THE GEOLOGY OF NEW ZEALAND 



15 



As a result of this double process many interesting features 

 have been produced in the development of the topographic forms 

 and especially valley systems on this complex land surface. In 

 some regions, especially those composed of the older pre-Notocene 

 rocks, the control of the drainage by structural features is indicated 

 by the reticulated plan of the valley systems, the association of 

 valleys with marked hues of faulting, or with zones of fault-breccias. 

 This is especially noteworthy where narrow masses of the softer 

 covering strata have become involved in the fault-zone, but some 

 control of drainage by fault-zones may be observed even in regions 

 in which the soft covering rocks only are exposed. Probably not 



Fig. 5. — Diagram to illustrate the evolution of the present topography. A, eflfect 

 of block-faulting of region covered by weak "Notocene" sediments; B, modern to- 

 pography resulting from denudation of above surface and aggradation in the troughs. 



only the somewhat reticulate character of the valley systems in the 

 argillites, etc., of the northeast of the South Island has been thus 

 influenced, but also the rectilinearly branching valleys which now 

 form the fiords traversing the gneisses, etc., of the southwestern 

 portion of the same island. Some of the more open, though still 

 long and narrow depressions may be due, not merely to the differ- 

 ential erosion of a strip of soft material among harder rocks, but 

 even to actual trough-faulting. The disposition of the covering 

 rocks about one lake (Te Anau) in the southwest of the South 

 Island affords proof of such in this particular instance. 



Glaciation. — The topography does not, however, depend 

 actively on structure and normal differential subaerial erosion, for 



