Reports and Proceedings — Geological Societg of London. 93 



Chalk escarpment. The greatest depth to which the channel has been 

 proved is at a boring in Hitchin, where the Gault was reached beneath 

 Drift at a depth of 68 feet below sea-level. That the channel flowed 

 northwards and belonged to a ' subsequent ' stream seems to be proved 

 by the fact that at Bragbury End, the only place where a southerly 

 stream could pass, the space between bare Chalk exposures is but 450 

 yards, and in about the middle of the space Chalk has been reached 

 within 50 feet of the surface (that is, about 200 feet above sea-level) 

 in a well dug a few years back. The channel must be older than the 

 Chalky Boulder-clay, which still partly fills it as far south as Langley, 

 and may have blocked it to the southward and given rise to the 

 features now presented in the drainage on the northern slope of the 

 escarpment. But the author is inclined to suggest that either glacier 

 ice or bay ice must have played no unimportant part in damming up 

 the old valley. The author suggests the existence of another channel, 

 in this case draining southwards, buried under the broad area of 

 Boulder-clay and gravel which lies immediately south of Stevenage 

 and to the north as far as Letchworth and Wilbury Hill. But a narrow 

 space of bare Chalk, at an elevation of 240 feet O.D. connecting large 

 areas east and west of it, precludes the occurrence of a channel farther 

 north than Letchworth. 



11.— Januanj 8th, 1908.— Sir Archibald Geikie, K.C.B., D.C.L., 

 Sc.D., Sec. U.S., President, in the Chair. 



The following communications were read : — 



1. " Chronology of the Glacial Epoch in Korth America." By 

 Professor George Frederick Wright, F.G.S.A. (Communicated by 

 Professor E. J. Garwood, M.A., Sec. G.S.) 



In the case of Plum Creek, Lorain County (Ohio), the study of the 

 activity of the stream and of the amount of work which it has done 

 since a certain stage of the Glacial epoch has yielded important 

 results. This stream began the erosion of its trough when the 

 temporary lake, held up in front of the ice, was maintained for 

 a considerable period at the level of its Fort Wayne outlet; it has 

 never had anything more resistant than Till to act upon. From 

 a given section, 5,000 feet long, it has excavated 34 million cubic feet 

 of Boulder-clay, removing it from exposed banks 1,600 feet long. 

 Twelve years' erosion of a 500 foot length of a part of the trough of 

 the stream under observation, and from banks 1,000 feet long, gives 

 a rate of 8,450 cubic feet per annum. Therefore, the removal of 

 34 million cubic feet from the 5,000 foot section would give a period of 

 2,605 years. Considerations tending to lengthen the estimate are the 

 former afforestation of the area and the increased gradient in the arti- 

 ficial cut-off. Those tending to shorten the estimate are the present 

 wider flood-plain, the time taken for forests to grow, and the probably 

 greater former water-flow. 



The erosion of the Niagara Gorge began considerably later than 

 that of Plum Creek, and probably dates from midway between the 

 disappearance of the ice from Northern Ohio and from Quebec. If 

 conditions have been uniform, the age of the Gorge would be 7,000 



