EVOLUTION OF IRONDEOUOIT VALLEY. 137 



main road ( next east of the Bay ) from the Float Bridge to Forest 

 Lawn. In fact no other source can be found for the fine silts com- 

 posing it. The peculiar acute angle between this mass and the main 

 east margin of the Bay depression (just north of the Ridge Road) 

 thus finds explanation as being the unfilled space between two dif- 

 ferent areas of deposition, while the steep and straightened back- 

 slope of the delta thus thrown laterally across a great chasm is 

 just what we wotild expect in such a case. Furthermore, the whole 

 structure lines up well with the Ridge Road spit across the Bay in 

 Irondequoit village. No other competent cause can be invoked for 

 this great obstruction blockading the ancient rock-valley for more 

 than two-thirds of its width, and for over a mile of its length. The 

 mode in which it was cut off from the main mass on the west will 

 be considered later. 



This main mass, in the town of Irondequoit. has been subjected 

 to a very pretty and perfect dissection by a group of closely spaced 

 "consequent" brooks that furnish as compact a little field for de- 

 tailed study as could be selected anywhere in the world. Digging 

 down through the silts at a point where these were spread somewhat 

 deeply over a broad hollow in the underlying rock and till surfaces 

 (see the accompanying paper on the rock topography) these brooks 

 have in most cases come to rest not far from the actual contact of 

 the silts upon the till. Here they find their downward progress 

 almost wholly checked by the firm stony "hard-pan"', but here thev 

 find also their greatest alimentation of ground water ; and so they 

 push their little valleys headward along the till stu-face by means of 

 the feeding springs, and develop lateral tributaries by the same 

 means. The new Durand-Eastman Park now includes a consider- 

 able portion of this area. 



Between this park and the bay lies a flat ridge of undissected 

 plain followed by the road and trolley to Sea Breeze (Lake Beach). 

 This ridge, though essentially a part of the delta sm-face. has be-, 

 come so largely through the smoothing down of the glacial kames 

 and earlier (Newfane) beaches already described. Across the sur- 

 face thus levelled must have been swept or carried in suspension the 

 silts that compose the detached mass east of the bay. The glacial 

 core of this ridge, which partakes of the nature of an inter^obate 



