What's At Fault?
Imagine the earth moving under your feet.
The earth near Cress Creek has moved many times in the past several million years. These movements, or earthquakes, occurred along faults, which are fractures in the earth's crust. The ground shakes when two pieces of the earth move past each other along one of these faults. For example, the ground beneath your feet was once level with the Snake River, before movement uplifted this ridge and dropped the valley floor.
There are four faults near Cress Creek. Faults provide avenues for water to flow to the surface. The water is heated by the earth's interior and alters the rock from it's natural gray color to various shades of red. The bright colored rocks near this fault are examples of this alteration process.
The Heise Fault
A fault is a line deep within the earth along which movement takes place. Where you are standing now was once level with the Snake River before movement uplifted this ridge with its sand and river gravel. Movement along a fault may be horizontal or vertical. The fine soil is called loess (LOW-ess), a rich mixture of sand, silt and clay deposited by the wind. The loess was blown from the Snake River flood plain southwest of here. The farmlands that you passed on the way to Cress Creek have plenty of loess in their topsoil.
Some rocks in this area have been changed by hydrothermal action—or when hot water came into contact with the rocks. (Hydrothermal is really a pretty easy word—hydro means water and thermal means heat) Minerals in the hot water changed the rocks into shades of red, orange and yellow. From where did the hot water come? It came from faults in the earth, such as the Heise Fault.