DRAINAGE EVOLUTION IN CENTRAL NEW YORK 669 



The second map exhibits the hypothetical Tertiary drainage. During later 

 Tertiary time all the drainage of the west half of the state was diverted west- 

 ward (subsequent) and northward (obsequent) into a great trunk stream that 

 occupied the Ontario and Erie valleys and probably drained westward into the 

 Mississippi basin. The cause of this radical reversion of flow was the great 

 thickness of non-resistant rocks in the Ontario district. In the vertical series 

 of strata between the Trenton and the Portage, on the Cayuga meridian, are 

 5,150 feet of rock, of which 4,500 feet are weak shales, 350 feet limestone, and 

 250 feet sandstone. 



The pre-Glacial divide was far south in Pennsylvania. The Allegheny sys- 

 tem poured north through the lower Cattaraugus valley. The upper Genesee 

 was tributary to the broad Dansville-Avon river, which almost certainly had 

 its northward course through the Irondequoit valley. The Susquehanna turned 

 west from the site of Lanesboro and Susquehanna villages along the strike of 

 the Chemung strata (which were less resistant than the overlying Catskill) , past 

 the sites of Binghamton, Owego, and Waverly, and then curved north through 

 the sites of Elmira and Horseheads and occupied the Seneca valley. The Che- 

 mung was the principal tributary from the west, as today, but it passed north 

 of Elmira instead of south, where it now lies in a post-Glacial cut. 



The Delaware and the upper tributaries of the Susquehanna were not di- 

 verted from their southwest courses. 



Along the Ontario lowland the Tertiary channels are almost entirely de- 

 stroyed or obscured by drift, but the valleys of Irondequoit, Sodus, Little 

 Sodus, and Fairhaven are trenches across the Niagara-Medina scarp, which 

 probably represent the northward pre-Glacial flow. Today only two large 

 streams pass across this rock ridge, the Genesee and Oswego, both in new 

 channels. It seems probable that along the belt of Salina outcrop the pre- 

 Glacial tributary streams flowed east or west as they do today. 



It is suggested that the "oversteepened" walls of the bottom sections of 

 the Finger Lakes valleys were produced by the rapid down-cutting of the 

 streams during the Tertiary uplift. 



The third map shows the principal stream flow as compelled by the ice- 

 sheets. A few strong south-leading valleys were enlarged or newly cut by the 

 concentrated glacial waters, and the Allegheny and Susquehanna systems were 

 turned to the south. In order from west to east the glacially developed valleys 

 are Cassedaga, Conewango, Ischua, Canisteo, Cohocton, Cayuta, Cattatonk, 

 Tioughnioga. These southeastward drainage lines, transverse to the primitive! 

 flow, were carved from numerous, short, subsequent valleys by the stream flow 

 forced to the southward by the ice-damming. Such flow was effective during 

 the advance of the ice-sheet, but stronger during the waning of the ice, and 

 probably more than one ice invasion has been concerned. 



On the Ontario lowland the forced drainage was west or east, alongside the 

 ice margin. In the Erie basin the later flow was all westward past the ice 

 front. In the Mohawk valley the drainage between Little falls and Rome was 

 turned from west to east. 



The water-parting which in pre-Glacial time lay in Pennsylvania has been 

 so changed by glacial flow that it now lies close to the Finger lakes. 



Between the two or more ice invasions lang epochs of warm climate prob- 

 ably permitted some development of valleys, which would be more or less 



