HISTORY OF THE RIVER 299 



upper Ohio drainage, with large additions from the melting ice, were 

 dammed, forming lake Monongahela, and the outlet was then either 

 across some of the divides in West Virginia, as suggested by Doctor 

 White,* or over the divide between the upper and middle Ohio drain- 

 age systems, at or near the line of the present stream. At different 

 times it may have discharged in both directions. During the retreat of 

 the ice following the Kansan stage, the line of discharge in the course 

 of the present Ohio was established as we now find it, and it continued 

 to cut until the streams of the upper Ohio region flowed below present 

 stream level. This cutting is quite narrow, and on the lower 12 or 14 

 miles of the Beaver it is bounded by almost vertical walls, but north of 

 Wampum this "inner" gorge is much wider and its walls are well 

 rounded. 



It is not yet determined whether the deep cutting of the " inner " 

 gorge followed immediately on the retreat of the Kansan ice-sheet, or 

 marks the work of a later portion of the time between the Kansan and 

 Wisconsin invasions, as there are scattered evidences on some of the 

 remnants of the bed of the north-flowing stream that seem to indicate 

 the old level had not been abandoned, at least permanently, until after 

 the time of the lowan loess. 



Following this stage of the river's history came the Wisconsin stage 

 of glaciation, with its immense accumulations of alluvial materials along 

 the drainage lines open to the southward, and this in turn was followed 

 by the present stream, still eroding the Wisconsin gravels. 



Figure 1 shows some of the features of the lower portion of Beaver 

 valley, and the relations of the present stream and the old north-flow- 

 ing stream to the various strata. The efl'ect of the great thickness of the 

 Homewood sandstone in restraining the cutting by the old river is 

 plainly seen. To simplify the map, no contours are shown between the 

 900 and 1000 foot lines. The 900-foot line clearly indicates the almost 

 absolute level of this portion of the old fluvial plain, and the 1000-foot 

 line shows how uniform was the width of the old valley. The contours 

 below 900 feet are only given at the mouth of the Connoquenessing to 

 indicate the existence of two benches at that point. At other places the 

 lower contours generally fall w^ithin the lines of the narrow " inner " 

 gorge. The contours near Rock point higher than 900 feet indicate 

 morainic deposits on the old fluvial plain. The top of the Homewood 

 sandstone at its type locality (Homewood) is somewhat uncertain. 

 Doctor White, from whose section I have taken the thickness at this 

 point, says the top of the Homewood sandstone is concealed. It rises 



* American Geologist, vol. xviii, pp. 368-379. 



