Pico Canyon Geology

Pico Canyon is the most westernly of all the canyons in the Newhall Oil District. The Pico Anticline is spectacularly exposed everywhere.

All of the productive Pico wells were drilled on the north flank of the anticline. The early oil men noticed what they called a "line of break". This is probably part of the axis of the anticline. No productive wells were found south of it. The "break" is visible from the hairpin curve if you look east at the exposed cliff (see photo below). Winterer and Durham (1962) also map a north-south fault through this area just west of the exposed "break". This fault could explain why the creek was able to cut perpendicularly through the anticline here. The fault would have weakened the rocks allowing water to erode through them.

As you drive down Pico Canyon Road and pass Stevenson Ranch Parkway you are already in the Pico Formation. When you park in the Mentryville pay parking lot you are still in the Pico Formation. On the geologic time scale (see chart on right), it is middle to late Pliocene. It was deposited in a shallow marine environment and consists mainly of mostly light gray siltstone and claystone, sandstone (sometimes pebbly), and conglomerate. The Pico Formation was first named in 1924 by W.S.W. Kew based on exposures in Pico Canyon, hence the name.

On the hike out of Mentryville, just past the bakery, you enter the marine Towsley Formation of early Pliocene age to late Miocene. This formation is also shallow marine and consists of mostly light gray to tan sandstone, medium grained to locally gritty and pebbly. There is also some claystone and siltstone. All the Pico wells were spudded in the Towsley Formation.

As you hike deeper into the canyon the Towsley formation rocks become older and older from north to south. The axis contains the upper Miocene Modelo Shale. Near the top of the road, just before you get to its end near the Odeen oil well marker, the shale is exposed. There are beds in the road with pieces broken off in the road.

View toward the west of the north flank of the Pico anticline showing the steeply dipping beds. The road is hidden at the bottom of the canyon. (2008)

After you hike out of Mentryville you will come to a gap where you first get a close-up look at beds of the Pico anticline. Here is a view of the right (north) side. Behind me is a bridge over the creek to the bakery trail. That trail was built in early 2009. (9/3/2023)

If you take the bakery trail and look closely, you may see these fossils. (1/26/2023)

Back on the main road and after you pass the gap, you will soon reach on old landslide on the left (south). It is easy to miss because it is now greatly covered with plant life. Photo taken in 2008, not sure when slide occured. (5/17/2008)

Before reaching Johnson Park, on the right (north) side of the road up the mountain side is this cave. (9/10/2014)

Zooming in and enhancing shows that there may be a vertical fault here. At the top of the cave you can see a triangular shape of rock which would indicate that the fault split into two sections and widened out. That caused the rock between the sections to fall out at the widest point creating the cave. Some of the above triangular rock may still fall out in the future. At least that's what I think (note - I'm not a geologist).

This looks like a nearly vertical fault on a curve on the right (north) side of the road before you get to Johnson Park. It's hard to see without the right sun angle. (2/21/2014)

First good view of north flank of the Pico anticline as you hike up the main road before Johnson Park. (4/27/2008)

After Johnson Park on the right (north) side just before the silver water tank are exposures of brown siltstone of the Towsley formation. (9/10/2014)

Zooming in shows the unique way they weather. This is a massive formation with little actual bedding, but with fractures. When it is exposed it breaks into blocks (like in the upper left). As it weathers, the sharp corners weather the fastest, leaving rounded shapes (like in the middle). This is a form of spheroidal weathering.

This bed of the Pico anticline (probably sandstone) high up on the right side of the road, weathered slower than the beds around it therefore it sticks out. It has also broken into pieces. (7/9/2013)

Looking closer, it almost looks like a man-made rock fence. (6/14/2008)

In this photo we are looking west from the road just before CSO 4 oil well site showing the overturned beds at the top of the ridges of the north flank of the anticline. (4/12/2008)

Nearly the same view from 1905 (From "The Santa Clara Valley, Puente Hills and Los Angeles Oil Districts, Southern California", George Eldridge and Ralph Arnold, United States Geological Survey, Bulletin No. 309, 1907)

Going up the creek bed south from the hairpin curve is this unusual feature. (1/21/2014)

