6 NEW YORK STATE MUSEUM 



The region mapped lies between 43° and 43° 20' north latitude 

 and 77° 30' and 77° 45' west longitude, and exclusive of Lake 

 Ontario comprises an area of- 239 square miles. The variation of 

 the magnetic from the true meridian in this section is 8^ west of the 

 true north. 



The hight of the lowest area on the map is on the shore of Lake 

 Ontario which is 247 feet above tide. The highest points are in the 

 vicinity of the Mendon hills, approximately 850 feet above tide. 

 The higher elevations are due to drift accumulations upon the Soft 

 Salina strata. 



The city of Rochester is situated nearly in the center of the map 

 and many fine rock sections are showm along the Genesee river well 

 within the city limits, while the numerous steam and trolley roads 

 running from the city make most of the other areas of the map 

 readily accessible. 



On the north, the area mapped is bounded by Lake Ontario. 

 From the northwest corner of the map the lake shore extends in a 

 southeast direction as far as Irondequoit bay, from which point a 

 northeast direction is assumed, making within the limits of this map 

 the most southerly projecting point of Lake Ontario in New York. 

 At the place where the lake has its greatest southern extension, 

 Irondequoit bay opens. This bay, representing undoubtedly an 

 unfilled preglacial channel, and though less than 5 miles in length, 

 is the southernmost bay of this lake. 



The "ridge road" or the shore line of ancient Lake Iroquois 

 passes across the map from both sides of the Genesee river in a 

 nearly east and west line, having a slight northerly trend. The 

 " ridge road," while a conspicuous topographic feature, is not con- 

 fined to any particular rock formation and is not to be confused as 

 to its origin with the ridges or escarpments due to the erosion of the 

 northerly edges of the southerly dipping rocks of this region. 



South from the " ridge road " the rocks of the Niagaran group are 

 spread over a considerable area, so that the rock terrace so well 

 developed in Niagara county, here finds expression in a much lower 

 one in which the rocks of Clinton age are chiefly concerned. This 

 terrace is best shown in the town of Greece along the line of the pres- 

 ent Erie canal. Southward the outcropping edge of the Lockport 

 dolomite rises in some localities above the general surface as a small 

 ledge, but is less conspicuous than the preceding one. At the ridge 

 or escarpment at Lewiston on the Niagara river, the Lockport dole- 



