The Sierra Nevada Files

It's easy enough to see rocks at the Earth's surface but how do geologists figure out what types of rocks are deep below - even hundreds of kilometers beneath the surface? This is an important question because critical geological processes, such as plate tectonics, happen deep within the crust (the upper layer of the Earth) and within the mantle (the layer beneath the crust). The main technique for examining Earth's architecture is called seismic tomography,  a method whereby seismic waves are used to probe the Earth's interior. At the most basic level, seismic waves travel slowly through warm and bouyant material but quickly through cold and dense material; therefore, by measuring the speed of these waves as they travel through Earth's interior, we can get important information about what's down there that would be, otherwise, impossible to obtain. Example of seismic wave velocities in the south Pacific. Blue indicates high velocities and red indicates low velo...

A paper written by one of my graduate students, Christina Tipp, has provided strong evidence that large swaths of the northern Sierra Nevada were completely buried by river sediments in the Eocene, ~30-40 million years ago. This study, published in the American Journal of Science , presents our first glimpse of the northern Sierran landscape in the Eocene and early Oligocene. Known as the 'auriferous gravels' or the 'Tertiary gravels,' these ancient fluvial sediments contained gold that was sought after by miners during California's Gold Rush. At the peak of the Gold Rush, the loosely cemented deposits were blasted by water cannons known as monitors and funneled into sluice boxes to recover the gold in a process called hydraulic mining. Hydraulic mining at the Malakoff Diggings, just north of the South Yuba River. The cliffs in the image are composed of ~40 million year old river deposits. Note people in the lower left of the image for scale. Photo credit: Universit...

I recently published a paper, in Geology , showing that canyons in the northern Sierra Nevada are much older than previously thought. Earlier work had concluded that the big rivers in the Sierra (e.g., the South Fork of the American River) had only begun cutting their canyons in the last ~5 million years; the youthful age of the canyons was used as evidence to support the hypothesis that the range had undergone uplift around the same time period. To test this idea, I used an old mining report, published soon after the Gold Rush by Waldemar Lindgren, to locate deposits of river sediment that date from the Eocene (~35-50 million years old). Because these ancient river sediments were the source of much of the gold, their precise locations were well-described. The picture below shows an Eocene river deposit near the North Yuba River. The Lindgren report describes many sites where these ancient gravels are only 100-300 meters above the bottom of the canyons. This means that incision o...