The dendritic (tree-like) pattern shown in this image of the Rocky Mountains gives clues about the underlying geology and topography of the region. The linear features are stream channels, where precipitation has struck the ground and flowed downhill. If a small amount of precipitation strikes a level, somewhat porous surface, it tends to be absorbed into the ground and flows underneath the surface. If a larger amount of precipitation strikes the same type of area, it will tend to create pools on the surface, where it is lost to the atmosphere via evaporation. If the ground isn’t very porous or the ground is already sloped, precipitation will flow over the surface as runoff, carving stream channels out of the rock. The high density of stream channels seen here tells us that the underlying rock is very hard and that the slopes are very steep; in addition, the Rockies do in fact receive significant precipitation, about 36 centimeters (14 inches) per year (mostly as snow). The tree-like pattern is caused by the fact that the surface is fairly homogenous and the rock is weathering at the same rate throughout. If the ground consisted of several rock types, weathering would be uneven, forming a trellis pattern.
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