The Northern Sierra Nevada is an Ancient Mountain Range

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 of the canyons began at least ~35-50 million years ago, not in the recent geologic past, and that there's been relatively little downcutting since. In the Google Earth image below, the yellow pin shows the location of one of these Eocene gravel deposits (yellow pin) in the canyon of the South Fork of the American River. The deposit is only ~160 m above the modern bed of the channel, indicating that this canyon was already quite deep ~35 million years ago.

The map below is from the paper. It shows the total amount of canyon incision, in meters, since the Eocene.

One implication of this finding is that the age of canyons cannot be used as evidence for recent uplift of the Sierra Nevada. Another implication is that the northern Sierra Nevada 35 million years ago would have looked similar to the modern range - in other words, it hasn't changed much. Although my study was focused on the northern Sierra Nevada because that's where these ancient river deposits are now found, Martha House and colleagues came to similar conclusions for the southern Sierra Nevada. For a 1998 study published in Nature, they used thermochronology (a method for measuring how fast bedrock is exposed to the Earth's surface by erosion) to investigate the paleo-topography of the southern Sierra Nevada and concluded that, by ~75 million years ago, the San Joaquin and Kings rivers had already cut deep canyons.