92 A SUMMER IN GREENLAND 



The main ravine at Atanikerdluk (Fig. 45) is 

 not only of special interest geologically; it also 

 affords a most remarkable display of dykes and 

 illustrates on a grand scale the relation between 

 scenery and rock structure. A stream flows among 

 jumbled heaps of boulders at the bottom of a 

 steeply inclined valley; the valley slopes consist of 

 natural embankments of loose, light yellow sand 

 mixed with milk-white rounded pebbles of quartz 

 detritus formed by the erosion of the sandstones 

 which are here the dominant rocks and in places 

 the talus slopes are replaced by exposures of the 

 rocks themselves, thick beds of sandstone with no 

 division into layers, thinner, well-stratified beds, 

 bands of shale, and an occasional seam of coal. 

 These sedimentary strata, having a total thickness 

 of many hundred feet, exhibit here and there on 

 an exposed plane of bedding a series of ripple- 

 marks, and afford other evidence of their origin 

 as sheets of sand and mud in shallow water and 

 among drifting currents. Many of the sandstones 

 are made up of thin layers, often rendered more 

 conspicuous by the presence of iron-stained bands, 

 which exhibit the well-known arrangement spoken 

 of as current-bedding : a series of layers sloping at 

 a certain angle is cut off by another set sloping in 

 a different direction. This frequent variation in 

 the lie of the thin beds is evidence of the deposition 

 of the sandy sediment in water with eddying and 

 shifting currents. Dark brown dykes cut across 

 the sands and sometimes intersecting dykes pro- 

 ject like part of a huge network; but the most im- 



