WATER-PLANES AND CRUST AL DEFORMATION 353 



met only with partial success. The terraces which appeared so sharp 

 and unequivocal from the railway station lost their character when 

 approached closely, and could be recognized as terraces only with 

 considerable doubt."' What error may be introduced through the 

 alteration of original forms is difficult, perhaps, to say. In the case 

 of beaches it would be small as compared with that resulting from the 

 variable height of a beach above the water-level. For terraces the 

 error would generally be somewhat greater, since the base of a bluff is 

 frequently obscured by talus, wash, etc. An error of i foot may be 

 assumed as due to this cause. 



It will be recognized, of course, that observations in the field, that 

 is, our so-called geologic facts, vary greatly in value, and in any 

 refined work should be weighted accordingly. Observations on the 

 ancient shore-lines, however, serve two purposes. The first is to 

 unravel the history of the glacial Great Lakes and as continuous 

 tracing of shore-lines is a very important part of this work, observa- 

 tions should be as numerous as possible. The second is to determine 

 the character of crustal movements in the Great Lakes region. This 

 is a different problem and the value of results depends upon the prob- 

 able error of observation. In refined work this error should be reduced 

 to a minimum, for which reason it would seem advisable to use, if 

 possible, only those observations that have small errors. 



5. Progressive climatic changes over long periods of time may 

 also have produced important variations in the water-levels of the 

 lakes. That such climatic changes occurred may reasonably be 

 inferred from the pronounced movements of the continental ice sheet 

 during that period. This factor may well have been of considerable 

 importance, but is at present undeterminable. It is simply presented 

 as one of the unknown factors in the problem of the history of the 

 ancient Great Lakes which might alter conclusions were its value 

 known. 



6. Secular changes in the water-level due to (i) show crustal move- 

 ments, and (2) changes produced by erosion in the elevation of outlets 

 are to be noted. Errors arising from these sources may eventually be 

 eliminated — ^when the history of the ancient lakes has been worked 

 out in fuller detail. Attention, however, may be called to one condi- 



I op. cit.; p. 263. 



