80 PHYSIOGRAPHY 



phases of weathering wear it away less rapidly than they wear 

 weaker rock. The result is that resistant rock remains as hills, 

 or even as mountains (called monadnocks), when the weaker rock 

 about it has been removed by erosion. Fig. 1, PI. XXI, p. 84 is 

 an example. In the West, similar elevations are often called buttes 

 (Fig. 79). If such an elevation has some expanse of surface at its 

 top, it is a mesa (Fig. 80). This term is applied to wide terraces, 



Fig. 80. Lime Mesa, southwestern Colorado. The mesa is the flat-topped 

 area near the middle of the picture. 



especially if high. If the hard rock which gives rise to an elevation 

 is a tilted bed of rock, the resulting elevation is long and narrow, 

 and is often called a hogback (Fig. 81 and PL XX, p. 81 ). 



Accidents to Streams 



Drowning. Streams are subject to many accidents. If the 

 land through which they flow sinks, as it sometimes does, they flow 

 less rapidly, or may even cease to flow altogether. If the lower 

 end of a valley sinks below sea-level, the sea-water enters and forms 

 a bay, drowning the lower end of the river and its valley. If the 

 streams along a coast end in bays, we infer that the coast has sunk, 

 and that its rivers and valleys have been drowned. Thus Delaware 

 Bay, Chesapeake Bay, and numerous other smaller bays between 



