5.1 Much of the landforms and landscapes we observe and admire are the result of fluvial erosion of regions underlain by sedimentary rock units, whether still flat-lying or now deformed. In this section we look at some of the common landscape features in such regions.
mesas and buttes. Many plateaus in arid or semiarid regions are surfaces capped by a particularly resistant horizontal sedimentary layer or sedimentary unit, underlain and overlain by softer, more easily erodible units. If the overlying weaker material is stripped away down to the level of the resistant unit, a plateau results. When fluvial erosion breaches the plateau and removes some of the weak rock underlying the resistant unit, broad areas of the plateau remain in partial or complete isolation from one another. Such a broad area is called a mesa (Figure 9-8). As the area of the mesa shrinks by wasting of the edges of the resistant layer and removal of more of the soft underlying material, the mesa becomes a butte (Figure 9-8).
Figure 9-8. Buttes and mesas. (From Lobeck, 1939)
cuestas and hogbacks. When a layer or unit of erosion-resistant rock, underlain and overlain by weaker rock, is gently dipping, fluvial erosion results in the resistant layer forming an asymmetrical ridge, called a cuesta, with a gentle slope parallel to the layer and a steep slope perpendicular to the layer (Figure 9- 9). The gentle slope is called a dip slope. If the dip of the resistant unit is greater, the ridge is more nearly symmetrical; in that case, the ridge is called a hogback (Figure 9-9). When the dip of the strata is even steeper, the dip slopes sometimes
Figure 9-9. Cuestas, hogbacks, and flatirons. (From Bloom, 1998)
Three of the most modern comprehensive treatments of geomorphology:
Allen, P.A., 1997, Earth Surface Processes. Blackwell Science, 404 p.
Bloom, A.L., 1998, Geomorphology; A Systematic Analysis of Late Cenozoic Landforms, Third Edition. Prentice Hall, 482 p. (especially Chapter 6)
Easterbrook, D.J., 1999, Surface Processes and Landforms, Second Edition. Prentice Hall, 546 p. (especially Chapter 15)
Two old books, with some outdated concepts but with superb illustrations (both photos and line drawings) of geomorphic features:
Lobeck, A.K., 1939, Geomorphology. McGraw-Hill, 731 p.
von Engeln, O.D., 1942, Geomorphology; Systematic and Regional. Macmillan, 655 p.
A classic textbook that covers a variety of geomorphic features and processes. Later editions are dumbed down.
Strahler, A.N., 1975, Physical Geography, Fourth Edition. Wiley, 643 p. + index and plates.
The classic book on fluvial geomorphology:
Leopold, L.B., Wolman, M.G., and Miller, J.P., 1964, Fluvial Processes in Geomorphology. Freeman, 522 p.