EVALUATION OF STllATKIIIAril IC CRITERIA 431 



cross-seetioiis of water carried by shore currents permit of local excava- 

 tion to depths of several fathoms. Where progressive subsidence permits 

 such features to become preserved, there may result lenses of marine 

 sandstones some tens of feet in thickness and marked by cross-bedding. 

 The slopes are not so steep, however, nor the cross-bedding on so large a 

 scale nor so dominant as in either river or dune structures. The marine 

 bars and channels are also relatively fixed and do not migrate over the 

 surface so freely as do river channels and desert dunes. 



In rivers where sands are being deposited the channel is subject to 

 meandering. It cuts laterally into the banks and scours down into older 

 floodplain deposits. The sands of the abandoned channels cut across the 

 bedding of the floodplain deposits on the convex sides of the meanders 

 and on the concave side are interlaminated with them. The river works 

 across the floodplain and buries channel structures widely in the fluvia- 

 tile deposits. The river bars work regularly downstream, being con- 

 tinually cut out above and deposited below. From these characteristics 

 of river action it is seen that the resulting sandstone lenses tend to scour 

 downward through the beds below and are marked by dominant cross- 

 bedded and flow and plunge structures. Gravel tends to be especially 

 concentrated along such channel bottoms. Lateral discontinuity of the 

 sandstone lenses is also a feature, the ancient channel deposits forming 

 a meshwork and giving sandstone courses rather than sandstone strata. 

 Where the course of the river is relatively fixed during progressive subsi- 

 dence, these channel sands and the sands of the natural levees may com- 

 bine into a dominant sandstone facies, which may correspond in geo- 

 logical horizon to a shale formation at no great distance. Such local 

 changes in facies mark the Siwalik formations and indicate the places 

 of exit of the Tertiary streams from the Himalayas. Wave action, on 

 the contrary, tends to spread out such a sand deposit over the zone where 

 the bottom is sufficiently shallow. 



Hiver currents roll and jump material along the bottom in but one 

 direction — a movement contrasted to the to and fro oscillating effects of 

 waves. Typical ripple-mark is therefore exceptional, but in time of les- 

 sened velocity current-mark, an effect approaching it, is produced. The 

 sand is caught between small back eddies on the bottom and the forward 

 current, which is slightly higher. The result is the formation of cres- 

 cents with gentle slopes facing upstream and steeper slopes facing down- 

 stream. The plan is more or less irregular and the individual ridges are 

 limited in length. Current-mark is in fact analogous to dune structure, 

 known as barchanes, rather than to the ripples nuule on the surface of 



