320 C. C. MOOK ORIGIN AND DISTRIBUTION OF THE MORRISON 



flowed across the swampy plains and deposited silt over broad floodplains. 

 Lakes were probably present and were the seat of deposition of many of 

 the fine-banded clays and sandstones. Deltas into these lakes, as sug- 

 gested by Eiggs, probably account for some of tbe coarser local sandstone 

 bodies. Some of the streams from the old mountains were much larger 

 than others and carried a greater load. From streams of varying size 

 and with different loads the deposits formed could not be uniform 

 throughout the whole area of deposition. Further than this, streams of 

 low gradient, such as those postulated, would deposit much of their ma- 

 terial before reaching a great distance from the original source. Ac- 

 cording to the evidence from the fauna and tlora, the depositional area 

 was not an arid one, certainly not through its entire extent. Eainfall 

 was probably fairly abundant, throughout much of the area at least, and 

 new systems of streams were produced on the plain itself. The basal 

 beds of formation were derived to a certain extent from the strata of the 

 formations underlying, as Avell as from the mountain areas. These 

 streams consequent upon the deposition plain would also carrv- a con- 

 siderable load, and in this way the formation would be built out from 

 itself. Material at the outer fringe of the area would therefore be much 

 younger than material deposited at the same distance above the base 

 nearer the source of supply. Shifting of channels would also result in 

 erosion of material already deposited. In this way a series of deposits 

 might be formed over a wide area, comparatively thin, with very great 

 lateral and vertical variation, and yet present the same kind of characters 

 over the whole area. Most of the formation being fine grained, the 

 streams were probably mostly sluggish. Occasional coarse iDeds, with 

 cross-bedding of the stream type, testify, however, to a considerable 

 amount of carrying power and a fairly swift current at times. 



The whole area probably remained nearly level throughout tbe whole 

 depositional interval, though there were probably slight irregularities. 

 The difference in thickness between the beds at the southwest and those 

 at the northeast, providing the base rested on a level surface, is not 

 enough to contradict this statement. The southwestern areas, near the 

 sources of the material, proljalily were, at times, slightly higher than the 

 northern and eastern areas. The sediments in the southwestern areas 

 contain a larger proportion of coarse material and may have been built 

 up above the level of the larger part of the plain. Tt is also possible 

 that depression took place in the southwestern part of the area in the 

 later part of Morrison time, allowing fine silts to be deposited farther 

 southwest. Tt is noticeable that the fine clays in the southwestern area 

 are confined largely to the upper half or third of the formation. 



