Spokane with the rest of North America is drifting west at the break-neck speed of about 1 inch per year. As we cruise west, we are colliding and overriding the Pacific floor causing all types of geologic havoc (Rocky Mtns., Cascade Mtns. and volcanoes, western earthquakes, etc.). This collision is not straight on but at an angle so that there is not only the expected east-west compression but also a sliding to the north of rocks on the western edge (California is coming our way!).

Back to the Natural Area. This model explains the orientation and motion of our faults, but why here? For that answer we have to look way back into earth history. The bedrock in the Natural Area is old, very old, maybe up to 1.5 billion years old! Back then we were on the western edge of an ancient North American continent, and to the west was the newly formed Pacific Ocean. Recent geologic mapping by the State revealed an occurrence of ancient (0.5 billion years old) Marine fossils preserved just to the west of Spokane. While the continental edge now is hundred of miles west of us, the deep roots of the older continent are beneath us and are even thicker to the east. As tensions in the Earth's crust transmit to the east from the plate collision off the coast of Washington, they run up against the old continental edge and release their energy by fracturing the local rocks. Maybe like waves crashing on a breakwater. So the next time you hike along one of these fault-caused ravines visualize if you can the changes in time that has affected this topography.

© Dishman Hills Conservancy
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