CRITERIA REQUISITE TO A GLACIAL AGE 75 



of the valley has been brought into working harmony. The earlier 

 readjustment plains may often be seen a few miles below the 

 terminal moraines on which the true glacio-fluvial plains take 

 their origin. At such points the valley is often filled from bluff 

 to bluff by a smooth plain of gravel of glacial aspect, and the 

 impression is apt to grow into great strength that this is the 

 true glacial flood plain. But as it is traced up toward the 

 moraine, terraces of higher altitudes and higher gradients are 

 seen to rise on the sides of the valley, and grow in strength 

 and continuity until they clearly disclose the earlier plain of 

 which they are the remnants, and this plain is found to merge into 



Fig. I. — ^Illustration of the successive formation of a true glacio-fluvial and two 

 adjustment plains. M, moraine at which the true glacio-fluvial plain heads ; H, head 

 of the valley train of true glacio-fluvial gravel : AAA down-stream portions of the 

 same; BBB, first adjustment plain, lower than glacio-fluvial plain at the right, and 

 higher at the left ; CCC, second adjustment plain crossing the horizons of both the 

 preceding. The portions of the adjustment plains at the left are liable to be mistaken 

 for the true glacio-fluvial plain. 



the moraine, while the other plain of lower gradient is found to 

 connect with a channel cut through the moraine. In such a 

 declared case, it is clearly seen that the plain of lower gradient 

 (and lower position at the moraine) is the higher plain a few 

 miles down the valley, and is there altogether likely to be taken 

 for the plain of true glacial date. Still farther down the valley 

 a yet later adjustment plain may be uppermost and prevailing, 

 and so liable to be misinterpreted. Errors of interpretation of 

 this kind are liable to be made within five miles of the moraines 

 on which the gravel trains head. 



Amount of the errors. — At such a distance probably the amount 

 of error in time may not be great, but we really do not know 

 how fast or how slow such a readjustment takes place. While geo- 

 logically rather rapid, it may be rather slow in terms of human 

 migration. In the nature of the case, the farther the locality is 



