258 



': he largest delta of this class observed in the region, at Bristol Springs 

 on the west side of the Canandaigua lake valley, was built in the Naples- 

 Middlesex glacial lake at one nearly static or slowly subsiding level. The 

 top, about one-half by one-quarter of a mile in area, is smooth and gently 

 sloping forward from the 1.200 to the 1,100-foot level. The surface ma- 

 terial is very coarse, containing rounded cobbles up to six inches in diam- 

 eter, often with little admixture of finer sediment. This delta was built 

 by a stream from the Bristol valley, which during the process must have 

 drained a loaded ice lobe and not a lake. 



Such simple, flat-topped, steep-sided deltas, resembling the bastion of 

 a fort or an abutment prepared by a daring engineer from which to spring 



Fig, 3. Naples Delta. Two Upper Levels. A kettlehole in the woods. 



the arch of a bridge, are formed rapidly by strong or torrential streams 

 and are composed of relatively coarse materials. From their striking and 

 characteristic form and position they may be called bracket deltas. 



Garlinghouse delta, a few miles south of Naples, does not project like 

 a bastion from the face of the valley wall but fills a niche a mile deep 

 and half a mile wide, the walls of which rise sharply 500 feet above its 

 surface. The niche now receives two or three insignificant brooks, but one 

 of them comes from a gap in the wall which opens northward to the upper 

 Honeoye valley. This gap probably once transmitted a strong stream from 

 the ice front but a few miles distant. This delta may be the only one of 

 its kind, and if so, belongs in a class by itself — that of niche deltas. 



