378 GLACIAL GEAVELS OF MAINE. 



the gravels. Here arise special difficulties. The gravels afford muoh posi- 

 tive evidence regarding themselves, but in accounting for the gaps in these 

 systems we have to rely largely on negative evidence. Probably no other 

 problem connected with the glacial gravels is so difficult of solution. It 

 will be seen that a discussion of the origin of the gaps in the discontinuous 

 systems will apply almost equally to the discontinuous portioDS of the osars 

 and broad osars. Indeed, if the streams which deposited the discontinuous 

 kameo had been longer, so as to extend farther north, nearer the ndvd, I 

 believe that toward their northern extremities the separate ridges would be 

 confluent and not distinguishable from the osars and broad osar terraces. 



The deposits forming the noncontinuous osars are of several quite dis- 

 tinct types. 



1. Marine deltas. The general characteristics of these plains have 

 already been described. The longer of the systems under discussion almost 

 always expand into one or two marine deltas; some of the shorter systems 

 also contain deltas, but more of them have none. The deltas are found at 

 intervals of several miles. Thus far I have not been able to find terminal 

 moraines genetically connected Avith the marine deltas, though there is 

 much reason to suspect this relation at the Waldoboro moraine; yet I have 

 often suspected their existence beneath the marine clays. These clays are 

 sometimes 80 to 100 feet deep, and large ridges might exist which are now 

 hidden by the clays. 



2. Broad solid ridges or gravel massives one-eightli of a mile or more 

 in breadth, separated by the usual intervals. They sometimes have a some- 

 what uneven surface, at other times are rather smooth and with slightly 

 convex surface. In external appearance they somewhat resemble the small 

 deltas, but the horizontal assortment of sediments is much less perfect. 

 They usually rise above the clay, and they do not pass into it by degrees. 

 Even at their southern border the material is often quite coarse, and never 

 finer than medium sand. The clays overlie these gravels at their bases and 

 are plainly a later deposit; at least these are their relations on the surface. 

 It is possible that in some cases a lower stratum of these plains passes into 

 the clays by horizontal transition ; but if so, the stratum showing that transi- 

 tion is buried out of sight. Often the exposures show conclusively that 

 there is no such transition, and nowhere is there proof of it. 



This sort of solid or massive hills closely resembles the plain before 



