Structure and landforms

When a valley or canyon is cut into uplifted, flat-lying, sedimentary rocks, stepped valley sides appear as the steep, often vertical, outcrops of the more resistant sandstones and limestones separated by the less steep shale slopes. The classic example of this is the Grand Canyon of the Colorado River. The most common homoclinal structures are sequences of sedimentary rocks having a general dip in one direction. These structures are produced in two main ways, either by the uplift of a sequence of off-lapping coastal plain sediments or as part of one limb of a large dome or fold. A common mode of rock failure is by folding into a series of anticlines and synclines . Folding is most prevalent where stresses are long-continued and where high confining pressures permit rocks to bend and flow, rather than fracture. Intense compression, particularly close to the surface or subparallel to incompetent beds (e.g. shales and silts), is commonly accompanied by failure along low-angle thrusts.