4Q 



PHYSICAL GEOLOGY 



surface exposed, they are 

 more vigorously acted upon. 

 Such places, too, encounter 

 water from two or more 

 directions and are more 

 likely to be affected by the 

 strongest solutions. The 

 greater weathering of the 

 edges, and especially of the 

 corners, causes them to dis- 

 integrate more rapidly, leav- 

 ing a spheroidal core of un- 

 weathered rock, embedded in 

 less compact, weathered rock. 

 When the rock is exposed to 

 the action of wind and water, 

 the unaltered, spheroidal core 

 is exposed (Figs. 15, 16). 



Differential Weathering. — 

 When rocks are not uniform 

 in character but are softer or 

 more soluble in some places than in others, an uneven surface may 

 be developed (Figs. 17, 18, 19) ; in deserts by the action of the wind 

 and in moist regions by 

 solution. Columns of 

 rock which have been iso- 

 lated in any way show 

 the effect of differential 

 weathering. In arid re- 

 gions the lower parts of 

 the columns (Fig. 20) are 

 worn away more rapidly 

 than the upper parts, be- 

 cause the drifting sand is 

 more abundant and effec- 

 tive near the ground, and 

 the bases grow smaller 



and smaller until the 



r- ,, , Fig. 17. — A bowlder showing differential 



monuments hnally topple weathering. The projecting portions are relatively 



QVer/i insoluble silica, while the main portion is limestone, 



Fig. 16. — Granite weathering under tropi- 

 cal conditions. Rhodes' Grave, southern 

 Rhodesia. The bowlders are residual frag- 

 ments of a sheet of granite that once overlay 

 the hill. (Photo. G. A. J. Cole.) 



