The exposed bedrock of the parks consists of shales and siltstones of the Early Triassic Moenkpi and Late Triassic Chinle Formations. The mud and silt, when later buried and compacted, transformed into shale and siltstone. The shales erode into a barren landscape, highly gullied. You can see different color stripes due to the different color beds of strata. The different colors are due to varying amounts of iron oxide and other elements.
Iron oxide gives a rusty red color to sedimentary rocks.
Some of the beds contain large amounts of a special clay mineral called bentonite. Bentonite is swelling clay, in that it absorbs water and expands, and when it dries out it contracts. Bentonite forms by the chemical alteration of volcanic ash. Thus, the sediment composing some of the beds was composed of volcanic ash. These beds, in the Chinlee, contains the majority of the petrified forests.