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Exploring the rocks of Roxy Ann Peak

Jump-start your imagination to envision the spaceship that Michael Rennie used ("The Day the Earth Stood Still," 1951-movie version) when deciding whether humans should be exterminated. Yes, it's Roxy Ann Peak in Prescott Park northeast of Medford.

The steep peak is the command module, while the less-steep saucer shape spreads out on either side.

Roxy Ann Peak is an intrusion (a geological "uninvited guest") that punched through older rocks on its way toward the surface. There is no evidence it actually made it to the surface.

How do we know it's an intrusion and not a lava flow? It's younger (30.8 million years old) than rocks around it (42.6 million years old) and, unlike lava flows, is massive, producing no extensive ledges.

Automobile traffic is prohibited in the park, and it will take two to two-and-a-half hours to walk the entire circuit. Look across the valley for wonderful views of the older Siskiyou Mountains. There's a fine view of the Table Rocks from the North Roxy overlook. Very few restrooms exist, so don't drink too much coffee. And for goodness sakes, don't wander into the poison oak!

On your hike you'll pass through several different rock types, starting with relatively open, gentle slopes and an orange-hued, sandy, clay soil developed from easily-weathered volcanic sandstone and claystone. The clay content can be tested by wetting the soil and squeezing it into a ball. If it forms a sticky ball, the clay content is very high. This soil, composed of shrink-swell clays, is conducive to landslides.

Up the road, more rock debris, crumbling from overlying lava flows to roll downhill, is mixed with soil. Harder, dark-gray lava flows form subtle benches populated by rocky debris and deep-brown soil.

A lava flow overlying volcanic pebbly sandstone can be seen in a road cut just to the west of the quarry on the southeast side of the park. Rounding that corner is a treeless slope inclined toward the east.

That inclination is due to tilting of the lava bench as the Siskiyou Mountains rose. The steeper-sloped intrusive rock is best seen along the road east of the peak.

If you wake up one day and don't see Roxy Ann Peak, let's just hope Michael Rennie puts in a good word for us. Klatoo, verada, nicktoe "…

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