DISCRIMINATIVE CRITERIA 197 



is not easy and often is impracticable, but there are some suggestive 

 features. 



The materials composing the subaqueous plain are finer and more uni- 

 form or better distributed than the subaerial or stream-laid deposits. 

 They have a smoother and more nearly horizontal surface, and usually 

 display a series of steps or terraces with definite cliff borders, produced 

 in the subsiding waters. These are sometimes of such dimensions as to 

 be indicated by 20-foot contours. A good example is seen in the lower 

 left corner of the Glens Falls (Kew York) sheet. On the other hand, the 

 upper aggraded part of the delta was affected only by the river flow and 

 will have no transverse cliffs, though it may be cut by the rejuvenated 

 stream into longitudinal steps or stream-banks. This suggests the im- 

 portance of criteria for discrimination between wave-cut cliffs and stream- 

 cut banks. 



The subaerial part of the delta has more continuous, average upslope, 

 the materials are more varied, and the surface locally more irregular, 

 more likely to display stronger distributary channels in the course de- 

 tritus. The lower reach of the cobble and the change to sand or finer 

 gravel may be taken as the limit of unchecked flow. 



As with most complex natural phenomena, it is easier to state theoretic 

 distinctions than it is to apply them in the field. The determination of 

 the initial marine plane on large deltas is only approximate and requires 

 checking by the study of neighboring features. The best practice is to 

 use the great deltas for approximate height, and then to make close 

 determination by study of the small deltas of the lateral streams and of 

 the shore features of the near-by valley walls. 



In districts of great tidal range, like the Bay of Fundy, the determi- 

 nation becomes more a matter of judgment based on experience than in 

 regions of steadier water level. 



INFERIOR VALLEY PLAINS 



The highest of the broad, conspicuous valley plains have sometimes 

 been accepted as the summit level of the standing waters. When the 

 stretches of open valleys, like the great valleys of New England, are in 

 question this decision must always be an error. The only detrital plains 

 tliat could be laid near the surface of the primitive estuaries must have 

 been related to the headwater deltas or to tlie deltas of lateral valleys, 

 and such plains were coarse materials — cobble, gravel, or sand — never silt 

 or clay. The valley plains of fine material were either accumulated in 

 considerable depth of water or as floodplains of an inferior level. 



