RIVER TERRACES. 63 



perceptibly higher near the stream. In other words, the vallej^ is filUng 

 with sediment. This is the condition of the stream and valley at the delta, 

 provided the flow of water is sufficient to cover the whole valley from side 

 to side. In a few places this is the present condition of the valleys, as, for 

 instance, the valley of the Crooked River for a few miles north of Sebago 

 Lake. Most of the more level portions of the larger valleys of Maine 

 must have been in this condition at the close of the Ice age. 



A strict classification would distinguish the flood plain of erosion from 

 that of deposition. Practically the two processes are intimately blended. 

 On the steeper slopes the flood plain is almost always due to erosion in 

 times of flood; on the gentler slopes it is composed wholly or in part of 

 matter deposited by the flood waters. It is often difficult to determine 

 which of the two processes has been more active. In field use, the term 

 "flood plain" implies the lowest river terrace which is now overflowed by 

 the river in time of flood, without regard to the origin of the terrace. 



Below the highest postglacial level of the sea (230 feet), we find the 

 larger streams bordered by a rather narrow flood plain, above which rise 

 one or more erosion terraces in the marine beds, or in the glacial sands and 

 gravels, or sometimes in till. Soon after we rise above 230 feet we find 

 one or more river terraces in the so-called valley drift. In addition to the 

 marginal terraces, several of the valleys show large ridges lying along 

 the axis of the valley. The largest and longest of these that I have 

 observed were found in the Kennebec Valley above Solon, in the valley of 

 the Little Androscoggin above South Paris, and in the Piscataquis Valley 

 above Abbott. The number of marginal terraces varies. In general, the 

 top of the central ridge has nearly the same elevation as the higher mar- 

 ginal terraces. Both .Jackson and Hitchcock report terraces in the upper 

 Kennebec Valley at elevations such that they must be higher than the 

 central ridges. These highest terraces are so obscure that I hesitate to 

 call them terraces. 



RECENT EROSION OF THE VALLEY ALLUVIUM AND OF THE GLACIAL SANDS AND 



GRAVELS. 



Before discussing the origin of the higher river terraces, it is necessary 

 to inquire what sort of geological work is now going on in the river valle3^s. 

 We can not declare that the higher terraces above the flood plain are due 



