176 W. H. HOBBS — EEPEATING PATTERNS IN STRUCTURE OF LAND 



be said to measure the amount of denudation, modified, however, by rock 

 composition and texture. Similarly, the rock folds and the fractures are 

 produced by what we commonly call orogenic movement — the tangential 

 rather than the vertical component of earth stresses — and the degree of 

 deformation resulting is, therefore, measured in terms of these forces. 

 Just as the positions of individual folds depend on inherent existing 

 structures — initial dip, thickening of formation, etcetera — so the localiza- 

 tion of the zones of excavation hy the denuding agents which attach the 

 surface is fixed hy fracture structures already existing at the time — a fact 

 of the first importance and one strangely overlooked by the modern 

 American school of geomorphology. 



Carrying our analogy one step further, the shape of the folds — whether 

 open or closed, symmetrical or overturned — ^will be determined largely by 

 the nature of the rock layer and by the stage of the process of deforma- 

 tion. So, too, it can not be too strongly emphasized that the shaping of 

 the erosion surface is an expression of the nature of the rock, the agent 

 of excavation, and the stage which has been reached in the process. It is, 

 however, largely independent of the position of valleys in the plan. The 

 conditions of the shaping may be largely read in the landscape patterns — 

 the character profiles; the position of valleys, on the other hand, in the 

 relief patterns brought out on the map. 



As already stated, the present paper is a summary made from a larger 

 study on which the writer has been engaged for a number of years. He 

 would greatly appreciate and gratefully acknowledge the communication 

 of facts or the reference to literature which bears on the subject. It is 

 suggested that studies of local relief in relation to fracture structures 

 made under proper oversight are eminently adapted for assignment to 

 students as thesis problems.^^ 



" See Harder, loc. cit., and Llnd, loc. cit. 



University of Michigan, Ann Arbor, Michigan, U. S. A, 



