MOOK, STUDY OF THE MORRISON FORMATION 



117 



The base of the formation is not shown in this section/ but judging 

 from another outcrop in the bank of Oil Creek, a few hundred feet east 

 of the quarry, the base of the quarry-floor sandstone is about 80 feet above 

 the calcareous sandstone which is here considered as immediately under- 

 lying the Morrison. 



The quarry-floor sandstone is cross-bedded; on the north side of the 

 gulch the cross-bedding is of the stream type, with the beds dipping 

 north; on the south side of the gulch the upper part of the quarry-floor 

 sandstone shows cross-bedding of the wind type. 



The heavy sandstone member below the quarry-floor sandstone is semi- 

 lense-shaped in cross-section and occupies a trough in the clays beneath. 

 The trough is a hundred feet or so in breadth and about 13 feet deep, the 

 sandstone which fills and covers it var^dng from 3 to 5 feet in thickness. 

 The contact between the clays and the sandstone in the trough is very 

 sharp. The only satisfactory explanation of this trough is that the clays 



Fig. 82. — Type of cross-bedding usually knoivn as the OBoIian type. 



were eroded and the sandstone deposited over them, by a stream of con- 

 siderable size. This means a stratigraphic break. While the channel in 

 the clays was being eroded and before the deposition of the sands filling 

 it, continuous deposition must have been going on in some other areas. 

 This break need not have been long, in fact was probably short, as the 

 same stream which eroded the channel probably deposited the sands on 

 suffering an increase of load or a decrease of volume or gradient. 



The gulch cuts directly across the old channel at this locality, and 

 therefore exposes its characteristics completely. Stream banks cut paral- 

 lel to old channels would not show their trough character at all, and banks 

 exposed obliquely to old channels in position would exhibit a long gradual 

 thinning out of the beds, without any pronounced lense-shaped cross- 

 section. Thinning out of this character is common throughout the entire 

 area of Morrison outcrops, and distinct lense-shaped sections, which are 

 only exposed under very favorable circumstances, are not rare. Stream 

 channeling and deposition are consequently especially characteristic fea- 

 tures of the Morrison formation. 



" See also figs. 8 and 9, p. 50. 



