PRIMARY UNIT OF THE SUBDIVISION 



133 



coast region (see figure 9)^ Speaking of the same coast of Greenland 

 at a point some three degrees farther to the north, von Drygalski^ shows 

 how individual clefts in the rock can be seen to be widening by frost 

 action and also extending their heads backward into the plateau. From 

 this stage, he says, it is but a step to the formation of the long extended 

 valleys, which, like the smaller ones, always leave upon one the impres- 

 sion of a long cleft. 



Figure 8. — Small Relief Unit determined hy individual and consecutive Joints 

 Holstensborg, West Greenland. (After Kornerup) 



Drainage Networks 



NORMAL NETWORK IN ROCKS DEVOID OF STRUCTURE PLANES 



The pattern of the relief as it is brought out upon maps is in large 

 measure expressed in the drainage network. The normal pattern of this 

 network upon an upland underlain by homogeneous rock which is devoid 

 of structure planes, should be arborescent or branching like a tree. Just 

 as the tree trunk and its branches usually fork so that branches and twigs 

 point slantingly upward toward the sun, so the branches of the stream 

 network are similarly inclined in the direction of the divide. Under 

 these conditions there must be a normal angle of junction to which 

 streams approximate (see figure 10). Such a normal arborescent drain- 

 age may be referred to as the normal drainage network for a maturely 

 eroded district. The intricacy of the branchings — the scale of the de- 



8 a. Kornerup : Loc. cit., pi. vi, fig. viii, and pi. viii. 



8 E. von Drygalski : Gronland Expedition der Gesellschaft fiir Erdkunde zu Berlin 

 1891-1893, vol. 1, 1897, p. 37. 



