A RECONSTRUCTION OF WATER PLANES 461 



for the U. S. Geological Survey. The method of measurement and of 

 assembling the data secured in this study is the same which had been 

 used in eastern Wisconsin in 1905. It was unnecessary in this case, 

 however, to spend much time in exploration; for Mr. Taylor had 

 selected a large number of localities where measurements could be 

 made most advantageously. Considering the shortness of the season, 

 therefore, the field covered was a large one, and the results obtained 

 were unusually complete. 



The old water planes, or imaginary surfaces of the extinct lakes, 

 are marked by a variety of shore features. One type which is common 

 on both the past and present shores is the cut bluff and bench. As 

 developed along the present shore of Lake Michigan, the steeply 

 sloping bluff or cliff rises from the water's edge, while the bench or 

 terrace at its base reaches out under water. The point at the base of 

 the bluff or the top of the bench is approximately the highest point 

 at which erosion by storm waves is effective. It is usually a little 

 above the normal lake level. Where bedrock cliffs instead of clay 

 bluffs form the coast, however, the bench is perhaps hkely to be a 

 little lower than lake level. There is a constructional variation here 

 of a few feet, but of only a few feet, in the case of Lake Michigan. In 

 making measurements on such a bench, to determine the altitude of 

 the old water plane, the base of the bluff was always taken. It is 

 the only determinable point that one can take as a standard. Care 

 was taken, of course, in making these measurements, to avoid places 

 where the bench had been built up by landslides, or by alluvial 

 wash down the face of the bluff, or where it had been gullied by 

 streams. A range of error of five feet would probably be quite enough 

 in this region to allow for discordances of benches due to original 

 constructional variation in height. 



Quite a different feature of topography is the beach or beach ridge 

 — a line of shore drift banked up by the waves at or close to the water's 

 edge, and rising only as high as storm waves can fling material. In 

 exposed places on the shore of Lake Michigan, beaches have been 

 observed whose crests stand fully six feet above calm water level. 

 As a rule, however, they stand only three or four feet above it. In 

 rare cases, where local conditions favor a heavy surf, the beaches 

 probably attain a height of eight or even ten feet. The crest of a 



