700 



A. H. PURDUE 



lower than that of the Boston Mountains. Its 

 streams are mature, the valleys comparatively wide, 

 and the topography in general presents the aspect 

 of much greater age than that of the Boston Moun- 

 tains. Professor C. F. Marbut, in discussing that 

 part of this region which lies in Missouri,^ claims 

 that it was base-leveled in early Tertiary times, and 

 the present cycle of erosion was instituted by an 

 elevation which dates from middle or late Tertiary 

 times. Be that as it may, the question as to whether 

 the region to the north of the Boston Mountains 

 ever suffered denudation to the extent of base-level- 

 ing does not particularly concern us here. The fact 

 of interest is that the denudation of the extensive 

 region to the north has been very great and the 

 topography is old, while that of the Boston Moun- 

 tains is limited and the topography young. 



It would appear that this difference in topogra- 

 phy cannot be attributed to the massive beds of sand- 

 stone at the top of the Boston Mountains, for these 

 same beds, while they have doubtless had a great 

 deal to do with preserving the region, formerly 

 extended over much if not all the denudated area to 

 the north. Besides, if we attribute the preservation 

 of these mountains to the character of the rocks com- 

 posing them, we are encountered by the question as 

 to why erosion has been so extensive to the north of 

 the region, removing the rocks over a large area, 

 leaving only here and there hills or circumdenuda- 

 tion, while in the southern part adjacent to the Arkan- 

 sas valley it has scarcely begun. 



I am able to account for the great difference in 

 the stages of erosion in the two regions only by con- 

 ceiving the Boston Mountain area to have been at a 

 lower elevation than the area to the north during 



^ Mo. Geol. Surv., Vol. X, pp. 27-29. 



