GEOLOGICAL IMPORTANCE OF SEDIMENTATION 563 



subsidence, the marine planation and the delta building during all 

 the time of accumulation, it being only ' necessary to assume that 

 sedimentation remained in excess of subsidence and that the shore 

 was a fluctuating line, of which this locality was continually on 

 the landward side. These mud-cracked strata form the basal 

 portion of the lower sandstones. Above them are 1,650 feet of red 

 buff, brown, and vermilion sandstones and shaly sandstones similar 

 in character, but not noted by Walcott as characterized by mud- 

 cracks. The similar characters and especially the color of the 

 iron oxide implying complete subaerial oxidation either before or 

 after deposition suggest a continuance of the continental conditions. 

 If deposited beneath the sea it would be expected that the continued 

 wave action which affects the sandy deposits of shallow seas would, 

 at least, in part, have separated the clay and iron from the grains 

 of sand, producing cleaner gray quartzitic layers. Such a change 

 is, in fact, noted in the compact, quartzitic, gray sand rock which 

 separates the lower from the upper sandstone of the Unkar. 



The question arises as to why the mud-cracks should be absent 

 from the 1,650 feet of lower sandstones, if the latter were really 

 of subaerial origin. The answer is that, even if the necessary 

 periods of desiccation were present between the river inundations, 

 mud-cracks need not necessarily arise. A sandy nature is unfavor- 

 able for their development, and the strata are much more sandy, 

 on the whole, than in the case of the Spokane and Marsh shales of 

 the Belt terrane. Again,- a canyon wall is not a favorable place 

 upon which to observe the bedding surfaces of gently dipping strata. 

 It is furthermore possible that mud-cracked strata which could be 

 observed by careful search were not noted, since Walcott was not 

 conducting the examination with that end in view as a principal 

 object. Finally mud-cracks which were noted were not always 

 recorded in this necessarily brief synopsis of the strata, at least 

 if they were off the line of the section, since on p. 515 of the article 

 cited it is stated that at Chuar Lava Butte numerous ripple-marks 

 and mud-cracks occur among the sandstones and shales covering 

 the uppermost lava flow, yet mud-cracks at this horizon are not 

 mentioned in the synopsis which has been given. 



Returning to the description of the Unkar stratigraphic section, 



