220 SMITH AND CUKTIS — CAMASLAND : A VALLEY REMNANT 



vertical. Camasland itself is in the trough of a gentle syncline, the axis 

 of which has a northwest-southeast trend. Observations of clip in the 

 massive sandstone are not easily made, but are suiliciently numerous 

 to clearly indicate this synclinal structure. 



The structure of Camasland is, however, more clearly shown by the 

 outcrop of the sheet of diabase, which has been folded with the sand- 

 stone. The intrusive nature of this sheet was conclusively determined 

 from a study of its upper and lower contacts with the sandstone. Wher- 

 ever the dip of the sandstone could be observed the sheet of diabase 

 was seen to be conformable, except at one point on the northeast side, 

 where the diabase breaks across the sandstone and connects with a 

 smaller sheet at a lower horizon. The latter sheet is not of great extent 

 and might be better described as a tongue which soon thins out. 



The synclinal basin is spoon-shaped, the point being on the north- 

 western side. Here the dips are very gentle and the diabase sheet is 

 thick. At the southwestern end of the basin the sheet had apparently 

 less thickness. Exact measurements of the intrusive sheet are difficult 

 to make, but approximately the diabase may be said to vary from a few 

 feet, as in the lower sheet or tongue, to over 500 feet on the northwestern 

 side of Camasland. 



The exposures of sandstone within the basin denned by the rim of 

 diabase are not numerous, but fortunately they are sufficient to clearly 

 indicate the conformity of the beds above the intrusive sheet with those 

 below. There is little or no metamorphism of the sandstone at the con- 

 tact with the diabase, a feature doubtless due in part to the pure quartzose 

 character of the sandstone. At a few points, however, apophyses of the 

 intrusive rock extend for a short distance into the adjoining sandstone. 

 Only one large dike, however, occurs in the vicinity of Camasland, and 

 the connection of this with the sheet could not be traced. This absence 

 of dikes is somewhat remarkable, since a few miles distant the Swauk 

 sandstone is cut by hundreds of diabase and basaltic dikes. 



Origin of Camasland 



The topographical and geological observations recorded above suggest 

 an explanation of this exceptional feature in a region of mature dissec- 

 tion. Camasland, standing high above the surrounding canyons, is in 

 reality a remnant of a former topography, and the alluvium-floored 

 basin owes both its origin and its preservation to the occurrence of the 

 intrusive sheet of diabase. 



It would be difficult to restore the older topography from this small 

 portion that remains. The topography of this part of the northern Cas- 



