STRUCTURE SHOWN BY TOPOGRAPHY. 213 



character. If the drainage system of the Hills were different the forms 

 of the surface would be different, but they would still give expression to 

 the character of the rocks and to the displacements. 



On the whole, the drainage system is well adapted to the display of 

 the intimate relation of rock structure and topography. A great number 

 of small streams radiate from the Carboniferous plateau, and flow independ- 

 ently, without combinations among themselves, until they pass beyond 

 the foothills. These streams are received by the forks of the Cheyenne 

 which embrace the district of the Hills. As a consequence, the facilities 

 for the transportation of debris are approximately equal in all directions, 

 and the rate of degradation by erosion is conditioned almost solely by the 

 constitution of the rocks. The inequalities of surface therefore — the results 

 of differential degradation — give a symmetrical and faithful representation 

 to the rock structure. 



In the northwestern extension of the Hills the case is somewhat differ- 

 ent The low prolongation of the uplift is crossed at right angles by the 

 north fork of the Cheyenne. A series of outcrops is produced each of 

 which circles about and returns to itself. On a map this arrangement 

 might appear very similar to that of the outcrops about the main uplift, 

 but it is essentially very different, and it depends no more upon the rock 

 structure than it does upon the disposition of the drainage. The displace- 

 ment at this point is essentially an anticlinal, and the river in traversing it 

 first descends stratigraphically, crossing successively lower and lower beds, 

 and then, having passed the axis, crosses the same beds again in inverse 

 order. The outcrop of each stratum runs to the right and left from the 

 point where it is first crossed, rising as it goes and curving in its course until 

 it becomes parallel to the river on each side of the valley. Then after pass- 

 ing the axis of uplift the two lines curve again toward the river, and 

 descending as they approach, finally meet under the water, having com- 

 pleted the circuit. The river has cut down through the upper soft zone, 

 the upper or Dakota hard zone, and the greater part of the middle soft zone. 

 The rocks of the latter alone exhibit complete circuits. The Dakota sand- 

 stone nearly completes its circuit, but the circle of this outcrop at one point 



