Southark Cartoon
Sisters-in-law Amy Frykholm (left) and Stepahi Frykholm.
Salida Hospital in the 1920s from Salida Regional Library Local History Archives.
Figure 1: Side view of normal fault.
Figure 2: Side view of reverse fault.
Figure 3: Plan view of surface of strike-slip fault.
Figure 4: Dipslip fault vein displaced down and laterally.
Figure 5: Side view of thrust fault.
Figure 6: Rio Grande Rift cross-section, not to scale.
This picture was taken just west of Poncha Springs. The flat surfaces of the low hill in the center foreground which slope to the right (west) are the caliche-supported surface discussed in the text. At some time in the last 150,000 years, the caliche formed beneath a flat surface, probably very similar to the floor of the San Luis Valley today. That surface was then displaced by a series of curvilinear faults that strike away from the camera, forming the wooded gullies. This displacement is normal, with each structure down on the left (east) and up on the right (west). The resulting parallel ridges can be clearly seen when driving south on U.S. 285 just past the Chaffee County Landfill. One's first impression is of large fossil sand dunes, but that is not the actual situation. Green's Creek, which also follows a fault, enters from the right. At the base of the slope is a portion of the major dip-slip fault that appears to be creeping, with the hill moving west and the camera position moving east.
A fault usually grinds the rocks on each side of it to bits. Thus faults are easily eroded and become swales, gullies, or similar linear depressions. Here, hot water that carried minerals (probably the result of igneous activity) invaded and poured through the fault area. The resulting structure was more resistant to erosion than the surrounding rocks, and they have been carried away. The result is this silcified fault breccia sticking out of the ground. This is part of the Wellsville Anticline Structure east of Salida.
This photo from near the mouth of Castle Creek east of Salida shows a reverse fault which has uplifted the rocks on the right. Notice that the beds on the left dip to the left. The plane of the fault strikes obliquely from left to right, and dips very steeply to the right. Displacement is about 14 feet.