760 .1. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



head, whereas the bare and yteep outcrops have superior resistance. In. 

 humid climates, consequently, areas of limestone advance to the peneplain 

 stage with great rapidity. In mountainous regions with semi-arid cli- 

 mate, on the contrary, they constitute formations nmch more resistant 

 than those of shale, standing out above them as more or less prominent 

 ridge-makers. 



Let attention be given next to the nature of ground-water action under 

 a peneplain. The conclusion would appear to be opposite to that which 

 has often been, advanced. It has been frequently postulated that a pene- 

 plain would become mantled with a very deep residual oxidized soil, and 

 that on uplift this disintegrated cover would be rapidly removed and 

 swept into depressions. The red color of the Triassic of the eastern 

 United States has been explained in this way by I. C. Russell and B. Willis 

 and it has been applied to the Potomac deposits resting on a Jurassic 

 peneplain. It seems clear, however, that the very thickness and coarse- 

 ness of these deposits and the great abundance of fresh feldspar in the 

 Triassic show that erosion went on with uplift and that the material was 

 not greatly weathered. Furthermore, beneath a typical peneplain there 

 should be iDractically no ground-water circulation. The pore s]mce of the 

 ground Avill be well filled with Avater, but there will be no head to give it 

 circulation, and as soon as the water becomes saturated all further solu- 

 tion stops. 



Peneplains, then, should possess a relatively thin regolith. The con- 

 clusion is of some importance in the explanation of the fresh and clean 

 rock surfaces below disconformities. These planes of separation extend 

 over great areas between the strata of different periods. They represent 

 land intervals, yet when the sea returns the new deposits almost invariably 

 rest on fresh rock. Some reworking of a regolith by marine abrasion 

 doubtless occurs, but the absence of a heavy debris as a bottom sandstone 

 and shale indicates in many cases that this is not important. 



All rules have their exceptions, and the truth or error of this general 

 principle, based on the laws of hydraulics, should not be regarded as 

 tested by an individual case. The preceding discussion has assumed a 

 homogeneous rock-floor, and this in many examples not the fact. First, 

 such a formation as the Dakota sandstone leads ground water to great 

 depths below baselevel, and even in advanced stages of the erosion cycle 

 enough head would still exist to give a flow, though feebler than in a 

 stage of higher relief. Second, waters sinking into residual uplands would 

 in places find their way far out under peneplains before rising to the sur- 

 face. Third, many of the irregular deeper parts of the weathered belt, 

 decomposed during the vigorous circulation of the mature stage of the 



