WATER-PLANES AND CRUST AL DEFORMATION 349 



former larger fluctuations in water-levels, and with other points that 

 will be taken up in categorical order, 



1. The exact position of an old water-plane is very difficult to 

 determine because of its variable relation to different beaches, ter- 

 races, and other shore forms. The common practice has been to take 

 the top of a beach ridge as fixing the position of the old water-plane, 

 "although it is of course recognized that the beach ridge probably 

 stood anywhere from 3 to 6 feet above the actual water-plane."^ If 

 in the case of a true beach ridge the actual water-plane will average 

 3 feet below the top of the ridge, elevations should be corrected by 

 this amount in order to bring them as near the true water-plane as 

 possible. If observations were entirely upon beaches this correction 

 would be a constant and might be omitted in considering the rela- 

 tive displacements suffered by the shore-fines. But as they are also 

 made upon terraces, spits, and bars, which require different assump- 

 tions in regard to the height of the water-plane, the above correction 

 should be appfied, otherwise the observations are not comparable 

 within the assumed limit of 3 feet. In the case of a cut terrace the 

 foot of the bluff is taken as fixing the position of the water-plane, 

 although the true water-plane may have been as much as 2 feet either 

 above or below that point. Where a terrace is cut in unconsolidated 

 material the actual water-plane would in most instances be above 

 the foot of the bluff and a correction might be applied to allow for 

 this. Where terraces are cut in rock its location would depend upon 

 the character and dip of the strata. In this instance a close study of 

 present conditions might furnish a correction that could be applied 

 in certain cases at least. 



2. The variability in the position of a beach ridge with respect to 

 the water-plane may particularly be noted. While a "normal" 

 beach may be 3 feet above the surface of the water, storm beaches as 

 much as 6 feet above the lake level may be considered as common, 

 and a few have been reported at still higher elevations. Unless such 

 beaches are situated so that their true nature can be recognized by 

 comparison with neighboring shore -line features, they may introduce 

 serious errors. One will be noted in the next paragraph. Storm 

 beaches are formed, of course, only on the exposed parts of the shore- 



I Goldthwait, op. cit., p. 100. 



