ROCK SCULPTURE BY WEATHERING

We are now to consider a few of the forms into which rock masses
are carved by the weather.

BOWLDERS OF WEATHERING. In many quarries and outcrops we may see
that the blocks into which one or more of the uppermost layers
have been broken along their joints and bedding planes are no
longer angular, as are those of the layers below. The edges and
corners of these blocks have been worn away by the weather. Such
rounded cores, known as bowlders of weathering, are often left to
strew the surface.

DIFFERENTIAL WEATHERING. This term covers all cases in which a
rock mass weathers differently in different portions. Any weaker
spots or layers are etched out on the surface, leaving the more
resistant in relief. Thus massive limestones become pitted where
the weather drills out the weaker portions. In these pits, when
once they are formed, moisture gathers, a little soil collects,
vegetation takes root, and thus they are further enlarged until
the limestone may be deeply honeycombed.

On the sides of canyons, and elsewhere where the edges of strata
are exposed, the harder layers project as cliffs, while the softer
weather back to slopes covered with the talus of the harder layers
above them. It is convenient to call the former cliff makers and
the latter slope makers.

Differential weathering plays a large part in the sculpture of the