Matilija Sandstone facts for kids
The Matilija Sandstone is a type of sedimentary rock found in Southern California. It formed during the Eocene epoch, which was about 50 million years ago. You can find this rock in Santa Barbara and Ventura Counties.
This rock is made of thick layers of sandstone, which is basically hardened sand. It contains tiny grains of minerals like feldspar and quartz. These grains came from older, harder rocks like granite. The Matilija Sandstone also has thin layers of siltstone and shale mixed in. It's very hard and strong, which is why it forms the tall, rocky peaks of the Santa Ynez Mountains near Santa Barbara and Ventura County.
Quick facts for kids Matilija SandstoneStratigraphic range: Eocene |
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Outcrop of the Matilija Sandstone in the Santa Ynez Mountains, California.
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| Type | sedimentary |
| Underlies | Cozy Dell Shale |
| Overlies | Juncal Formation |
| Thickness | 0–2,500 ft (0–762 m) |
| Lithology | |
| Primary | Sandstone |
| Location | |
| Region | Coastal southern California |
| Country |  United States |
| Type section | |
| Named by | T. Dibblee |
Contents
Where Can You Find This Rock?
The Matilija Sandstone was first studied and named near Matilija Hot Springs. This spot is along the Ventura River, about 10 miles (16 km) northwest of Ojai.
At this location, the rock layer is about 2,500 feet (760 m) thick. It includes sandstones that formed both in the ocean and on land. The sandstone layers are made of well-sorted quartz and feldspar grains.
You can see the Matilija Sandstone along the top of the Santa Ynez mountain range. It stretches from the western end of the mountains near Point Arguello all the way to Ojai. It also goes east into the Ventura County backcountry, where creeks like Piru and Sespe cut through it. This rock forms the highest points in the Santa Ynez range. It dips down under younger rock layers like the Coldwater Sandstone and Cozy Dell Shale near San Marcos Pass.
How Thick Is It?
The thickness of the Matilija Sandstone changes a lot. It's around 2,500 feet (760 m) thick where it was first named. Near La Cumbre Peak, it's about 2,000 feet (610 m) thick. It gets thinner to about 500 feet (150 m) under San Marcos Pass. But then it gets thicker again, reaching about 2,000 feet (610 m) at the high peaks of Santa Ynez and Broadcast Peaks. Further west, it thins out to less than 300 feet (91 m) near Point Conception.
Giant Boulders and Mudflows
The Matilija Sandstone is the source of the huge sandstone boulders you see along creeks and beaches in Santa Barbara and Goleta. You can also find these boulders on hillsides. These giant rocks, which are too big to be moved by today's floods, tumbled down the mountains during ancient storms and mudflows. These events happened during the Ice Age and were incredibly powerful. Matilija boulders were also carried in the mud and debris flows during the Montecito mudflows in January 2018.
What Grows on Matilija Sandstone?
When Matilija Sandstone breaks down, it doesn't create much soil. Because of this, only tough plants like chaparral usually grow on its slopes. Some higher, north-facing slopes might have a few pine and fir trees.
Under the Ocean
This rock formation also exists deep underground in oil fields offshore. This shows that the formation dips steeply under the Santa Barbara Channel. For example, at the Point Conception Oil Field, the Matilija Sandstone is at least 2,000 feet (610 m) thick and starts about 4,300 feet (1,300 m) below the surface. In another offshore gas field, it's found about 10,500 feet (3,200 m) below the seafloor!
How Did This Rock Form?
About 50 million years ago, during the early Eocene, the land that is now Santa Barbara County was covered by the sea. During this time, sand and mud settled on the ocean floor. The sea level kept changing, sometimes getting deeper and sometimes shallower. When it was shallow, sand was deposited, forming sandstone. When it was deeper, mud was deposited, forming shale. The Matilija Sandstone formed during one of the earliest long periods when the water was shallow, between 48.5 and 46 million years ago.
Something amazing also happened: the piece of Earth's crust where this sandstone formed has actually rotated! It spun about 90 degrees clockwise since it was created. It moved from a spot near where San Diego is today to its current location.
After reaching its present position, the entire Santa Ynez Mountain Range was pushed up. This happened mostly in the last few million years, due to huge forces from Earth's tectonic plates. The Pacific Plate and the North American Plate are slowly moving against each other, causing the land to crumple and rise. The Matilija Sandstone and other rock layers, which were once under the sea, were lifted more than 4,000 feet (1,200 m) from their original positions. In some places, this pushing was so strong that even young rock layers, formed only a few million years ago, are now tilted almost straight up!
Ancient Life in the Rock
While the Matilija Sandstone has many tiny fossils, it doesn't often contain larger ones. However, in the Santa Rosa Hills, scientists have found many shells of molluscs, including bivalves (like clams) and gastropods (like snails).
Why Is This Rock Important?
The Matilija Sandstone is important because it can hold oil and natural gas. Geologists group it with other nearby rock layers as a potential "play" for finding these resources. The oil and gas likely came from older, organic-rich shale layers found deeper underground.
Since this deep rock layer hasn't been fully explored, scientists are still guessing how much oil and gas it might contain. The area that could hold oil and gas is very large, covering about 1,500 square miles (3,900 km²) offshore. The biggest amount of oil and gas found so far in the Matilija Sandstone is in the Molino Offshore Gas Field, discovered in 1983.
