Speight. — Physiography of the Cass District. 151 



almost all the down country in the angle between this river and the Wai- 

 makariri was covered, and at the height of the glaciation the ice reached 

 through the main gorge of the latter river as far as its junction with the 

 Kowhai. Erosion modified the shape of the valleys, and deposits of angular 

 material, such as the moraines at the lower end of Lake Pearson and be- 

 tween that lake and Grasmere, as well as the great mass of angular debris 

 which forms St. Bernard Saddle, marked stages in the recession of the 

 ice-flood. 



It must not be assumed that all of this was due to ice which invaded 

 this territory from the Waimakariri. The basin itself must have acted in 

 some, measure as a collecting ground, especially the Upper Cass Valley and the 

 hollows of the Craigieburn Mountains ; but the area lies too far to the east 

 of the main divide to allow of any great accumulations of snow, most of 

 which would have been intercepted by the higher range to the west. The 

 Craigieburn Mountains do show, however, the shell-shaped hollows at high 

 levels, some of which certainly held small glaciers of the " corrie " type. 

 On this range they are usually filled with debris at the present time, which 

 is frequently piled in rough ridges across the lower end of the hollow — 

 either a mbrainic accumulation from the glacier period or due to the rolling 

 of loose stones down the frozen snow slopes from the exposed ridges above. 

 In the wetter regions, where plants have a better opportunity for esta- 

 blishing themselves, they form ideal spots for the formation of colonies of 

 alpine plants. The immediate vicinity of the Cass Station is not altogether 

 favourable for examining them, except as to the accumulations of debris, 

 but a short journey to the mountains near the Hawdon enables them to 

 be seen in typical development. 



When the ice retreated and left the vicinity of the Cass considerable 

 changes took place in the drainage. These resulted chiefly from two 

 causes : First, the deposit of morainic material caused interference with 

 the normal directions of streams, an interference which might have been 

 slowly overcome had not other causes accentuated it. Transitory lakes, 

 such as Lakes Sarah, Grasmere, and Pearson, probably occupied inequalities 

 in the floors of the valleys, and these M^ould have discharged along the 

 original lines of flow — approximately, at any rate. Now, Lake Pearson does 

 this, but it is slowly disappearing owing to infilling from the sides, and also 

 to its cutting down the dam which blocks it towards Broken River, large 

 areas of drained lake bottom being there visible. Lake Grasmere, how- 

 ever, being more within the sphere of iiifluence of the other cause, has 

 reversed its direction, and has not proved competent to lower the barrier 

 at its southern end. The second important factor has been the over- 

 deepening of the main bed of the Waimakariri in the vicinity of the 

 Cass by the excavating action of the powerful ice-stream which moved 

 down the main valley. The ice here would be much thicker than that 

 in the valley near the middle Cass, and, as power of excavation depends 

 directlv on the thickness of the ice, the floor of the main stream 

 would be lowered much below the level of that of the Cass. This over- 

 deepened portion was in all probability occupied by a lake after the ice 

 retreated up-stream towards the Bealey, until the rock bar in the neigh- 

 bourhood of the mouth of the Esk and below it was removed by erosion. 

 When this occurred the effective base-level of all the streams near the 

 Cass would be lowered also, and it is easy to understand, if the ice 

 had not already formed a gap in the rock bar at the mouth of the Cass, 

 why it was cut through, and, if the gap bad been formed by the ice or 



