TERTIARY PENEPLAIN AND FARMINGTON LOWLAND PLAIN. 373 



isolated positions beyond the boundaries of the escarpment appear to be 

 remnants of the steep slope at a time when it extended out into the low- 

 land much further than at present. These solitary mounds often pre- 

 sent a striking contrast to the surrounding country and form conspicuous 

 features of the local landscape. 



FA RMING TON LOWLAND PLAIN. 



The median plain which occupies a large portion of the Mine la Motte 

 country is a part of a broad zone that extends over a much more exten- 

 sive area. It is limited on the east by the irregular cherty escarpment 

 of the upland forming the divide between the Mississippi and Saint Fran- 

 cois rivers. On the west the porphyry hills interrupt its continuity. 

 The general elevation of the plain is about 1,000 feet above mean tide. 

 This level is 700 to 800 feet below the horizon of the great peneplain 

 (figure 1). 



<. ^ T *• T i. i^ -^ L. 1 1^ y 





Figure i. — Geological Cross-section of Farniington lowland Plain. 



The rocks of the Farmington plain are soft, or at least succumb much 

 more easily to meteorologic influences than do the fine grained crys- 

 tallines and the chert-bearing dolomites. The principal features are the 

 outcome of rather vigorous erosive action upon limestones and sand- 

 stones and the general surface is rolling. Wherever the calcareous beds 

 predominate, the topographic outlines are rounded and greatly softened ; 

 where sandstones occur the relief is bolder. In the lowland the river 

 valleys have a tendency toward a broad, open ty^^e, in contradistinction 

 to those of the upland, where narrow, contracted gorges prevail. The 

 extent of the plain, however, is not confined to the areas of limestone 

 and sandstone, but, with some modifications, covers also the granites. 

 The latter, disintegrating much more readily than the porphyries, leave 

 the areas occupied by the fine grained rocks standing far above them. 

 The surface relief of the granite district is noticeably more rugged than 

 that of the sedimentary areas, but the comparative resistances of the two 

 kinds of rock are not so diverse but that the areas occupied by them 

 may all be grouped together. 



The lowland is manifestly a plain of denudation. It is the product of 

 a former cycle whose work was interrupted before completion. In point 



