Posted 21 April 2016 by A.G. Sylvester ©
This part of US 101 lies in the Santa Maria basin, which, technically, is part of the Coast Ranges. The boundary between the north edge of the Transverse Ranges and the south edge of the Coast Ranges is approximately marked by the Santa Ynez River, which US 101 crosses at Buellton (exit 139).
The rocks you’ll see along the highway are very different in kind and age south and north of the Santa Ynez River (exit 139): Cretaceous marine shale and sandstone are south of the river; Pliocene and Pleistocene terrestrial sedimentary rocks predominate north of the river. Some geologists have mapped a major strand of the Santa Ynez fault along the south edge of the river to explain this great difference, but one geologist maintained that the contact is an unconformity now buried beneath the river sediments.
The Santa Ynez River once transported much sand to the sea. Longshore currents then conducted the sand around Points Arguello and Conception to Santa Barbara-Ventura beaches until the large water storage Bradbury dam was completed across the river in 1956. Two lesser dams upstream also trap some of the river water and sand, so that the river at Buellton is usually dry during the summer. As a consequence, Santa Barbara beaches are increasingly sand-starved.
Between Buellton and Palmer Road, US 101 follows the course of Zaca Creek through a broad anticline in the Purisima Hills consisting mostly Monterey Shale exposed here and there in stream cuts. Beyond the turnoff to CA 154 (exit 146), the highway makes a broad left bend to proceed WNW along the trough of the Los Alamos syncline all the way through Los Alamos (exit 154) to Palmer Road. The north limb of the syncline dips gently southward. Monterey Shale is overlain here and there by almost flat-lying, weakly consolidated, alluvial sediments deposited from streams draining the San Rafael Mountains on the eastern skyline. You may see some shale outcrops in abandoned railroad cuts a few miles north of the CA 154 turnoff, but otherwise, the rocks, such as they are, are covered with vineyards. The south limb of the syncline dips more steeply, but the rocks are obscured by vegetation.
From Clark Avenue (exit 164) to Betteravia Road (exit 169), US 101, surrounding houses, and premium agricultural fields lie on Holocene sand dune deposits. The sand has been transported to the sea by the Cuyama/Santa Maria River and then blown back ashore by strong westerly winds.
You cross from Santa Barbara County into San Luis Obispo County before the bridge over the Santa Maria River at PM 100. The cliff on the north side of the river exposes fluvial sandstone deposited by the Santa Maria River.
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Roadside Geology of Southern California 