TIIK DAKOTA lilDCiE. 177 



and the Red Valley to avoid anticipatinj^ a considerable share of that which 

 belong-s most properly here. A certain amount of rei)etition is therefore 

 inevitable in presentin<^- the relati«)n of the groui) to the topoj^raphy. 



As a rule every monoclinal ri(l<^e contains a hard bed and a soil one. 

 The hard bed forms one side of the ridge and its crest; the soft bed ai)penr8 

 at the base of the other side and underlies the hard. In tlie case of the 

 monoclinal that surrounds the Hills, and to which the title "foothills" has 

 been applied, the cai)ping- hiinl bed is the Dakota sandstone, and the under- 

 lying soft bed is partly Jurassic and i)artly Triassic. But for the hardness 

 of the sandstone there would be no ridge. ]5ut for the softness of the Ked 

 Beds and Jura, it would not be distinguished from the general arch of the 

 Hills. It is the result, under the laws of erosion, of the contrast between 

 the two. 



Where the dip is great the base of the ridge is narrow and both sides 

 are steep. Where it is small the base is broad and the side capped by the 

 sandstone becomes a sloping plateau. On the east and west sides of the 

 Hills the dip is so great that the ridge character is strongly marked. At 

 the south end of the uplift the southward dip is very gentle, and the result 

 is a plateau bounded on the north edge by a cliff. As the ridges apj)roach 

 the plateau they converge in direction and their lines of strike are continued 

 as the limiting lines of the jdateau until they meet. This gives to the 

 plateau a triangular form. It is not, however, a plane triangle, but is arched 

 from east to west. It pictures forth in a most striking manner the slow 

 sinking of the axis of uplift beneath the quiet strata of the plain. Through 

 this plateau the South Cheyenne has cut its way from west to east, form- 

 ing the defile known as the Big Canon. 



At the north end of the uplift — or rather at the northwest end, for the 

 axis of uplift curves around until it has that din^ction — the structure is 

 similar in its main points, but it lacks the simplicity exhibited at the south. 

 The form of the uplift is less regular and it is complicated by volcanic dis- 

 turbances. It is broader and it is carried a little higher. The Belle Fourche 

 crosses it just as the South Cheyenne does at the south, but it has eroded 

 a deep, broad \alle}- instead of a narrow canon, and the symmetry of the 

 terminal sandstone plateau is thereby destroyed. Nevertheless the plateau 



12 BH 



