GLACIATION OF AN ICELANDIC VALLEY 



127 



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seems inexplicable, except through the action of an advancing 

 glacier, thus showing, with the earlier advance supplied by the ground 

 moraine below it, a twofold advance and retreat of the ice during 

 the period represented by this sedimentary series. 



Above the contorted tufaceous sandstone is almost 100 feet of flat- 

 lying ripple-marked sandstone, alternating between layers of fine clay 

 and sandstone, the latter 

 often spotted with pebbles 

 of vesicular basalt. The 

 clay layers were generally 

 about an inch in thickness, 

 while the sandy layers were 

 often a foot thick and 

 seemed to represent the 

 annual flood of the stream. 

 The ripple marks, by the 

 steeper slope of the ripple 

 ridges, showed a south- 

 westerly direction of flow. 

 In one place the sand- 

 stone was cut by a basalt 

 dike which probably served 

 as a feeder to some of the 

 upper lava flows. 



The third moraine in 

 this section did not exhibit 

 any marked peculiarity. Like the second, it was overlain by tufaceous 

 sandstone, in this case nearly flat-lying and not ripple-marked. 



About 5 miles down the valley, to the westward from the main 

 section, there is a good exposure of what is probably the second 

 moraine of the first series. Here the overlying contorted sandstone 

 is entirely lacking, and the sandstone above the moraine is more 

 evenly bedded, shows no ripple marks, and is much more conglom- 

 eratic, containing several subangular bowlders, which at first sight 

 gives it the appearance of morainal material. 



Higher up on the cliff the third moraine is well exposed in a small 

 gorge, and consists of about 10 feet of indurated moraine, contain- 



FiG. 5. — Ripple-marked sandstone above the 

 second moraine. 



