Differencial Erosion

Differencial erosion occurs whenever and at whatever level
rocks are exposed to erosion. Mountains and valleys are
examples but it can also be manifest in hand sample.

The first example is from a dry creek bed in East Hempfield, PA,
the second is from a beach on the east side of Horsetooth reservoir
near Fort Collins, CO.

Differential erosion does not depend on overt differences but occurs where there are no obvious differences in a sample as in this marble from a lake in the Alps