The Mint Canyon formation was formed by an ancient lake and delta during the Miocene Epoch. The climate was much more moist than it it today. During this time the lake gradually filled up with silts, boulders and other deposits. After it filled the deposits were compressed and were thrust up during the Coast Range Orogeny. Many of the surrounding mountains were formed in this same orogeny.
These photos show how the formation reacts to erosion. The first photo shows bluffs and buttes formed from the harder, more resistant sandstone and conglomerates of the Mint Canyon Formation. Sandstone create the cliffs while the siltstone and shale create the slopes.
The second photo shows close- up what wind and water erosion have done to these rocks.
The third photo shows how the formation reacts when wet shale underlies the resistant sandstone. A landslide occurs as a result of this mix. This is called a dip-slope slide.
In the Mint Canyon formation there are three facies. These are the Topset beds, Foreset beds and the Bottomset beds. They are formed from the delta that was once here.
There are indications of uplift and renewed erosion around here that most don’t notice. The off ramp at Sand Canyon Road off State 14 is located on a river terrace. A river terrace is an old level that the river was once at. Other indications are in the rock itself. Most of the terraces have either been destroyed by erosion or the building of the highway. Near where Soledad Canyon Road crosses the highway, there is a large cut in the hill, as shown below. The boundary between the Mint Canyon Formation and the terrace deposits is visible near the center of the picture where reddish beds meet the tan beds.
There are reddish, rocky beds that sit atop the lighter colored Mint Canyon Formation. These reddish beds show signs of water transport – rounded cobbles, a relatively flat bedding plane. They are the deposits of the ancestral Santa Clara River. These deposits have been uplifted to this level, left high and dry by a downcutting river. Once you know what to look for, it can be rather fascinating to see. Of interesting note, rhyolite clasts found within the Mint Canyon Formation have no local source. Their source is found in the Chocolate Mountains near Mecca, California. These two locations were offset by the San Andreas Fault.