LAND FORMS, THEIR DEFORMATION AND FORMATION 



55 



and estuaries), glacial troughs (fiords), and elevations 

 (islands, peninsulas, and headlands). Underwater 

 topography is irregular, and shoals and banks are 

 usually present. 



Initial wave action works to straighten this shore- 

 line. Waves start cutting off the points of land and 

 depositing detached materials in more sheltered bays 

 or similar areas. 



Youfh. The points of land are subject to some ir- 

 regular cutting which often leaves isolated elevations 

 (chimneys or stacks) in front of the wave-cut clifTs. 

 The cliff's may be excavated to form clefts, sea caves, 

 and arches. However, the general tendency of erosion 

 is continually to wear away the projecting land in a 

 direction parallel to the over-all straight trend of 

 the shoreline. 



The products of erosion are deposited in the bays 

 and any other indentations of the shoreline. These 

 deposits frequently accumulate near the mouths of 

 these bays. Here, they grow from the shore, forming 

 first spits, and then bars as the sediments almost 

 close a bay. 



Submafurity. This additional stage is recognized 

 when headlands are cut back and the bars almost 

 completely close off the bays. However, many bays 

 may already be blocked by bars and isolated from the 

 sea; others may even be filled with land and bar sedi- 

 ments, and so actually be part of the land. 



Mafuriiy. A mature shoreline, after submergence, 

 is no different from one after emergence. The mature 

 shoreline after submergence fills out to the bars and 

 the typical underwater profile develops. 



COMPOUND SHORELINES 



Most shorelines are not as simple as previous con- 

 siderations would imply. In most areas, both sub- 

 mergence and emergence are superimposed on a 

 single coastline. Such complexes are called com- 

 pound shorelines. As one might suspect, their life 

 history is dependent upon the particular sequence of 

 past emergence and submergence. 



WIND 



The wind was considered as a phenomenon of the 

 atmosphere. Its erosive action is mainly from sand 

 and any other carried particles that mold the land 

 forms typical of arid landscapes. Wind deposits soil 

 and sand in heaps called diinei and finer dust in wide- 



spread, more or less evenly distributed deposits. 

 Wind also excavates the landscape and forms caves, 

 arches, blow holes, broad plains, and wind-scoured 

 valleys. This excavation action also leaves residual 

 "pockmarked" rocks, pedestals, mushroom rocks, 

 table rocks, arches, and undercut cliffs. Wind even 

 affects the particles it carries or moves. For example, 

 sand grains are cracked, pitted, and chipped to the 

 point of having a frosted appearance, and rocks tend 

 to become beveled. Therefore, wind erosion is from 

 the blowing away of materials, and from the sand- 

 blasting action of windblown particles. 



Wind can cause extensive economic damage. For 

 example, it can cut telephone wires, pit glass, and 

 remove paint. Also, a violent, heavily laden dust 

 storm can ruin agricultural and other lands and even 

 be a hazard to human life. 



DUNE FORMATION 



Wind blowing over level sand will cause an oval 

 sand pile to accumulate. As a symmetrical oval is 

 formed, there is a progressive decrease in velocity on 

 the pile side away from the wind, the leeward side. 

 This leeward velocity decrease soon stops sand from 

 being carried beyond the crest of the dune, and even- 

 tually the dune has a gradual windward slope and 

 steep leeward slope. The process of dune formation 

 also causes dune migration inland, because formation 

 and further activity consist of sand being blown up 

 the windward slope and depositing at the summit or 

 just beyond on the leeward slope. 



Wind direction and velocity are responsible for the 

 variety of dunes (Figure 4.18). Parallel ridges that 

 may or may not be interconnected are oriented either 

 across or with the direction of prevailing winds. A 

 second type, individual, arc-shaped or crescent- 

 shaped barchanes form in many arid areas. They occur 

 where prevailing winds are from one direction and are 

 oriented so that the arc is centered on the wind direc- 

 tion and the open ends of the arc are pointed leeward. 

 A third type, seijs, are half arcs that form where two 

 prevailing wind directions exist. 



DESERTS 



In dry climates, wind erosion is sometimes a promi- 

 nent factor in the sculpturing of the landscape (Fig- 

 ure 4.19). This is so in spite of the fact that rainfall 

 usually comes as cloudbursts and leaves its mark in 



