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While there are countless smaller layers visible throughout the park, there are three main
formations to know, which contain these smaller (unnamed) layers. The Lower Vasquez Formation is
the oldest, with geologists estimating an age of approximately 25.6 to 23.6 million years old, and a
thickness of approximately 1,300 meters or 4,200 feet. The Vasquez Formation lays on top of the Lower
Vasquez Formation. It is approximately 5,800 meters (or 19,000 feet) in thickness and is approximately
20 million years old. The newest formation visible in Vasquez Rocks is the Mint Canyon Formation,
which is approximately 1,550 meters, or 5,000 feet, in thickness, and 10-16 million years old.

After the deposited layers of sand and sediment were “lithified” (turned to stone), large amounts
of earthquake activity cause major disturbances to the previously flat stone. Due to the park’s location
along the Elkhorn Fault, an offshoot of the massive San Andreas Fault, earthquakes have been a very
stable occurrence for millions of years, where constant movement and pressure have led to extreme lift.
Typically, the rocks point toward the north, at angles ranging from around 40-52 degrees. It is estimated
by geologists that parts of the formation also go down to incredible depths: approximately 22,000 feet,
or around 4 miles.

These steep formations erode as they are exposed to the elements, and because the varying
layers of the sandstone are composed of alternative soft and hard layers, they erode at different rates.
This creates a layered effect of sometimes sharply angled rocks that rise prominently toward the sky.
These types of jagged formations are sometimes referred to as “hogs back ridges” due to their jagged,
sometimes vertebral-looking shape. As you walk around the park, take notice of these sharper rocks,
but also observe how those nearby may be a softer, more rounded shape- it all depends on the types of
weathering that each particular part has encountered and how susceptible they are to wearing down.

The Rocks range in size and angle severity, but for reference, the “main rock”, located in the
middle of the park and the most iconic of any parts of the formation, rises above the ground below over
150 feet. Though not the highest point in the park, it certainly demonstrates the incredible height and
angles that the formation features.

As with anything of such an ancient age, they have persevered but are always changing. Exposure
to the elements breaks and shapes the rocks, which complete its circle and eventually becomes sand
once again. Through erosion, the rocks become smaller as centuries pass, and they become rounded and
smoothed from wind and rain. It is interesting to think about what the rocks looked like 500, 5,000 and
5,000,000 years ago, and what they will look like in the distant future.

Useful Geology Terms & Photographic Examples

Whether you are studying this geology for a class, or simply interested in learning more about the park
around you, these geological terms can help you better understand the aspects of Vasquez’s Rocks
interesting geology.

Alluvial Fan: Sediment, including sand and rocks, that wash down from the mountains (in this case
the San Gabriel Mountains) and are deposited over plains or valleys. The word “fan” indicates that these
deposits are typically fan shaped, coming down from many avenues on a mountain/several mountains,
to a smaller, more narrow bottom point in the valley floor below.

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