LAND FORMS, THEIR DEFORMATION AND FORMATION 



47 



occur anywhere that a stream once cut through an 

 elevation as a water gap. 



Ahhough cuestas are common along old coastal 

 plains, they also can occur inland. There, cuestas 

 develop when sedimentary rocks of differential resis- 

 tance are tilted at a low angle. This low angle of tilt 

 distinguishes a cuesta from a related and similar- 

 appearing land form, a hogback. In addition, cuestas 

 have the general appearance of block mountains (see 

 below). 



INTERIOR PLAINS 



Inland level terrain may be the result of erosion to 

 a peneplain, elevation of an arm of the sea, drying of 

 a lake, flow of lava, or deposition of glacial till or 

 stream deposits at the base of a mountain. These 

 plains are called young if they have not developed 

 their stream patterns, and mature when a drainage 

 system has been established. Usually, the drainage 

 pattern is treelike. If the life cycle is typical, full 

 maturity is characterized by rolling low hills and 

 mature streams. 



PLATEAUS 



The life history of plateaus is in part a function of 

 climate (Figure 4.13). However, in either dry or wet 

 environments it starts from an essentially flat but 

 elevated plain that has permanent or temporary 

 streams. 



During the early stages of the plateau, erosion, 

 especially by streams, forms valleys in the old plain, 

 leaving plateaus as highlands above the valleys. In 

 moist climates, the valleys are V-shaped. The gentler 

 sloping walls are usually covered by soil, talus, or 

 other rock debris, so that the various rock strata are 

 obscured. This slope debris remains through most of 

 the life of the plateau, because greater moisture is 

 allied to denser vegetation, and the vegetation helps 

 to hold the debris over the bedrock. Also, the vegeta- 

 tion and slope debris provide protection from erosion 

 for the underlying strata. In arid regions, the greatly 

 reduced plant cover allows faster removal of erosion 

 products, hence more activity of weathering and re- 

 duction of land features by jointing. Plateaus in dry 

 areas are characterized by sheer walls having exposed 

 layers from which blocks of rock are removed by 

 jointing. 



Maturity occurs when only the peaks of mountains, 



erosion mountains, represent the old plain surface. 

 The peaks are a consequence of valleys forming a net- 

 work and leaving only mountain-shaped remnants of 

 the old landscape. Because the stream network is 

 constantly growing, both young and mature streams 

 must occur in a mature plateau. 



Mature plateaus in humid and arid climates dis- 

 play the same basic differences as young plateaus in 

 the same environments. In arid areas, erosion is 

 mainly by jointing and causes steep, steplike, valley 

 walls and small, flat, mountain summits. The much 

 restricted summit platforms represent the level of the 

 previous plain. In humid areas, the valley walls are 

 covered by debris and vegetation, but the summits are 

 usually distinct exfoliated domes. The domes may or 

 may not be covered by vegetation. 



Old age, a peneplain, occurs when there are only 

 remnants of the old plateau but never its original 

 surface. These remnants are most prominent in arid 

 areas where they form mesas, buttes, pinnacles, and 

 needle rocks scattered on the flat peneplain. Any 

 flat-surfaced elevations that might exist represent 

 layers that were far below the original plateau surface. 



During old age, or even before, a new cycle of 

 plateau life history can start from uplift or other 

 causes. When such rejuvenation occurs, a later stage 

 of life history may be seen superimposed upon an 

 earlier stage. 



DOME MOUNTAINS 



Circular, oval, or elongate isolated mountains can 

 form in many ways. For example, masses of salt can 

 gather and produce small, inconspicuous mounds; 

 liquid magma from beneath the earth's crust may rise 

 through a line of crustal weakness and accumulate 

 beneath the earth's surface in a small (laccolith) to 

 large (batholith) pool; and erosion can isolate 

 rounded masses from the surrounding terrain. 



When first formed, domes, if of sufficient size and 

 in a moist enough environment, develop a radial 

 drainage system. In youth, the headwaters of these 

 streams form a pocket in the summit of the dome. 

 This entire headwaters system produces a circle or 

 oval of infacing cliffs that is cut by the radiating 

 streams. In maturity, rings of such infacing cliffs, 

 called hogbacks, continue to develop within existing, 

 expanding rings. Also within individual rings, the 

 radiating streams give rise to tributaries that later 

 join in a closed ring. With the peneplain of old age. 



