Rainbow Basin features a variety of geologic items to study. It is located in the Mud Hills, northwest of Barstow. The canyon displays a number of sedimentary rock layers, and a number of geologic structures that warp the normally horizontal layers.

.You will notice that the rock layers are tilted in different directions depending on where you are. This contorting of rock layers is caused when the earth’s crust is compressed, usually due to movement along a fault. If the layers are dipping downward in a bowl-shape, it is a syncline. If the layers are slopping upward in a dome-shape, it is a anticline. The axis of either a syncline or anticline is an imaginary line through the middle of the high point (for anticlines) or low point (for synclines).

The fault responsible for most of the warping is the Calico fault system. This system is very much like the San Andreas fault system as it travels through the San Gabriel/San Bernardino Mountains. There is a bend in the fault that keeps the fault from moving sideways, so that movement along the bend is upward. This is what caused the Mud Hills to raise above the desert floor.

In sedimentary rock, the rock layers are horizontal as they form. Sedimentary rocks layers can form in a variety of environments. They can form on the bottom of oceans, in shallow lakes, in volcanic areas, or in deserts (to name a few). Sedimentary rock layers on the bottom of a series of sedimentary rocks are older than the rocks on top of them. Generally, where there is one sedimentary layer, there are a series of layers grouped together. If most of layers were formed in the same environment or around the same time, they are grouped together and called a formation. The sedimentary rocks layers in the canyon are part of the Barstow formation. Most of the rock is mudstone and sandstone that formed in a shallow lake that covered the region about 10 million years ago. Interspersed within these layers are several layers of extremely white rock. These were deposited from volcanic activity which spread ash throughout the region. In contrast to the slow growth of the lake layers, the ash layers that formed these white rocks were deposited in a very short period of time. Once the layer was deposited, a new layer of lake sediments began forming on top of it. The formation is topped by a rather recent layer of sand and gravel only 1 million years old. This leaves 9 million years between the two rock types. When a period of the geologic rock record is missing, this is called an unconformity. There are three types of unconformities: 1. Angular unconformity - The beds beneath lie at an angle to those at top. 2. Nonconformity - Sedimentary layers atop structureless igneous rocks. 3. Disconformity - A missing gap in the rock record, but hard to spot because the rock types are similar. Many times, only by comparing the age of the fossils in a layer can you tell if there is a disconformity.

Some of the mudstone displays cracks, telling geologists that the lake that this rock was formed in sometimes dried up, forming mud cracks that eventually hardened into rock.

You should notice the different colors of the rock layers. Red rock layers show the presence of iron in the rocks that have rusted by exposure to air and water, just like any iron object would that was exposed to the environment. The green color many times indicates the presence of copper, however the green rocks in this canyon come from clay minerals that form during the weathering of volcanic rocks.