The durable volcanic neck protrudes high above the surrounding terrain. A glance at the chapter-opening photo of Joshua Tree National Park shows an additional example of this phenomenon, called differential weathering.
The results vary in scale from the rough, uneven surface of the marble headstone in to the boldly sculpted exposures of bedrock in New Mexico’s Bisti Badlands (Figure 2).
Differential weathering and subsequent erosion are responsible for creating many unusual, often spectacular rock formations and landforms. Many factors influence the different rates of rock weathering. Among the most important are variations in rock composition. More resistant rock protrudes as ridges or pinnacles, or as steeper cliffs on an irregular hillside (see Figure 3).
The number and spacing of joints can also be a significant factor (see Figure 4).