RIPPLE MARK 



<)15 



verses its direction twice daily. Consequently all of tJie current ripple- 

 mark in a given set of river-laid sands will have the steep slope on the 

 same side, whereas in beds laid down under the action of tidal currents 

 the steep slope will face in opposite directions with about equal' frequency. 

 The variable direction of the steep slope of asymmetric ripple-mark may 

 thus be depended on to distinguish marine from river channel deposits. 



Cross-bedding may be considered in connection with ripple-mark, be- 

 cause it probably represents in many instances one phase of a phenomenon 

 called sand waves, which are nothing more than current-made ripple- 

 mark of mammoth proportions (figure 10). Sand waves appear to be 

 formed instead of ripple-mark when the current is overloaded witli sedi- 



FiGURE 10. — ''Sand Waves/' or mammoth Ripple-mark^ Avon River, Nova Scotia 



ment. The crests are often 15 to 35 feet apart and rise from 2 to 3 feet 

 above the troughs. They are formed on the beds of most streams which 

 1 are heavily laden with sediment and along most coastlines which are sub- 

 ^ ject to the action of powerful tidal currents. A set of beds which have 

 I been laid down in a zone characterized by sand-wave formation may noi 

 I preserve the outlines of any single set of sand waves, but the steep forest 

 beds which are formed as these waves travel with the current will be pre-- 

 served as the familiar cross-bedding so common in many coarse sand- 

 stones. Cross-bedding is thus characteristic of both river and tidal cur- 

 rent-laid beds. The direction of the inclined bods in the river deposits 

 .. should, however, possess a degree of uniformity not shared by those which 

 .Jl,have been produced in marine deposits under the influence of a current 



