RESTORATION OF ERODED STEATA. 205 



away from the Hills and toward the Plains. But although the Dakota 

 sandstone is assumed to be parallel to the Carboniferous limestone beneath 

 it, its dip is not necessarily the same as that of the nearest outcrop of the 

 Carboniferous. On the whole its inclination is somewhat gentler, and in the 

 variations of this inclination we find recorded local details of the form of the 

 great displacement. To restore the Dakota sandstone over the whole of the 

 oval area circumscribed by its outcrop taxes the imagination more heavily 

 than the restoration of the Carboniferous, and the tax increases with one's 

 appreciation of the magnitude of the quantities dealt with. The space to 

 be filled has a length of nearly ninety miles and a breadth half as great, 

 and the total thickness of the formations that were removed from it — the 

 Dakota sandstone, the Jura, and the Red Beds — is about 900 feet. Never- 

 theless the evidence is such as no geologist would hesitate to accept, and to 

 restore the uplift to its full proportions we must carry these formations over 

 the entire area of the Carboniferous, actual and restored, building on every- 

 where a thickness of from 800 to 1,000 feet. Over the Red Valley the 

 thickness to be added is not so great, for a part of the Red Beds, and in 

 some places a part of the Jura also, there survive, so that only the missing 

 portions need be supplied. The amount to be added varies from 300 feet at 

 the foot of the Dakota cliff to 900 feet at the margin of the Carboniferous, 

 and its precise value at any particular point is to be determined by a study 

 of the local outcrops and the local dip 



The work of restoration does not end with the reconstruction of the 

 Dakota sandstone. Outside of the foothills the view shows a number of 

 monoclinal ridges smaller and less persistent than the Dakota ridge, but 

 agreeing with it in the fact that the composing strata are inclined away 

 from the uplift. These ridges mark the position of the harder members of 

 the upper Cretaceous rocks, and testif} 7 to the fact that those rocks were 

 affected in common with the Dakota sandstone by the upheaval. We do 

 not know their full thickness, because in the neighborhood of the Hills 

 they have everywhere lost something by erosion, but whatever that thick- 

 ness was — and it was not less than 600 feet — it must be rebuilt over the 

 area of the Hills iu order to restore the uplift to its full proportions. 



Let us assume that we know the thickness of each of the formations 



