Pilot Rock, protruding like a hitchhiker's thumb on the Rogue Valley skyline, is our very own facsimile of Devil's Tower. Much of historical value has been written about this local landmark, yet its natural origin is little discussed.
Pilot Rock represents the remnants of magma (hot, liquid rock) that cooled in the throat of a small, 25.6-million-year-old volcano. The soft, long-gone bulk of the volcano and the harder rock the magma punched through are composed of volcanically fragmented and chaotically deposited debris of many different sizes.
Collectively, the poorly mixed material, resembling pebble-rich concrete, is known as "breccia" (pronounced brecha), a fine Italian word. Breccia is analogous to your grandmother's Christmas fruitcake — and is probably just as hard.
The crumbly nature of breccia is easily seen on the steep approaches to Pilot Rock, acting like ball bearings when walked on (so be careful). Breccia, being weaker than the solidified magma, eroded over time like a melting Popsicle, leaving behind the "stick" (Pilot Rock).
Quite noticeable are prominent vertical columns (columnar joints). While it might appear a gigantic bear scraped the sides of the rock to form such features, the natural creation of these columnar joints is equally intriguing.
When magma cools, it contracts (hence must occupy less volume), resulting in cracks that propagate up and down from the cooling surfaces that join to form four- to six-sided columns.
Note that the columns aren't vertical. They're inclined 20 degrees eastward, tilted during progressive uplift of the Klamath Mountains.
If you reach the Rock (I recommend a half-day hike), you'll notice it's speckled with small, dark-green crystals (augite) and some elongated black crystals (hornblende). These crystals are best seen through a magnifier (Š la Sherlock Holmes but without the Deerstalker hat). The hornblende is very unusual in local volcanic rocks, signifying an unusually water-rich magma. The easily scratched, (silky) white crystals found in small irregular masses in the rock or in cracks are zeolites, which in larger quantities are used in water softeners. Recall, however, that Pilot Rock occurs in the Cascades-Siskiyou National Monument, so collecting samples is verboten.
The top of Pilot Rock can be accessed along a moderately challenging crack leading to spectacular views in all directions. However, be careful. The ascent isn't recommended for people suffering a wee touch of vertigo — or for the very clumsy.
Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at firstname.lastname@example.org.