Foree section of the Sheeprock unit.
The red and gold layer on top of the green occurred from an eruption 28.7 million years ago from a volcano over 80 miles away. It is up to 150 feet thick in some areas. Each of the light colored ledges below are ash or tuft, from individual eruptions. These can be dated with great accuracy, as they contain the mineral sanidine. Sanidine crystals contain a radioactive isotope of potassium. (K) which is formed as the lava cools. As time passes, it decays into argon (Ar) which becomes trapped in the crystals. As the decay rate of the unstable element is known and is constant & measurable, the age of the rock can be determined.
There are over sixty layers of basalt lava flows in the unit, some as deep as 52 feet.
This is a composite of several photos. Although it's reduced in resolution here, it's still best viewed by clicking on it. The formation in the first photo is found right of center in the panorama below.