PHYSIOGRAPHY OF THE BOSTON MOUNTAINS 697 



the original water divide of the plateau, making the water 

 divide as it now exists, a very zigzag line. In the western part 

 of the state, the south-flowing streams are the stronger, and as a 

 rule are robbing the White River basin of territory in this 

 locality. Further east, in the middle portion of the region, the 

 north-flowing streams are the stronger, and seem to be encroach- 

 ing upon the drainage area of the Arkansas, while in the east- 

 ern part, the south-flowing streams head very near the north 

 escarpment of the plateau. 



The rocks of the region are mainly unmetamorphosed sand- 

 stones and shales, those at the base being of Lower Carboniferous 

 age, and those at the top belonging to the Coal-measure series. 

 These alternating hard and soft rocks have produced the ter- 

 races on the hill slopes, which are so characteristic of dissected 

 regions of horizontal strata. As these terraces are often of con- 

 siderable width, and are favorable horizons for springs, they are 

 inviting to the farmer, and can be located miles away by the 

 small farms on the mountain sides. 



The low region to the north of the Boston Mountains is one 

 of great denudation. From its northeastern part, all the rocks 

 have been removed above the Ordovician, leaving those exposed 

 at the surface. West and south of this is a region from which the 

 Upper Carboniferous rocks have been removed, leaving those of 

 Lower Carboniferous age at the surface. Standing up promi- 

 nently on the latter are numerous hills of circumdenudation, 

 composed of remnants of the horizontal strata of the Boston 

 Mountains, and serving as living witnesses to their former 

 extent. The height of these outliers very closely approximates 

 that of the plateau of which they were formerly a part. This 

 uniformity in height between the various parts of the dissected 

 Boston plateau and its outliers suggests a peneplain, and herein 

 lies the physiographic problem of the region. 



In a region of folded or inclined strata the determination of 

 a peneplain becomes a question of comparative ease, for in 

 those cases denudation will have reduced both hard and soft 

 strata to practically the same level, the peneplain intersecting 



