POST-JURASSIC HISTORY OF THE NORTHERN APPALACHIANS 695 



nificance of gravels found on terraces. As Professor Johnson points out, the 

 mere presence of such gravel is in no sense a criterion of marine as opposed 

 to fluviatile origin. But when taken into connection with the size of pebble 

 and the relations of the terrace to the highlands behind, then such gravels 

 may become of significance. Bonney has shown that Alpine rivers of moderate 

 length carry pebbles from four to six inches in maximum diameter up to dis- 

 tances of twenty miles and more from the Alps, over the Piedmont. On these 

 Maryland terraces no such relations of mountains and grade existed as would 

 give the rivers such carrying power. The gravels were found in scattered 

 pebbles up to six inches in diameter on the highest remnants of a flat plain 

 facing the sea and not within valley walls. Neither is the general height of 

 the country behind such as could account for the presence of gravel of this 

 size. 



Professor Johnson speaks further of the possibility of fitting certain assumed 

 terraces to any part of an irregular profile. Again, as an abstract proposition 

 Professor Johnson makes a good point, but one which has been abundantly 

 guarded against in this investigation and which is ruled out by the evidence. 

 The terraces up to the 1,700-foot level are sufficiently well preserved over cer- 

 tain regions to give a markedly level sky-line, and these levels can be detected 

 at intervals on resistant formations along hundreds of miles of the Atlantic 

 slopes. The two higher terraces are less well preserved, and the conclusions 

 in regard to them rest therefore not so much on their internal evidence as 

 from the broader relations of these terraces, on the one hand, to the well- 

 preserved ones at a lower elevation on the seaward side and, on the other 

 hand, to tne different character of the topography on the side of the mountains, 

 especially the sharpness of that line diagonal to the structure which separates 

 the highest terrace from the still higher and mountainous uplands. 



Professor Johnson has spoken also of the distinction in form which should 

 show between monadnocks and rock-stacks. These distinctions are largely 

 features which would be lost long before the residuals had themselves disap- 

 peared. On the higher terraces the long exposure to subsequent subaerial 

 activities has smoothed down all slopes, whatever their origin, to such grades 

 as are in adjustment with the later history. 



Mr. M. R. Campbell has spoken of the different degree of warping which 

 would be implied in consequence of the acceptance of these terraces as sur- 

 faces of marine planation. It is true that if these be accepted, since they are 

 greater in number than the baselevels previously used, it would mean a re- 

 view of the correlation of the baselevels of the continental interior and would 

 probably require the assumption of less warping in a new interpretation. On 

 the interior slopes of the Appalachians the erosion has been presumably en- 

 tirely subaerial, but the higher levels were largely developed on softer forma- 

 tions than where they are preserved on the Atlantic slope, with the result that 

 subaerial peneplanation may there have been better developed, with the con- 

 sequences in drainage history which have been previously held. 



Mr. Darton has suggested that we do not know how much erosion has served 

 to lower any particular hill and consequently that there is doubt thrown on 

 the reconstruction of ancient terraces by the method of projected profiles. In 

 answer, it should be stated that such a hypothesis must, of course, he in the 

 geologist's mind in looking at every region, but that it may be readily tested 



