4 Transactions. 



II. Physiographical.* 

 (A.) General. 



The lower portion of the Cass Valley alone concerns us here. This area 

 consists of a flood-plain about 7 km. long by 2-5 km. wide. The actual 

 river occupies a comparatively small strip in the centre of the plain, while 

 on either side there extend, east and west, a series of low, flat terraces. 



Strictly speaking, the term " bed " is applicable only to that part of 

 the valley actually covered by the flowing water. But the Cass, like most 

 New Zealand rivers, is continually liable to flood, at which time the river 

 covers a much wider strip than usual. The river, moreover, even normally, 

 flows as a network of anastomosing streams, and in time of flood these 

 channels become wider and meet, until there is a considerable breadth 

 of running water. In this paper the term " river-bed," following the col- 

 loquial usage, is used to include that part of the valley liable to be covered 

 with water in times of severe floods. This " river-bed " is bounded on 

 either side by a broad expanse of flat land, chiefly tussock-covered, termed 

 colloquially " river-flat," but in this paper " terrace." This area is at no 

 time subject to complete inundation, and probably only in times of very 

 severe floods will the lower-lying portions of it be slightly submerged. 



Now, it is the whole flood-plain which supports the cushion- and mat- 

 plants dealt with, but the chief area, the richest and the most interesting, 

 is the river-bed itself, only a few species of the cushion types growing on 

 the terraces. 



The river-bed consists of various areas varying from bare shingle to a 

 well-defined plant-association. These areas merge into one another, but 

 certain well-defined stages occur, and for purposes of comparison and re- 

 ference it has seemed advisable to adopt some scheme of classification of 

 the various portions of the river-bed. The lowest grade may be termed 

 " bed proper." Above this are various grades which, becoming more and 

 more consolidated and more and more peopled by plants, finally merge 

 into the mature " terrace." These grades are termed " transition terrace," 

 and are placed in three grades, called respectively transition terrace 1, 2, 

 and 3. It must be clearly understood that these grades merge into one 

 another, 



(B.) The Bed Proper, and the Terraces. (Plate I, figs. 1 and 2.) 



The bed proper is obviously the simplest grade to define ; it is not neces- 

 sarily the lowest part of the valley at any cross-section. It consists of a 

 mass of water-worn greywacke rock the constituents of which vary from 

 fine sand to boulders 0-25 m. and more in diameter, the larger ones pro- 

 jecting strikingly above the general surface. It is through such shingle 

 that the river flows, being confined to various channels which are continually 

 changing. An old channel may be deserted for a new one ; and the older 

 one, if not used considerably during frequent floods, soon commences to 

 merge into transition terrace. The chief point about the bed proper is its 

 absolute freedom from a plant-covering. 



The transition terrace, grade 1, is the first stage after the unpeopled bed ; 

 it is sparsely covered with plants. It is that part of the river-bed no longer 

 liable to be used as a channel except in case of moderate floods. It has a 



* For a detailed account of the physiography of the area adjacent to the River 

 Cass, see Speight, 1916. 



