Regional Erosion

Slope erosion. Early theories suggested that mountains gradually eroded down to form plains over very long stretches of time—the sides and tops of the mountains were eroded to gentler slopes at similar rates, gradually losing elevation and approaching base level. Today it is more accepted that many slopes erode according to parallel retreat. Instead of eroding to gentler angles over time, slopes maintain their original steepness as they erode progressively back from river channels (Figure 1).

Figure 1

Parallel Retreat

The climate, rock type, and rock structures are the most important factors in slope erosion. As previously noted, rocks of different compositions weather at different rates—harder rocks form steeper slopes and generate coarser‐grained, more angular talus slopes that are resistant to weathering and erosion. Sand and clay slopes erode more easily and are less steep. Certain climates accelerate soil creep and chemical weathering, which result in more rounded topography. Rock structures such as folds, bedding planes, and faults can also affect the slopes that result from weathering.

Drainage patterns. A stream and its tributaries form geometrical arrangements called drainage patterns. A drainage pattern can be greatly influenced by the geologic formations through which it passes. A treelike dendritic pattern develops in a rock type that erodes uniformly, such as granite. A radial pattern, which resembles the spokes on a wheel, occurs when the streams originate on the flanks of conical mountains. A trellis pattern consists of parallel main streams with short tributaries on either side that form in areas of tilted sedimentary rocks that create parallel ridges and valleys. Bedrock that is regularly fractured or jointed in 90‐degree angles can create rectangular patterns, which have distinctive right‐angle bends.