402 THE JOURNAL OF GEOLOGY. 



topography of lake shores has already received attention from 

 one skilled in reading geological history in the relief of the 

 land' and the study of existing shores in the light of what has 

 already been done in that direction should enable even the begin- 

 ner to avoid falling into serious error in interpreting ancient rec- 

 ords of the same nature. 



To be able to discriminate clearly between shore features and 

 somewhat similar glacial phenomena, it is necessary to become 

 familiar also with the topography of glacial deposits. Fortu- 

 nately in this study also a guide is at hand ^ which, in connection 

 with field observations, should soon train the eye to discriminate 

 the shapes assumed by moraines and the deposits of glacial 

 streams from all other topographic forms. 



In examining the records of former lakes it will soon be 

 observed that, in many instances, where the highest of a series of 

 ancient beaches is obscure and indefinite, the topographic expres- 

 sion above and below a certain horizon, and also the character of 

 the surface material, whether of the nature of lacustral clays and 

 sands or of glacial debris, residual clay, etc., above and below 

 the same level, are significant, and enable one to map the outline 

 of a former water body with considerable accuracy. 



In tracing ancient beaches and terraces, their forms and inter- 

 nal structure need to be recorded, so that the fact of their being 

 true shore records may be made plain to others. The elevations 

 of various well-defined points throughout the extent of an 

 ancient shore should be carefully measured, for, as will be noticed 

 below, although originally horizontal, they have, in many 

 instances, been elevated or depressed, owing to broad general 

 movements of the earth's crust. The continuous tracing of indi- 

 vidual shore lines for as great a distance as possible is highly 

 desirable, especially in a wooded country, in order to be positive 

 as to which ridge or terrace measurements of elevation relate, and 

 and also for the purpose of observing the nature of the changes that 



' The Topographic Features of Lake Shores, by G. K. Gilbert, in Fifth Ann. Rep. 

 U. S. Geological Survey 1883 -4. 



^ Preliminary Paper on the Terminal Moraine of the Second Glacial Epoch, by 

 T. C. Chamberlin, in Third Ann. Rep. U.S. Geological Survey, 188 1 -2. 



