Keeping one eye on the road, another on an unruly child and a third on the yellow-brown cliffs on the northeast side of the Rogue Valley, one might wonder how those cliffs originated.
Those prominent features east of Phoenix are the Payne Cliffs, and to the southeast they are the Van Dyke Cliffs, Pompadour Bluff and Songer Buttes.
This geologic unit, the Payne Cliffs Formation, is quite different — and younger (Eocene, 43 million years old) — than the marine Hornbrook Formation, the topic of my Aug. 10 column.
It is composed of pale-brown, non-marine sandstone and conglomerate (cobble-rich rock) deposited on the uplifted and intensely eroded Hornbrook Formation. The Payne Cliffs Formation extends from Sams Valley on the north to southwest of Buck Rock (southeast of Ashland), where it abruptly ends. The eastward-tilted formation, 2,300 meters thick, forms the middle reaches of the mountains northeast of the valley. These rocks represent extensive but shallow, sediment-choked streams flowing not only from the Siskiyou-Klamath Mountains but also from as far away as Idaho.
Idaho you say? Absolutely. Study of a distinctive mineral, muscovite — which is flat, shiny and pearly-white — indicates most didn't come from nearby sources, but they're the same age as those in Idaho's granitic rocks. Furthermore, those irritating white quartzite and metamorphosed volcanic cobbles littering the landscape on the valley's northeast side, many older than rocks of the Siskiyou-Klamath Mountains, are from Idaho, as well.
How did they get here?
During the Eocene, the imposing barrier of the Cascade volcanic range was just a twinkle in Mother Nature's eyes. As sea levels dropped, a very gentle westward-inclined slope extending from Idaho to the Siskiyou-Klamath Mountains was exposed. During a period of intense erosion, the unimpeded flow of energetic rivers carried an astounding amount of debris and spread the deposits over the landscape. Today's analogy would be numerous wide, shallow Yukon Rivers criss-crossing the countryside to bring pieces of wood (now petrified), carbonized leaf fragments and the occasional unfortunate animal into our area. Volcanic rocks mixed with the river deposits in the youngest part of the formation.
Most exposures are on private land but can be investigated on the northwest side of Emigrant Lake and along Highway 66 south of the lake. Also seen are cross beds. No, they aren't angry rocks, but unlike the parallel-layered marine Hornbrook Formation rocks, these were deposited by streams cutting into previous sediment at an angle, producing features similar to, but smaller than, those observed in Zion National Park.
Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at firstname.lastname@example.org.