View of PCO Hill to the east looking at the "break". This is at the hairpin curve opposite the CSO 4 well site. CSO 2 was located here. No productive wells were found to the right (south) of the break. You can see folding, faulting, and all sorts of contortions of the beds. (12/29/2013)

View of the break from further away. (8/21/2013)

Here is part of what is probably part of a pecten that I found in a pebbly sandstone in the Towsley Formation within view of CSO 4. There were a few other shell fragments in this same piece. Unfortunately, the pecten is too incomplete to be positively identified. (5/26/2008)

In the same bed as the pecten, I found this vertebrate bone fragment. Many bones have been found in the Towsley Formation in Elsmere Canyon, so it would not be unreasonable to find one in Pico Canyon. It is probably from a whale but, like the pecten, there is probably not enough material left for identification. (6/14/2008)

Some concretions fell out of the road cut here. A piece broke out of one. You can see more in the road cut that are still in place. Often the "seed" of a concretion is a fossil or fossils. Not so with these. (5/23/2023)

Here is some fossil charcoal in sandstone I found next to the road. The charcoal must have washed out to sea after a fire indicating that at least this part of the Towsley Formation was reasonable close to shore. (5/24/2008)

As you get higher up the road, there is a nice exposure of the north flank of the anticline. These beds were originally horizontal when they were deposited in a shallow sea. You could spend a lot of time studying this. (10/16/2014)

Closer view of these beds. This is the Towsley Formation. (5/17/2008)

Another view of the beds. (12/1/2014)

Still another view. (4/6/2013)

The beds on the right are alternating shale and sandstone. Suddenly, on the left, the beds change to a large unit of conglomerate. (5/24/2008)

Close-up showing satin spar gypsum beds probably formed in the shallow sea beds when deposition took place. A definition of Satin Spar (from various sources): "A form of gypsum and is classified as an evaporite because it forms when dissolved ions crystalize from evaporating bodies of water. An example would be in a shallow sea basin, lagoon, desert, hot spring, delta, or anywhere else that immobile water is evaporating from without replenishment, resulting in high concentrations of ions. The ions eventually crystalize into minerals either through precipitation or from all of the water evaporating leaving behind a thin layer of gypsum salt. Over millions of years, these thin salt layers can accumulate, building into thicker layers of gypsum. Over time these thin layers will turn into what are called beds and they will become buried resulting in compaction and exposure to high temperature where the beds undergo a process called lithification - where a stone is formed. Satin spar often occurs in seams, some of them quite long, and is often attached to a matrix or base rock." (11/27/2014)

I found these external mold fossils of bivalves in sandstone on the ground below the left part of the exposure shown above after some rain in March of 2021. The bottlecap shows their size. They are probably a species of lucina. It it hard to tell which bed they came from. It appeared to be high up the cliff face and petered out before it reached the bottom. (3/13/2021)

Heavy rains of January of 2023 caused more external mold fossils to fall from the cliff at the same location. (1/8/2023)

View of the left part of the roadcut. At the bottom of this face is where the fossils were found. (3/9/2008)

If you take the Hughes Canyon trail (between PCO Hill and Christian Hill to the east), which joins the main road around here, you will see more of the north flank at the bottom. These beds eroded in an interesting way. The sandstone beds have different degrees of hardness causing them to weather like this. This trail extends down to Johnson Park. There are many geologic features down this trail. (4/5/2008)

Same beds from farther away (4/5/2008)

Staying on the main road, something unusual is happening here. We are beginning to reach the axis of the anticline. The north flank will fold over and the slope of the beds will change direction further up the road. (3/18/2012)

Here is a fault where we start to hike in the south flank of the anticline. The fault runs diagonally through the photo crossing the road at my black backpack. (5/17/2008)

South flank beds as we close to the end of the road. (10/25/2022)

Concretion in south flank beds near last photo. (10/25/2022)

Finally, as we near oil well Odeen 1, we are in the south flank. This view is toward the east showing south flank beds. (5/15/2008)

At Odeen 1 marker. This photo was taken in 2008. Today the marker is hard to see because of all the plant growth there. (5/15/2008)