C. A. Cotton — Block Mountains in Nevy Zealand. 255 



during the^ period preceding the deformation. The approx- 

 imate position of such a nonburied area of the iindermass or the 

 source of the material forming tlie covering strata furnish 

 evidence for the direction of possible antecedent streams. 

 Without such knowledge only streams which cannot be placed 

 with certainty in any other category may be classed as possibly 

 antecedent. The relation of valley directions to the trend of 

 elongated blocks may give information. If, as in Otago, the 

 deformation has produced longitudinal tectonic features — par- 

 allel elongated arches and troughs or long tilted blocks sepa- 

 rated by fault angles — both true consequent and anteconsequent 

 drainage will follow generally longitudinal courses, though per- 

 haps breaking across here and there from one linear series of 

 depressed areas to another. Antecedent drainage, on the other 

 hand, may cross the longitudinal features diagonally or trans- 

 versely. 



4. Sequential Forms. 



Davis has recognized two main elements in the form of a 

 simply tilted block mountain, the back slope and the front or 

 scarp. These are the two main elements in the whole initial 

 landscape of a region of tilted blocks. 



In a region that has been affected by diverse movements, the 

 elements of the initial landscape are horizontal areas (high or 

 low-lying), back slopes (areas of surface with a more or less 

 uniform and gentle slope), fold surfaces (areas of steeper slope, 

 not necessarily uniform), and fault scarps. There will be 

 transitions from fold surfaces through what may be termed 

 fold scarps to fault scarps. The present problem is to trace 

 the development of topographic features from an initial surface 

 comprising these various elements, upon which a system of con- 

 sequent drainage becomes established. After postulating an 

 initial relief caused by instantaneous deformation, it is possible 

 to consider the effects of erosion and a continuation of the 

 deformation going on simultaneously. 



In the stage of extreme youth, the block fronts (fault and 

 fold scarps) will experience the mo&t rapid changes of form 

 and, as a result of slumping and the formation of consequent 

 gullies, these w^ill supply the largest quantity of waste. Very 

 early, also, the perhaps closely spaced consequent streams of 

 considerable length, which w^U have come into existence all 

 over the relatively large areas of inclined block surfaces, will 

 be actively engaged in grading their courses (see tig. 2). 



From the rapidly deepened consequent valleys, insequent 

 tributaries will be developed, and probably also subsequents. 

 Since the spacing of the consequents alone may be close and 

 the texture of dissection becomes liner when insequents and 

 subsequents have also been developed, and since entrenchment 

 of the whole system beneath the sloping surface of the weak 



