land. Areas of weak rock are wasted to plains, while areas of hard
rock adjacent are still left as hills and mountain ridges, as in
the valleys and mountains of eastern Pennsylvania. But in such
instances the lowering of the surface of the weaker rock is also
due to the wear of streams, and especially to the removal by them
from the land of the waste which covers and protects the rocks
beneath.

Rocks owe their weakness to several different causes. Some, such
as beds of loose sand, are soft and easily worn by rains; some, as
limestone and gypsum for example, are soluble. Even hard insoluble
rocks are weak under the attack of the weather when they are
closely divided by joints and bedding planes and are thus readily
broken up into blocks by mechanical agencies.

OUTLIERS AND MONUMENTS. As cliffs retreat under the attack of the
weather, portions are left behind where the rock is more resistant
or where the attack for any reason is less severe. Such remnant
masses, if large, are known as outliers. When

Note the rain furrows on the slope at the foot of the monuments.
In the foreground are seen fragments of petrified trunks of trees,
composed of silica and extremely resistant to the weather. On the
removal of the rock layers in which these fragments were imbedded
they are left to strew the surface in the same way as are the
residual flints of southern Missouri. flat-topped, because of the
protection of a resistant horizontal capping layer, they are
termed mesas,--a term applied also to the flat-topped portions of
dissected plateaus (Fig. 129). Retreating cliffs may fall back a
number of miles behind their outliers before the latter are