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Keep on rocking in the valley

Southern Oregon's landscape has been changing for millions of years

If this huge Bear Creek Valley had a theme song, it would be "Worlds in Collision."

Though it seems peaceful, sometimes pristine, the valley we live in has a checkered geological past and a potentially violent future. It's a melting pot, a big crossroads and meeting ground of many geological energies and traditions.

In short, a lot is going on here but in very slow motion. If you stand at valley bottom, say on the Bear Creek Greenway, you're looking at sprawling volcanic lava flows that created Grizzly Peak and Pompadour Butte to the east.

Look to the west and you're taking in old Pacific Ocean seabeds that came all the way here from New Zealand 160 million years ago to create Wagner and Anderson Buttes, said Emeritus Prof. Monty Elliot, now retired from Southern Oregon University.

Elliot presents "The Geology of the Rogue Valley," at 7 p.m. Wednesday at North Mountain Park Nature Center.

In fact, the whole Pacific Ocean floor is trying to get under our skin in a big way, pushing inland below Western Oregon and forcing granitic masses to the surface. That's what Ashland is built on - and it stretches to the Siskiyou crest. It's that crumbly, pebbly stuff you slide on when you hike the west hills trails.

To top it off, the valley floor is smoothed out by natural erosion that carved the surrounding hills. And it's split by a significant fault, the Siskiyou Summit Fault, that runs from near Mt. Ashland east to Interstate 5 (it runs right through Callahan's Lodge), then suddenly jogs north, running down the railroad tracks to Emigrant Lake, creating vents for many hot springs, such as Buckhorn Springs, Colestin and Lithia Springs - fount of the famed mineral-rich but dubious-smelling Lithia water.

The fault creates Siskiyou Pass, the natural opening to the south end of the valley, by slipping upward on its north and west side and downward on the rest.

This valley was once Pacific Ocean seabed, with the coastline

at the present Idaho-Oregon border. The Rogue Valley emerged about 200 million years ago, serving as a playground for dinosaurs of the Jurassic Era. Their bones and those of the much later megafauna, such as mammoths, have frequently been stumbled on here by road and home builders.

The defining energy of the region comes from the offshore San Juan de Fuca plate slamming into and pushing under the continent from Cape Mendocino to Vancounver Island. Part of the global system of shifting plate tectonics, its energies underlie the folding and crumpling of the basaltic Klamath mountains about 160 million years ago.

People often think Mt. Ashland is a volcano. It's not. Pressured by subduction of the coastal plate, it intruded upward into the Klamath Mountains about 150 million years ago.

What are the Klamath Mountains? They're just about everything you see off to the south and west of Ashland. There really are no such thing as the Siskiyou Mountains, said Elliot. That's just a word the early trappers and pioneers put on them. They're either Cascades or they're Klamaths.

About 100 million years ago, this area became submerged in the Pacific again, an event that laid down almost a mile of marine sediment here (called the Hornbrook formation) before it ended 65 million years ago. Since then, the valley uplifted and erosion has laid down another two-mile thick sediment, called the Payne Cliff formation.

Looking east from Talent, you see the Payne Cliffs themselves (named after a pioneer family that settled in their shadow) - remnants of ancient sandstone and gravel streambeds.

On Grizzly's lower slopes you see more remnants - Pompadour Butte, next to Dead Indian Memorial Road and that semi-circular, sandy-looking outcrop directly across the freeway from Ashland. Hiking the area, you can find an abundance of rounded river rocks.

Both the Hornbrook and Payne Cliff beds began tilting up toward the northeast 17 million to 12 million years ago and, mixed with later lava flows, now form our valley floor. This tilting is what cracked the Klamath Mountains, creating the Siskiyou Summit Fault.