If we dusted off our time machine and traveled to the Bear Creek Valley 100 million years ago, we'd find ourselves bobbing like corks on a vast ocean. At that time, the Cretaceous period, ocean waters covered a third of earth's present land area. The ocean spread eastward from here to what is now western Utah. Offshore to our west, oceanic plates plunged fitfully beneath the Klamath-Siskiyou Mountains. They generated great earthquakes that lifted the land in sporadic jerks and jolts. Rapid erosion caused by this upheaval deposited gravel, sand, silt and clay in an ancient sea whose lithified remains are exposed at lower elevations in our valley and, along Interstate 5, arch nearly to the Siskiyou Summit. These rocks define the Hornbrook Formation, 1,200 meters of distinctive, greenish-gray sandstone and dark gray mudstone.
While treading water, we might see the old mashed and metamorphosed rocks of the Klamath-Siskiyou Mountains poking up above sea level like the head of a curious gopher. A few rivers that drained those mountains eroded gold, depositing it in cobble-strewn rivers and along gravel beaches. Later bipedal excavators afflicted by an acute and incurable malady known as gold fever would root like hedgehogs among those deposits.
An ancient beach is exposed in the steep road cut along I-5 south of Neil Creek. There the discerning eye can observe light-colored sandy and cobbly beach deposits lying directly on granite. I'd suggest not hammering on the cliff face unless you want to loosen an overhanging "poison rock" — so named because it takes only one drop and you're dead.
The ocean deepened as sea levels rose up to 200 meters higher than today, shrinking the land area. Great sheets of storm-generated sediments are seen in sandstone and mudstone that gently settled from the water as storm waves passed. These now-tilted rocks are exposed in patches along I-5 and the valley's southeast side.
Sea levels fluctuated, resulting in deposits of finer sediment alternating with an influx of more sandstone and conglomerate bearing white quartzite cobbles. The latter are seen as ridges southeast of Emigrant Lake and on the valley's southeast side. As the ocean deepened and the shoreline became far away, only mud and rare surges of thin, very fine sand could make it into the basin. This gray mudstone unit is not only thick but soft, easily allowing present-day Bear Creek and its tributaries to carve our beautiful valley.
Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at firstname.lastname@example.org.