428 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



the bottom profile is in better adjustment to the waves the inequalities 

 are much less. For example, south of Long Island the bottom profile 

 deepens smoothly to 30 fathoms and beyond. In small bays free from 

 tidal races the same smoothness of bottom is noted, but in much shal- 

 lower water. It is seen, therefore, that sharp channeling and flow and 

 ])lunge structures are not features of wave action, but of current action, 

 and tend to be smoothed out in open-water bodies. Although the profiles 

 of sand beds on the subaqueous plain tend to be smoother in detail than 

 those of the subaerial plain and of gentler slope, there are, nevertheless, 

 decided inequalities over larger distances. The sediments are dominantly 

 swept in certain directions and concentrated bottom currents are thought 

 to prevent deposition in others. Such effects are not, however, to be 

 studied from the shores. The conspicuous effects of an ocean surf on the 

 shore impress the imagination, but have little direct relation to the mak- 

 ing of sedimentary structures on the floor of the water bod}'. Where 

 waves drag on a shallowing bottom and throw up bars a cross-section of 

 the resulting bedding would show sandstone lenses whose upper surface 

 is convex upward with gentle slopes and cross-bedded structure. Chan- 

 neling by currents, on the contrary, cuts into the beds below and under- 

 cuts the sides, giving curves which are convex downward. Currents also 

 build shifting bars with convex upper surfaces in places of slack water. 

 The slopes of channels and river bars are, however, steeper and the de- 

 posits in the slack water are more local and irregular. 



It is one of the important principles of sedimentation that the beds of 

 sand which are laid down, and not later disturbed, are the results of the 

 heaviest storms. These churn up the shallower bottom, loading the 

 water with sediment and moving part of it to a greater depth of water 

 whither minor storm waves can not transport the sand. Here the sand 

 is laid down gently and without indication of the commotion which is 

 reigning elsewhere. A sandstone bed may thus be deposited during a 

 single storm and give the appearance of rapid sedimentation, when in 

 reality years may elapse between the deposition of successive coarse beds. 

 During such storms, although the sand is worked out to unusual depths, 

 the silt of those depths has also been greatly stirred and is in part worked 

 farther seaward, in part settles back in place. The resulting bed from a 

 single storm, owing to this stirring, will show a sharply defined surface 

 to the sand, frequently ripple-marked, witness to tlie culmination of the 

 storm, on which another bed of mud or silt will come to rest and record 

 the following period of lessened wave action. This is normal flagstone 

 bedding. 



