EVALUATION OF STRATIGRAnilC CRITERIA 433 



bedded structure, giving flagstone bedding, but excessive wave action on 

 a shallow bottom by developing waves of translation throws up bars and 

 islands, beyond whic;h the waves reform with lesser amplitude. In such 

 cases beach action, shown by marked and irregular cross-bedding and 

 even eolian effects, may be expected to become developed in places within 

 the more regular formations of the flat bottom. The river currents, on 

 the other hand, give an elongated structure to sandstone lenses, and tend 

 to develop current-marked surfaces and more pronounced cross-bedded 

 structures, instead of evenly bedded and ripple-marked sands, as the nor- 

 mal accompaniments of deposition. Waves tend to restrict the coarsest 

 material to the zone of the shore, but finer gravel may be spread over the 

 contiguous bottom in smooth even sheets. Rivers tend to concentrate 

 such gravels into discontinuous courses. 



Lenticular thickenings of marine sands should normally be convex 

 upward and show flat cross-bedding. Chaniie] sands, on tlie contrary, 

 are more irregular, and although convex on tlie upper surface are more 

 commonly convex at bottom, cutting through the underlying deposit 

 and showing steep cross-bedding. These are the extremes of difference; 

 but, on the other hand, evenly bedded almost structureless sands with 

 minor cross-bedding may arise from the action of either sea or rivers. 

 Although in extreme cases it is thought that sandstones of marine and 

 fluviatile origin may be distinguished, it is clear that comparative studies 

 of the two need to be carried out with a greater refinement than has 

 heretofore been done, and that it is the character and dominance of a 

 particular type of cross-bedding which is of significance rather than the 

 mere presence of the structure. 



Effects of sheet-flood deposition. — Many aggrading streams overloaded 

 with sand exhibit at low stage shallow braided channels within the main 

 channel. At higher water the main channels may likewise form a 

 braided system, and at- highest water the whole floodplain may be cov- 

 ered by a shallow moving water body, simulating on a larger scale the 

 effects seen in the channels at low water. The effect is well illustrated 

 in miniature by the waste banks from coal or ore washing plants. It 

 seems to be emphasized over those piedmont slopes or deltas where aggra- 

 dation is actively going forward and where the streams are always over- 

 loaded. In such cases the channel becomes an unimportant feature and 

 sheet-flood effects arise. 



It is such conditions which seem to be required to produce the great 

 depths of regularly bedded and widely extended sandstones which mark 

 certain continental deposits. The beds may vary from thin to thick. 

 They succeed each other without interlamination of clays and commonly 



