OEIGIN OF LARGEE COMPLEXES. 467 



south. Thus the main supply of water from the north would be cut off 

 before the ice at the sides of the reticulated ridges melted, and in this 

 region of short hills the supply of frontal or overwash matter was small 

 and due to local action. But in the valley of the Saco River from Hiram 

 to Steep Falls the sedimentary plain that borders the river presents in cer- 

 tain places just such a structure as would result if reticulated ridges were 

 subsequently overlain by much frontal matter, at the same time being more 

 or less washed away and reclassified. The valley is inclosed by such high 

 hills from Steep Falls northwestward that there was no late diversion of 

 glacial waters out of the valley, while numerous glacial rivers have left 

 gravels showing that they flowed into it. These are true ice-channel gravels, 

 not overwash, and the plain of the Saco is therefore an intermediate forma- 

 tion between the frontal or overwash apron and the reticulated ridges, and 

 contains both of those formations. 



One method of the formation of reticulated ridges has been observed 

 by Professor "Wright at the Muir glacier and by Professor Russell at the 

 Malespina glacier — one form of the overflow gravels suggested above. A 

 frontal or overwash sheet of gravel is first deposited over the thin marginal 

 ice. During the subsequent melting, channels are cut in the ice beneath 

 the gravel by streams, apparently of local origin, and the overlying- gravel 

 tumbles from both sides into the channel, where it is more or less water- 

 washed and stratified. It is highly probable that ridges having a pellmell 

 internal structure often originated in substantially this manner, and it is one 

 of the means employed to produce the mounds and hollows of the moraines. 

 The clogging of the mouths of subglacial tunnels woiild bring- the streams 

 to the surface of the terminal slope, like those of the Malaspina glacier, 

 when they would deposit on the marginal ice a more or less ridged sheet 

 of gravel, which would become a jumble of ridges, mounds, and hollows 

 during the unequal melting of the subjacent ice. But while admitting this 

 as one of the methods of the formation of reticulated ridges and kettle- 

 holes not forming a jiart of the delta plexus, I regard it as subordinate in 

 rank to sedimentation in connecting ice channels by the great osar rivers 

 themselves, for most of these ridges are stratified and must have been 

 formed basally, not on the ice. Where large ridges are composed of coarse 

 material and are stratified, we can evoke only the largest and most rapid of 

 glacial rivers, not local brooks undermining sheets of clay. 



