Lake terraces can be seen at the base of the Lomerias Muertas and above the San Benito River. This rich San Juan Valley soil was once the bottom of a Pleistocene lake. Photo by Jim Ostdick.
Lake terraces can be seen at the base of the Lomerias Muertas and above the San Benito River. This rich San Juan Valley soil was once the bottom of a Pleistocene lake. Photo by Jim Ostdick.

This article was contributed by retired Earth Science teacher Jim Ostdick.

The Pleistocene Epoch (2.6 million years ago to 11,700 years ago) is well-known for its Ice Ages and interglacial periods that produced large freshwater lakes in the western United States. Some of the most famous of these lakes are Salt Lake in Utah and Lake Lahontan in Nevada, but there used to be hundreds of them, now dry due to changes in climate and the shifting of Earth’s crust.

San Benito County was also a player in these processes. If you were standing at the current location of Fourth and San Benito Streets in Hollister (elevation: 289 feet above sea level) in late Pleistocene times, you would have needed scuba gear and plenty of air in your tank. Ancient Lake San Benito would have covered you in over 100 feet of water!

Pleistocene Lake San Benito extended for 30 miles from Morgan Hill in the north to Tres Pinos in the south, temporarily filling up the interior of the region to the 400-foot contour like a giant, plugged-up bathtub. It receded very slowly, leaving behind the smaller Lake San Juan at the 200-foot contour, which lasted approximately 50,000 years and deposited that rich San Juan Valley soil, for which we are eternally grateful.

And then what happened, you ask? And how did the lakes form in the first place? And what genius figured this whole thing out? Like a lot of things on the Central Coast, to get those answers we have to consult the work of legendary, prolific state geologist Olaf P. Jenkins, a longtime resident of Pacific Grove. Jenkins lived well into his 90s, published a treasure trove of papers and maps, and spent most of his career crawling around the state of California chronicling, classifying, and explaining this crazy, faulted, folded, 3-D jigsaw puzzle we call home. In 1973, Jenkins published an article in California Geology that untangled the mystery of the silty lake terraces that surround San Benito County and the fertile soil that spearheads our agricultural economy. I will summarize that article in the paragraphs below, but you can read the whole thing and learn more about Olaf by clicking on the attached PDFs at the bottom of this article.

Currently, the San Benito River drains 530 acres in San Benito County, from its headwaters southeast of Santa Rita Peak 109 miles northwest to its mouth at the Pajaro River north of San Juan Bautista. Joining the Pajaro River there, it flows through Pajaro Gap and into Monterey Bay at Zmudowski Beach by the Pajaro Dunes. For most of the year, the river appears dry, but beneath the surface, water follows its natural path toward the sea. All along the base of the Lomerias Muertas on the northern edge of San Juan Valley, a clearly visible line of green trees and vegetation marks the current location of the riverbed.

In the 20th century, while scouting locations to mine sand and gravel deposits, finer-grained lake deposits were found interspersed among the gravels in the bluffs along the river and in terraces around the edges of the valley. A blue clay marker bed was found atop river deposits in many well bores. From the location of this marker bed and the gravel beneath it, Jenkins and others deduced that the course of the San Benito River had changed over time. The river used to exit the valley through a gap that was located a half-mile southeast of San Juan Bautista. It drained into Elkhorn Slough and then into the Monterey Bay submarine canyon at Moss Landing. Movement along the San Andreas Fault caused massive landslides, which choked off the gap and halted the movement of water toward the sea. This caused river water to back up, forming Lake San Benito and leaving behind terraces at the 400-foot contour like bathtub rings surrounding the valley. These “bathtub rings” can be seen today as terraces above the San Benito River and in other places along the 400-foot contour.

Over time, with continued fault movement and constant erosion, a new gap formed at the current location we call Pajaro Gap near Aromas. Lake San Benito water drained down to the 200-foot level and remained for some 50,000 years, slowly depositing the silty lake bed that is now the farm land of San Juan Valley. The path of the San Benito River shifted north during this time, seeking its way to the bay and joining with the Pajaro River to cut the new gap even more.

Steep landslide scars can be seen today while driving along Highway 129 toward Chittendon and the San Andreas Fault, reminding us that the processes that acted in the past are still operating today. Will massive landslides ever close Pajaro Gap and refill Lake San Benito, you ask? I would not lose any sleep over it. They might try, but in my humble opinion, some TNT and a fleet of Caterpillars would give them a pretty good run for their money.

Somewhere in geo-heaven, Olaf P. Jenkins is having a good chuckle.