Differential Weathering And Erosion
As erosion progresses, by whatever means, some rock units will be more resistant than others. These will be left exposed at higher elevations, and may protect underlying rock. Depending on the orientation and shape of the resistant unit, and the agents of erosion acting on it, various landforms may develop.
Although easily attacked in humid regions, limestone is a resistant rock in arid areas. Its crystalline structure gives it strength, much like solid lava. Where nearly horizontal, both of these rock types are found capping, and protecting, softer rocks beneath them in much of the American southwest. When erosion breaches the resistant unit, it may cut down through the soft rock below very rapidly, leaving isolated islands of resistant cap rock. As this continues, the protection these cap rocks provide preserves tall, nearly vertical landforms called buttes, if they are small, or mesas if they are larger. Monument Valley, near the four corners of Utah, Arizona, New Mexico, and Colorado has dozens of spectacular examples.
If a resistant unit dips at a moderate angle, it will often hold up an asymmetric hill. A small one is called a hogback, and a bigger one, usually on the scale of counties, is a cuesta. Rivers, cutting down through such layers, leave a notch as they cut. If a river continues to flow through such a notch it is called a water gap; if not, it is called a wind gap. Delaware Water Gap; between New Jersey and Pennsylvania, is a classic example. Sometimes people get the wrong idea about how these form, figuring that the hill was there first, and then the river cut through it. Usually, however, the main river cuts the resistant rock and the softer rock on either side of it at the same time. Tributaries to the main river, however, erode the soft rocks down to near the elevation of the river, while the more resistant rocks remain higher on either side.
A resistant unit may also be nearly vertical, because sometimes it is formed that way. Molten rock can flow into vertical cracks deep beneath the surface, and then solidify into resistant igneous rock bodies called dikes. When surrounding rocks weather away, the resistant rocks can form vertical walls extending many miles. The Spanish Peaks of south central Colorado have classic swarms of such dikes dominating the topography.
Vertical walls can form from resistant sedimentary units which have been rotated into a vertical orientation. Rock climbers work out at Seneca Rocks, in West Virginia, the remains of a resistant sandstone unit which is now vertical.
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