Examples of basic principles of geology exposed near San Ysidro, NM

Nicolas Steno (1638–1686) recognised these four basic principles of geology:

A: Original horizontality — most layered rocks were deposited as horizontal deposits

B: Lateral continuity — these horizontal layered rocks often extended for great distances

C: Superposition — younger rocks are overlain on older rocks

D: Cross cutting relationships — can be used to determine which rock unit is younger

All of these relationships are evident in this photograph from the east limb of the San Ysidro Anticline. The white layer is the Cretaceous Dakota Formation. The horizontal sediments capping this angular unconformity (orange line) are part of the Sante Fe Group. For more information on the geologic history, stratigraphy, and structure here, check out our virtual geologic tour of the San Ysidro area.

To get to the San Ysidro Anticline, turn west on Cabezon road just south of San Ysidro, NM.  There is a parking area for the White Ridge Bike Trails near the nose of the anticline.