The following is from Santa Barbara Day Hikes by Ray Ford
There are eight main rock structures underlying the Santa Ynez Mountains. From the oldest to youngest they are: Juncal Formation; Matilija Sandstone; Cozy Dell Shale; Coldwater Sandstone; Sespe Formation; Vaqueros Sandstone; Rincon Shale; and Monterey Shale. All are composed of sandstone, shale, or interbeds of shale and sandstone. A small amount of conglomerate is also found in the area.
The Juncal Formation is composed of alternating layers of sandstone and shale that are 4,000 to 5,000 feet thick. They are of the Eocene Age (58 to 36 million years ago). The shale predominates and weathers easily, forming rounded clay hills. The more rapid erosion of the shale interbeds leaves the sandstone jutting out as prominent ledges or ridges. The layers accumulated in a cold, deep sea which supported little marine life. Because the soils formed by the Juncal shales are of poor quality, they support little but brushy growth on steeper slopes that have a southern exposure. Near the crest, such as behind Montecito, the shale has weathered to rolling, rounded hilltops that have a grassy cover, a sharp contrast to the jagged Matilija formation which is just to the west. The Juncal Formation is prominent in the upper Santa Ynez Valley and is the major rock structure in the Red Rock area.
Matilija Sandstone is the thick, resistant layer of sandstone that forms the 3,985 foot high La Cumbre Peak. It is 2,000 feet thick at this point. This sandstone is grayish-white, weathers to a creamy buff color, and is extremely hard. This makes it highly resistant to erosion, and allows it to form the most rugged, craggy, and scenic strata found in the Santa Ynez Mountains. The sandstone was laid down in the later Eocene period and its origin is of granitic rock eroded from inland sources. After being washed into the ocean the granite was decomposed by underwater currents and spread out over the ocean floor as a uniform blanket of sand as the basin subsided. The Matilija formation doesn’t contain any fossils because the cold, inhospitable marine environment that emerged during the formation of the Juncal shales continued to prevail.
The upper part of Tunnel Trail passes through Matilija Sandstone and the upper end of Rattlesnake Canyon ends at the base of this formation. Just above the large meadow you can see a large wall of sandstone. This is Gibraltar Rock, a popular climbing area. The narrow, upper part of San Ysidro Canyon is also created by this sandstone.
COZY DELL SHALE
Formed in the upper Eocene, this formation is composed almost entirely of shale. Cozy Dell Shale is almost 1,700 feet thick and disintegrates readily into small fragments. This causes it to form markedly recessive topography, most graphically the deep saddles you can see in between the Matilija and Coldwater sandstones. It is dark gray and weathers to a brownish-gray or olive gray color. Cozy Dell Shale was deposited as a fine mud 35 to 40 million years ago when the Eocene sea reached its maximum depth. While the Coldwater and Matilija sandstones form spectacular peaks and cliffs, the Cozy Dell saddles have their own gentle grace. This shale is exposed in several areas, most notably along the connector trail leading from Rattlesnake Canyon to the Tunnel Trail and the saddle between Cathedral and La Cumbre peaks. The rolling, grass-covered knolls on the crest above San Antonio Creek are also composed of Cozy Dell Shale.
Coldwater Sandstone is the thickest of the marine sandstones found in the Santa Barbara area. Its resistant layers form the pyramid-shaped Mission Crags in the mountains directly above the Botanic Gardens. Averaging 2,700 feet in thickness, it is composed mostly gray-white sands which weather on the outside surfaces to a buff color. Coldwater Sandstone contains a composition of approximately 20% siltstone and shale which can be seen in between the much thicker sandstone layers. The main part of the layer is composed of granitic sands washed down into an Eocene sea during a period when geologic activity was causing the sea to retreat. Most likely, the sandstone was deposited when the Santa Barbara basin was nearly full. This shallow marine environment was most likely much more favorable to the development of life. The shallow, brackish seas fostered the growth of large beds of oysters, a fossil found frequently in the Coldwater Sandstone. Though not quite as resistant as Matilija Sandstone, it is extremely hard.
The sandstone forms the picturesque ledges, cliffs, and boulder fields found at Lizard’s Mouth and the Playground. Most of the rock exposed along the upper Jesusita Trail is Coldwater Sandstone, as are the formations in lower Cold Springs and San Ysidro canyons. Where it lies along the base of the Santa Ynez Mountains, Coldwater Sandstone forms beautiful narrow canyons that feature large pools and waterfalls. The most well known of these narrows is at Seven Falls in the west fork of Mission Canyon.
The Sespe Formation is composed of interbedded shales, sandstones, and conglomerates that total 3,000 feet in thickness. The rock is primarily reddish-brown or maroon due to the high content of iron oxide found in it. The Sespe Formation is the only non-marine layers of rock found in the Santa Barbara area. It accumulated on a nearly level plain as the sea became choked with sediment. Eventually the iron oxidized to become the rusty color it is today.
The Sespe Formation is found along the lower part of the foothills and comprises many of the rolling hills found in the Goleta area. Where there is a large percentage of clay in the strata, it weathers to a loamy soil which supports grassy slopes, many of which have avocado orchards on them. The red conglomerates are readily visible in the first few miles of Highway 154 above the San Antonio bridge crossing. The formation is also present along lower Jesusita Trail and there are outcroppings of it throughout the Santa Barbara and Montecito foothills.