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ROCKS SPEAK Over hill and dale along I-5 Comment By Jad D'Allura for the Mail Tribune MailTribune.com By Jad D'Allura for the Mail Tribune Posted Dec. 5, 2014 at 2:00 AM Updated Dec 5, 2014 at 2:53 PM By Jad D'Allura for the Mail Tribune Posted Dec. 5, 2014 at 2:00 AM Updated Dec 5, 2014 at 2:53 PM » Social News One doesn’t have to be bored while traveling along I-5 north from Medford. There’s a lot of geology to see spicing up the most jaded traveler’s trip.  Keep an eye out on the changing landscape and road cuts — all but the driver of course, else you end up as a hood ornament on a Peterbilt truck. As you leave our valley, the canyon walls steepen and the terrain roughens. You’re entering the 173- to 136 million-year-old hard rock of the Klamath Mountains seen from here to just south of Roseburg. The Klamaths consist of old oceanic sediment, volcanic and granitic rock, and serpentine that are exotic immigrants to North America. All were swept in on tectonic plates and mashed onto the continent like bugs against a windshield.  Those collisions caused intense heat and pressure to metamorphose and harden softer rocks. Look for greenish rocks that were once volcanic lava and moderate to dark gray rocks that were sandstone or shale deposited in long-vanished oceans. All have been squeezed like a cheap accordion during “docking” (collision). Many are twisted from horizontal to vertical orientations and torn along massive faults. Imagine time-lapse photography showing these exotic land masses chugging toward us only to become part of North America during successive collisions like cars during a slow-motion pileup.  Just past exit 66, the road climbs a steep grade toward Sexton Summit. Trees are more widely spaced than elsewhere. That’s because they are growing on less-nutritious serpentine soil. Serpentine, metamorphosed mantle rock twisted to the surface during collision, is exposed as shiny green rocks in the freeway cut. South of Roseburg (exit 119), the topography gentles as we enter the Siletzia terrane. It’s a triangular-shaped block of oceanic crust with its apex pointing to the east and its largest section extending from here to the southern part of Vancouver Island!  It “docked” against North America about 50 million years ago. It, an LIP (Large Igneous Province), was much thicker than most oceanic crust and failed to “subduct” (go beneath) North America.  It got stuck in the subduction zone like a lie in a politician’s throat. Stress caused the crust to break to the west forming the Cascadia Subduction Zone, jamming Siletzia’s wedge into our continent. The dark, iron-rich lava of this terrane erupted beneath the cold ocean, where it cooled rapidly into blobby shapes that resemble large “pillows”. These pillows, looking like clusters of huge black grapes, are visible on the steep road cut at Roseburg. These pillows, however, are notoriously difficult to fluff. Docking caused crumpling of the crust to produce large undulations (folds) in overlying dark-colored marine shale and thin sandstone layers. Those can best be seen climbing the grade just past Sutherlin (exit 135). The layered rock slowly changes orientation inclined downhill (south) at the base of the grade to crest at its top, then on the downgrade dip to the north. Reminiscent of a gentle rollercoaster ride, the rolling hills and valleys near here are manifestations of these folded rock layers.  Traveling closer to Eugene, note that younger sandstone layers become thicker. That’s because sediment eroded from uplifted Klamath Mountains and the east began to cover the old Siletzia oceanic crust, finally creating the fertile Willamette Valley. Hopefully, that once-tiresome drive to the north just became a little more interesting. If so, please direct your comments to [email protected]. Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. By Jad D'Allura for the Mail Tribune MailTribune.com By Jad D'Allura for the Mail Tribune Posted Dec. 5, 2014 at 2:00 AM Updated Dec 5, 2014 at 2:53 PM » Comment or view comments Reader Reaction » STAY INFORMED Email NewsLetter Sign Up Today Sign up for our newsletter and have the top headlines from your community delivered right to your inbox. 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