rll8 NEW YORK STATE MUSEUM 



jet determined, but it is probably two or three feet to the mile 

 and possibly more. Consequently today the plane of the ancient 

 lake surface is not level, but rises to the northward. That plane 

 was determined by the Fabius outlet. If the bottom of that chan- 

 nel be taken as 1280 feet, and if we assume the depth of water 

 over the intake as 20 feet, we will have 1300 feet as the altitude 

 of the lake surface at that point. The corresponding plane at 

 Cazenovia is now at a higher level, perhaps about 1320 feet, 

 depending on the unknown rate of uplift. At the parallel of the 

 Perryville outlet the plane would be still higher. 



Variable elements. The student of these phenomena will need 

 to be on his guard against several sources of error. In correlat- 

 ing outlet channels with delta terraces or other static water 

 phenomena, the following points must be kept in mind. 1) Down- 

 cutting of the outlets. The first flow of water over any of the 

 long-lived channels must have been at a level somewhat higher 

 than the later flow, more specially if the channel was in drift 

 or soft material. 2) The depth of water over the wasteweir, or 

 head of the channel, is to be considered in determining the hight 

 of the lake surface. 3) The hight of water surface was subject 

 to considerable variation, as in modern lakes. This variation 

 would be greater in the smaller lakes, where the difference 

 between the winter and summer melting of the glacier would be 

 a larger proportionate factor. 4) The deltas usually have several 

 levels or terraces, caused partly by changes in the lake level, but 

 specially by the lowering of the water surface as the lake was 

 slowly drained away. 5) In the southern ends of the valleys 

 terraces may be found to correlate with the two or more outlets. 

 6) The relation of the delta plain to the water surface varies 

 somewhat according to the coarseness of the detritus and the 

 rapidity of its deposition. The head of the delta should be a 

 few feet above the water level, and the point or front under the 

 same level. 7) Failure to find expected phenomena does not 

 always prove their nonexistence. And the positive nonexistence 

 of features which are theoretically expected does not necessarily 

 prove the absence of the agency. Shore phenomena^ in particular, 



