80 NEW YORK STATE MUSEUM 



courses were determined by the contours of the deposits left by 

 the retreating ice sheet, and do not correspond with the old valleys 

 in position but cross them at varying angles. The present rock- 

 bound portions of their courses are due to the uncovering of hill- 

 tops and divide summits of the preglacial topography, which lay 

 buried beneath their modern courses when they first assumed 

 them. When they occupy, or cross, old valleys, they have not yet 

 been able to cut down to their rock bottoms and have only par- 

 tially removed the drift filling. The modern valleys are not as. 

 large, as deep, nor as mature, and the surface relief is not as 

 great as before the appearance of the ice. 



Spruce creek presents some interesting features. All the upper 

 part of its course closely follows the pre-Cambrian edge. This 

 contact forms a natural drainage line because of the southwest- 

 erly slope of. the resistant pre-Cambrian surface uncovered by the 

 retreat of the Beekmantown inface, and there must certainly have 

 been a preglacial stream here. Brigham has noted the corres- 

 ponding position of Black river, which follows this contact for 

 miles. 1 The present divide between Spruce creek and Black creek 

 (an affluent of West Canada creek which flows to the northwest 

 along the contact line) is a moraine ridge near the north limit 

 of the sheet. Both these streams are in their old valleys, though 

 where the preglacial divide was is uncertain, the writer however 

 suspecting that it was at or near Diamond hill, and that the 

 present upper part of Spruce creek is in the old Black creek valley, 

 the drainage now being reversed. However that may be, the gorge 

 at Diamond hill is modern, either because of the cutting down of 

 a col, or because the stream is there turned aside out of its old 

 valley. Just below, the valley is blocked by a moraine, and to the 

 eastward of the gorge no rock shows at the surface for a mile, so 

 that we are not limited to the supposition of a col at this point 

 to explain the present course of the stream. From Diamond hill 

 to the fault line the stream is occasionally out of the old valley, 

 specially at Salisbury Center, thence its course to its mouth is 



a Op. cit. p.186. 



