Speight, Cockayxe, Laing. — Mount Arrowsmith Dixfricf. 341 



by experience in the fiord region of the south-west of New Zealand, where 

 the landscape-features are apparently inexplicable on any hypothesis whicli 

 denies to glaciers the possession of marked powers of excavation, though it is 

 possible that we are not aware of all the factors which control this power. 



7. Changes in' Drainage in the Rakaia Valley. 



The case of the change of drainage in connection with the Lake Heron 

 Valley has been referred to several times previously. In the pre-glacial 

 river system the Cameron undoubtedly drained south toward the Ashburton, 

 and in all probability a small tributary ran north to the Rakaia from a 

 divide between that river and Lake Heron. This divide was lowered by 

 glacier-action, and the drainage was reversed. The change resulted largely 

 from two causes — viz., (1) the piling-up of a barrier across the Lake Heron 

 Valley at -the south end of the lake, and (2) the lowering of the bed of the 

 Rakaia by the erosion of its great valley glacier. The Lake Stream has 

 thus been given a high gradient, and it has therefore rapidly removed any 

 barriers that may have existed to the north of the lake, and has degraded 

 the rocky ridge which formed the lake's containing-wall on that side, so 

 that its size has been materially reduced from that which it had immediately 

 succeeding the retreat of the glaciers. The great swamp north of Lake 

 Heron is the old bed of that lake in its extended form. 



The overdeepening of the bed of the Rakaia also allowed the small rock- 

 bound lake at the junction of the Lake Stream and the main river to be 

 emptied as well. 



In other parts of the Rakaia Valley drainage anomalies occur, the 

 most remarkable being that of Lake Coleridge. This lake occupies a 

 valley parallel to that of the Rakaia, down which the Wilberforce River 

 and glacier once flowed. A great terminal moraine was left by this glacier, 

 which was also added to by a lateral one from the main Rakaia, across the 

 south end of the lake, blocking the drainage in that direction. The effect 

 was accentuated by the lowering of a saddle in the ridge between the upper 

 end of the lake and the Rakaia Valley, so that at one stage the glacier 

 flowed over this and reduced it so far that the Wilberforce River deserted 

 its old bed and joined the Rakaia ten miles further up stream. The lake 

 followed this direction too, and now discharges at its upper end by a small 

 stream which joins the Harper River and flows into the Wilberforce. The 

 Harper River also once flowed down a valley to the east of the Lake Cole- 

 ridge Valley and parallel with it ; but it, too, turned over a low saddle in 

 the direction of the Rakaia, and flowed through the Wilberforce gap. Other 

 small streams have behaved in a similar way, and have turned from their 

 own valleys through gaps in the ridges that once separated their own valleys 

 from the Coleridge Valley. These gaps were in all probability formed 

 primarily by the action of corrie glaciers in the manner described previously. 

 These remarkable changes in drainage seem to have been made possible by 

 the overdeepening of the Rakaia Valley by its more powerful glacier below 

 the level of the parallel valleys, thus providing a lower temporary base- 

 level for the tributary streams. The same fact is also true to a modified 

 extent of the valley to the east of Lake Coleridge, which has been eroded 

 deeper than they have by the large glacier which occupied its floor. 



While the Rakaia has been gaining in the upper part of its course as a 

 result of glacier interference, it has lost lower down. Between the Rock- 

 wood Hills and the Big Ben Range there is a wide valley which was occupied 



