Differential weathering of limestone and chert, San Rafael Valley, Utah

This rock has a weathered form that is not atypical of weathering and erosion in the high desert regions of southern Utah. The slow ‘etching’ of the rock over time has the effect of accentuating differential resistance to erosion. The limestone (light grey) gives way much faster than the chert layers (orange). The tendency of the chert to fracture through jointing is quite apparent, in strong contrast to more subtle deformation mechanisms in limestone at this scale of observation.

I collected this in the mid-1990s on the San Rafael Swell in southern Utah, within the Colorado Plateau.  I like the contrasts, in resistance, texture, and hue.  I like the fact that it is a miniature example of erosion of landscapes.  I have found that this rock particularly grabs the attention of non-geologists.

Age & Formation
Jurassic Carmel Formation, ~170 Ma.