In lofty mountain ranges there may not be a single valley without its traces of landslides, so common there is this method of the movement of waste, and of building to grade over-steepened slopes. Rock Sculpture By Weathering We are now to consider a few of the forms into which rock masses are carved by the weather. [Illustration: Fig. 13. Differential Weathering on a Monument, Colorado] =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. [Illustration: Fig. 14. Honeycombed Limestone, Iowa] [Illustration: Fig. 15. Cliffs and Slopes on North Wall of the Grand Canyon of the Colorado River, Arizona] 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_ (Fig. 15). Differential weathering plays a large part in the sculpture of the 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. [Illustration: Fig. 16. Taverlone Mesa, New Mexico] 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. [Illustration: Fig. 17. Monuments, Arizona