CRITERIA FOR DETERMINING WATER LEVELS 191 



wholly or even mainly on features of wave construction or wave erosion — 

 classes 2 and 3. At the initial or summit level of the scale vel waters 

 with the short life of the latter, these are the rarest of features, but Avhen 

 found are, of course, conclusive proof of standing water. The same is 

 true of number 7; but it must be understood that marine fossils never 

 mark the initial Avater level and are usually far below, 100 or 200 feet. 



For close determination of the standing water surface, heavy deltas, 

 those of vigorous and well loaded streams, are not so precise criteria as 

 bars and cliffs. But their practically unfailing occurrence at the highest 

 or initial water level makes them more useful over large areas, and for 

 long distances in comparison of far-separated valleys they are sufficiently 

 accurate. 



Because of their more common occurrence, the classes 3 to 6 are more 

 useful, taken together, than class 2 for the geologist with experience in 

 shoreline study; but they are less definite and often equivocal, and ex- 

 perience and skill are required for confident use. They may vary in char- 

 acter with the lithology, the nature of the drift, and the topograjohy of a 

 district. When well determined they have value at least as showing 

 minimum uplift, which is all that classes 2 and 7 give. The best use of 

 these features is for confirmation of the shoreline over intervalley areas 

 or between more definite features. 



SUMMIT DELTAS 



By far the most useful class of shore features in locating summit water 

 levels is stream deltas, the deposits built at the debouchure of streams in 

 static waters. They are practically certain of production at the initial 

 level of the receiving water body, are scarcely ever wholly destroyed, and 

 are quite unmistakable. Any possible conditions that might inhibit con- 

 struction of deltas in the far-inland valleys must be exceedingly rare. 



In the large sense this class of deposits might include the broad valley 

 plains of river detritus built in lowering waters in the downstream sec- 

 tions of an uplifted main valley. But the term as here used is meant to 

 cover only the more or less localized bodies of coarse detritus which have 

 an evident genetic relation to the work of some living or extinct stream, 

 without distinction of shape, size, or composition. It is a matter of origin 

 by interaction of flowing and static water. Perhaps an addition to our 

 terminology is desirable. 



The mistake has been made of regarding broad sand or silt plains as 

 marking the highest or summit level of the standing waters. Except as 

 lateral floodplains, such fine detritus is not dropped near the water sur- 

 face. The broad, smooth plains are nearly always of inferior level. In 



