FOEMATIOlSr OF EIDGES OF AQUEOUS SEDIMENT. 451 



All over the State the reticulated ridges are usually rather coarse. In 

 western Maine the ridges transverse to the general course of the glacial 

 river are on the whole rather finer in material than the ridges parallel with 

 the course of the river; but this rule is not univei'sal. Indeed the reticu- 

 lated kames seem to defy all rules. 



9. Their internal structure. Many of the reticulated ridges, especially 

 near the north end of the plexus, have the steep slopes and roof-like top 

 characteristic of the pellmell ridge. All the excavations in Maine which I 

 have examined show more or less distinct stratification. No very distinct 

 layers can be expected where the materials are very coarse, and the thick- 

 ness of the beds formed by a single flood might be many feet. 



WAYS IN WHICH A RIDGE OF AQUEOUS SEDIMENT CAN BE FORMED. 



1 . Subaerially, in the way in which streams carrying much sediment 

 build up delta channels. Thus, the Mississippi River, the Po, etc., near 

 their mouths are flowing on top of a ridge composed of their own sediments. 

 This ridge is really composed of two ridges which form the banks of the 

 stream, but when the amount of sediment is great the ridges coalesce at the 

 bottom and the river flows in a depression on the top of a single broad 

 ridge. They seldom rise very high before the stream abandons the old 

 channel and makes for itself a new one at the side of the old, thus spread- 

 ing out in the well-known fan shape. 



2. Wholly within channels of the ice, either subglacial or superficial, 

 the subsequent melting of the ice leaving the sediments rising above the 

 adjacent ground. 



3. At the sides of rapid streams where they enter comparatively still 

 water. A good modern instance of this kind of ridge may be seen at 

 Kingman (see p. 98). This is, in fact, only a subaqueous example of 

 the same process as that by which a subaerial stream in its delta builds up 

 two-sided channels. The rapid glacial streams, both subglacial and super- 

 ficial, would form large ridges on each side of them as they entered the sea 

 or a lake. This is well exemplified by the gravels near East Monmouth 

 (see p. 189). 



4. When a small stream bearing much sediment entered a body of 

 still water, the two ridg-es formed at the sides would soon coalesce at the 



