l893-] FAIRCHILD — GEOLOGICAL HISTORY OF ROCHESTER, N. Y, 215 



THE GEOLOGICAL HISTORY OF ROCHESTER, N. Y. 

 By Herman LeRoy Fairchild. • 



To interpret from the rocks the long geological story of this 

 locality some fundamental geological principles must be clearly in 

 mind. 



The vast majority of all sedimentary rocks (to which group 

 belong all those of Rochester, except the superficial drift) are layers 

 of detritus formed in the sea, and derived from the decay and wash 

 of neighboring land. Such accumulations are possible through the 

 oscillations of level, which may slowly carry great continental areas 

 under the sea, to re-elevate them, probably, at a later time. The 

 thickness of strata may bear a direct relation to the amount of 

 subsidence. This does not signify that the continental areas and the 

 oceanic basins ever change places, but simply that submerged areas 

 of a continent are no less part of the geological continent in that 

 they lie below sea level. Whether or not a portion of the continent 

 be dry land depends upon slight changes of attitude toward the 

 plane of water surface. 



The character of sediments indicates the conditions under which 

 they were formed. A gravel deposit means strong currents and 

 consequently shallow water, and a near source of supply. Finer 

 material, as sand, indicates less velocity of water, probably greater 

 depth and greater distance from land. A pure quartz sand indicates 

 a great amount of movement, immediately or remotely, necessary to 

 remove by trituration, assorting and solution the other minerals of 

 the original crystalline composite rock. A stratum of shale (indurated 

 silt) can be deposited only in slack water, and if of great horizontal 

 extent indicates considerable depth of sea and probably long trans- 

 portation. 



Shallow water deposits are frequently marked by sinuosities due 

 to waves. Exposure above water at low tide is shown by several 

 features, as shrinkage-cracks, rill-marks, rain-pits. A near shore-line 

 is proven by cross or oblique bedding, and other phenomena 

 produced by variable currents due to changing winds and tides. 

 Climatic conditions, and to some extent de[)th, are indicated by the 

 nature of the organic remains. 



Limestone rocks are produced by the comminution of the calcar- 

 eous framework of low organisms. Such rock may be formed upon 

 the place where the animals grow, the detritus mingling with the 

 larger masses, as in the coral limestone now forming upon reefs in 



