began forming 3,700 years ago. Sometime between 1540 and 1630 A.D., the creek was offset by about feet in an even larger earthquake. Two other older creek beds lie northwest along the San Andreas Fault. The first creek bed was created around 13,000 years ago when climate change formed the creek on a large active alluvial fan. The second bed was created about 11,000 years ago. The creek has been carefully studied by geologists to find a correlation between the offset and historical events, such as earthquakes, that have occurred along the San Andreas Fault. Although Wallace Creek is not the only creek that has been offset by the San Andreas Fault, it is the most spectacular. Access State Route 166 passes the south entrance to the Carrizo Plain, and State Route 58 crosses through the northern portion. Connecting them is the narrow Soda Lake Road, the only dependably passable road through the plain—but even this may become impassable during or soon after a rain since the middle portion of it is gravel. Geology San Andreas Fault The most prominent geologic feature of the Carrizo Plain is the San Andreas Fault, which runs along the northeast side of the plain, at the base of the Elkhorn Scarp. The section of the fault in the Carrizo Plain is the oldest section along the entire fault zone. Displacement on the San Andreas is responsible for the development of distinctive features, including shutter ridges, diverted or decapitated stream channels, and sag ponds. One feature relating to the San Andreas Fault and aligned to it in the Carrizo Plain is the Dragon's Back pressure ridge. Other faults The Big Spring Fault, the San Juan Fault, the Morales Fault, and the White Rock Fault are small faults that run parallel to the San Andreas Fault along the Caliente Range on the western boundary of the Carrizo Plain. Soil taxonomy The parent materials for soils in the Carrizo Plain are predominantly alluvium deposits. Alluvium is soil that has been deposited by rivers or flowing water. The Paso Robles formation is a Pleistocene aged alluvium deposit that reaches up to thick near the San Andreas fault and thins out towards the north and west. The Paso Robles formation is a well known aquifer that has been reliably productive for ground wells throughout the area. The upper layers of soil are more recent alluvium. This recent layer is