EETICULATED EIDGES WITHIN ICE CHANNELS. 461 



other places they are so broad and deep that they contain lakelets and must 

 reach very nearly to the till. If we suppose that the central parts were 

 kept clear of sediment by swift streams, these transverse bars of gravel 

 connecting the main ridges are still to be accounted for. 



In southwestern Maine we find in the complexes large steep-sided 

 ridges extending for long distances (1 to 3 miles) without noticeable change 

 in average size and without becoming confluent, except by occasional trans- 

 verse bars or low ridges and many terraces. The contrast in structure 

 between the ridges of the delta, which become broader and more confluent at 

 their bases and show horizontal classification of sediments, and the ridges 

 of the large plexus, which show little assortment of sediments but continue 

 for miles of nearly unifoi'm sizes and with steep lateral slopes, is very great 

 indeed. The delta plexus is an intelligible formation; why should not all 

 these ridges broaden toward the south and become finer in composition if 

 they were all deposited in the sea or other large body of water? Think of 

 the enormous rivers required to flow between two ridges 50 to 100 feet high 

 and one-fourth of a mile apart and yet keep the space between them so 

 clean of sediments that the deeper hollows are 100 feet deep, alternating 

 with transverse bars rising- almost to the tops of the lateral ridges; and all 

 this, too, without the ridges broadening or the sediments becoming finer 

 southward over distances as great as the breadth of the reticulated plexus 

 of even the largest marine delta. The theory that the reticulated ridges 

 were formed by unequal deposition in open bodies of water accounts well 

 for the plexus at the proximal ends of marine or lacustral deltas, and for 

 some of the reticulations in 'lakes or broad channels relatively small to 

 the flow of the river, where there was no horizontal classification of sedi- 

 ments, or only an imperfect one, but it breaks down in face of the larger 

 complexes and those not connected with deltas, such, for instance, as those 

 occurring in the course of the osars on northern slopes and at considerable 

 distances from the sea. Here the reticulated ridges were often as plainly 

 deposited between ice walls as were any of the osars. 



Let us now take the case of the most complex and best-developed 

 plains of reticulated ridges to be found in the State — those lying west of 

 the Saco River in southwestern Maine. They are situated in a region where 

 the rocks are mostly granitic and the till is consequently abundant. The 

 country is hilly, hence probably favorable to the englacial till getting up 



