278 G. r. WRIGHT— POSTGLACIAL EROSION AND OXIDATION 



tion. Even a cursory examination of these streams can not fail to impress 

 the observer with the small amount of work which has been done by 

 them; while in the case of Plum Creek, in Oberlin, an unusual oppor- 

 tunity has been offered for definite calculations. 



Plum Creek is a branch of Black Eiver, draining an area of 25 or 30 

 square miles. At Oberlin the elevation of its bed is 800 feet above sea- 

 level, or approximately 235 feet above Lake Erie, which is distant 10 

 miles in a direct line. The descent from Oberlin to the falls in Black 

 River at Elyria, 8 miles distant, is 100 feet, or 12 feet to the mile. But 

 the bottom of its trough averages 17 feet below the general level of the 

 country. This trough is entirely one of erosion, the original stream hav- 

 ing begun its work 250 feet above the level of Lake Erie and 50 feet 

 above the 200-foot shoreline, which is 5 miles to the north. The region 

 is so deeply enveloped in till that the underlying rock is nowhere exposed 

 in the bed of the creek. The entire trough has been eroded in till, so 

 that there is no complication of rock barriers requiring an indefinite time 

 for removal. 



A section of this trough 5,000 feet long, where it had been least modi- 

 fied by artificial interferences, was found to average 400 feet in width 

 and 17 feet in depth, showing that a total amount of 36,000,000 cubic 

 feet of till had been removed by the stream from tliis section since the 

 beginning of its flow. 



Opportunity to obtain an approximate estimate of the rate of erosion 

 effected by the stream was fortunately furnished by the town's taking 

 possession of its trough for a reservoir and their turning the drainage 

 around through an open ditch, 14 years ago. We have thus been able 

 accurately to determine the rate at which this stream removes the mate- 

 rial under the new conditions produced by this change in its course. On 

 measuring a section of this new channel, 500 feet long, and noting from 

 year to year its enlargement, it was ascertained tliat the original ditch, 

 which was 21 feet wide at the top and 10 feet at the bottom, had in 13 

 years been enlarged by the erosion of the stream to a width of 51 feet at 

 the top and 17 at the bottom, showing that, from this 500-foot section, 

 8,450 cubic feet of material had been removed every year. 



But, to make the estimate applicable to the older section of 5,000 feet, 

 where the erosion represented the entire work since the opening of the 

 Fort Wayne outlet, it was necessary to go into this 5,000-foot section and 

 measure the length of the sections where the present stream is impinging 

 against the original till bank and eroding it under conditions similar to 

 those existing in the 500-foot cut-off. As will be seen on slight reflec- 

 tion, in the 500-foot cut-off* the stream is acting directly upon 1,000 feet 



