When chugging up the Siskiyou Summit, motorists whose engines are overheating might wonder why the summit is the highest point along all of Interstate 5.

When chugging up the Siskiyou Summit, motorists whose engines are overheating might wonder why the summit is the highest point along all of Interstate 5.

The answer lies in the geologic framework of the summit. It's a well-known fact that hard rocks are more difficult to erode than soft rocks. The Bear Creek and Colestin valleys are underlain by soft and easily eroded mudstone of an ancient ocean. Long ago, the Bear Creek and Colestin valleys were connected. So why aren't they connected now?

The culprit is the Siskiyou Summit Fault, which runs southwest to northeast along the steep slope rising from the Colestin Valley through Callahan's Lodge and ends about a mile southwest of Buck Rock. Movement along this fault has offset the Bear Creek and Colestin valleys by more than six miles.

The fault is an oblique fault with both up-down and side-to-side slippage. To get an idea of how this fault moved, take a slice of bread and tear it by moving your right hand down and toward your body while moving your left hand away and up. Now put peanut butter and your favorite jelly on the bread and consume it. Waste not, want not.

The fault moved hard granite northwest of the fault against hard volcanic rocks to the southeast. This motion formed a land bridge — an erosion-resistant corridor — for both plants and animals to migrate between the Siskiyou Mountains, Cascade Range and Great Basin, producing the unique and varied flora and fauna of our region.

Traveling south on I-5, take the Mount Ashland turnoff. Speckled granite is present before and along the off-ramp. Erosion along the fault caused the sharp bend in the road at the end of the off-ramp, opposite of which reddish, clay-rich soil, a paleosol (old soil), is exposed. The paleosol was developed on deeply weathered volcanic rock when the region's ancient climate was changing from subtropical to temperate. Volcanic rock is seen along the Mount Ashland road until the road dropping into the Colestin valley appears. The fault cuts through the gap, juxtaposing white volcanic ash on the south against granite across the gap.

Is the fault active? Probably not. A 27-million-year old ash deposit laps over the northeast end of the fault, marking the last time it moved. Although the dominant tectonic stresses of this region aren't favorably oriented for reactivation, like any old, broken, but healed bone, it remains a zone of weakness. However, rest assured: Any rumbling you might feel when traveling over the summit isn't caused by an earthquake. It's likely a flat tire.

Coming up: Jad D'Allura will talk about geology during a two-part hike-and-learn event Friday and Saturday, June 28-29, organized by Friends of Cascade-Siskiyou National Monument.

From 6 to 7 p.m. Friday, D'Allura will give a talk at Southern Oregon University, Science Building, Room 171, about the geology of the Hobart Bluff area of the monument. At 9 a.m. Saturday, D'Allura and Armand Rebischke, CSNM botanist, will lead a hike to Hobart Bluff. D'Allura will talk geology on the way out, the group will lunch on the bluff, and Rebischke will talk botany on the way back. The hike is limited to 25 people.

For reservations, email cascadesiskiyou@gmail.com.

Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at rockit@dishmail.net.