r 



MOOK, STUDY OF THE MORRISON FORMATION 167 



Grabau makes the following statement regarding flood-plain sediments : 

 "From the nature of the deposits on river flood plains, perfect and often 

 very fine stratification is to be expected. This may be considered as char- 

 acteristic of typical flood plains." The Po, Ganges and Hoangho are 

 given as examples. Davis (1900, 9) states that the proportions of fine 

 to coarse materials in these rivers is very great. 



In general, levelness of surface is a characteristic of flood-plains; the 

 material may vary from coarse to fine, the former usually occurring in 

 greater abundance near the source and the latter at a distance from the 



Fig. 94. — A trihutary of the Grand River, near Mack, Colorado. 



Streams of this character were probably abundant in the Morrison area during the 

 deposition of the formation. 



source; and overlap away from the source of supply is characteristic. 

 The strata deposited will often approach horizontality over considerable 

 areas. Thinning out and replacement of beds is com~mon. Footprints 

 and similar structures are often found. 



Color of Sediments 



In moist or pluvial climates with moderate vegetation, the soil is apt 

 to be bluish. Vegetation prevents a high degree of oxidation. "In 

 seasons of dryness, when the amount of vegetation is small, the iron of 

 the sediments of deltas and alluvial fans may become thoroiighly oxidized. 



