THE TWIN LAKES. GLACIATED AREA 289 



sides of the river, and by the fact that the gravel east of the river 

 contains material recognizable as derived from the valley of Lake 

 Creek, and not found east of the Arkansas. Through this sheet of 

 gravel the Arkansas cut its valley, and then 300 to 400 feet into the 

 rock. These higher gravels are therefore older than the rock-valley. 

 But it is with this older series of gravels that the outer moraine is to 

 be correlated. Fig. 3 shows the relations that hold along the whole 

 stretch of the old moraine north of the river. A is the later moraine; 

 B, the earlier moraine; C, the upper gravel-terrace. On the rock- 

 terrace at D, scattered bowlders or thin gravel veneer mark the 

 former eastward extension of the terrace. E is the gorge gravel of 

 younger date. C is the wash-plain to the outer moraine B, just as 



Fig. 3.- — Profile from lower Twin Lake northeast to the Arkansas. Symbols as 

 in Fig. 1. 

 Horizontal scale, i inch=2,ooo feet; vertical scale, i inch=5,ooo feet. The section is located on the map. 



the gorge gravels are to the later moraine. These relations of the 

 moraines to the gravels show that the periods of ice-advance indicated 

 by the two moraines are separated from each other by a time interval 

 sufficient for the Arkansas to cut its gorge 300 to 400 feet below the 

 floor of its broad, graded, preglacial valley. When one comp:res 

 the work done in this period with that done by the river in postglacial 

 time, in which it has not been able completely to clean out of its val- 

 ley the gravels of the later glacial period, one can get an idea of the 

 relatively long time separating the two periods. 



B. Differences in weathering of the two moraines and their asso- 

 ciated wash-plains. The great difference in age of the two glacial 

 advances is further shown by the difference in the amount of weather- 

 ing of their deposits. The later moraine, as seen in cuttings, is every- 

 where fresh and unweathered. The large stretch of older moraine 

 north of Lake Creek does not show sections which will enable one to 

 make a comparison with the later drift. The western of the two 

 occurrences south of Lake Creek, however, shows a long section 

 where the moraine is crossed by an irrigating flume. The moraine 

 here is thoroughly disintegrated, bowlders crumbling to pieces 



