Marshall. — Geological Notes. 561 



The valley of the Rock Burn proved to be in all its features 

 one of the typical glacial valleys of the west of Otago. Bounded 

 by precipitous mountains rising to 6,000 ft. or 7,000 ft., on the 

 sides it exhibited well the typical U shape of such valleys. At 

 no point is this valley entered by a tributary at its own level. 

 Its waters are fed by tributaries that enter its valley over water- 

 falls. They are true hanging valleys. The floor of the valley is 

 comparatively flat — about a quarter of a mile wide ; it is covered 

 with dense forest except in its upper portions, and everywhere 

 it is strewn with immense angular blocks of rock, some morainic 

 in their origin, others have fallen from the frowning cliffs above. 

 Covered as they are with dense growth of ferns and forest, they 

 offer formidable obstacles to the traveller, and the difficulties 

 taken together often limit the rate of progress to half a mile per 

 hour, even when strenuous exertions are made. From time to 

 time steps in the floor of the valley were met with ; over these 

 the stream rushes with impetuous fury, but has cut but a narrow 

 gap in the rock of which the floor is formed. These steps are a 

 result of a sudden increase in the erosive power of the glacier 

 which filed out the valley. The increase is usually due to the 

 addition of a further quantity of ice from a small tributary, which 

 made a material difference to the weight of ice, and hence the 

 power of erosion of the glacier beneath the junction of the tribu- 

 tary. The valley is about fifteen miles long, and in this distance 

 its bed descends from an altitude of 3,520 ft. to 1,150 ft. where it 

 enters the Dart. The valley terminates in a cirque, with pre- 

 cipitous walls on the north and east, but on the west the slope is 

 more gentle. 



On the east side the snout of a glazier terminates at an eleva- 

 tion of 1,200 ft. above the valley-floor. The glacier supplies the 

 greater portion of the water of the Rock-burn Stream. The east 

 side of the valley is usually 65° in slope. It is often a flat, bare 

 surface coincident with the foliation plane of the schist. 



The west side of the cirque is comparatively low, rising to only 

 4,490 ft. The west, therefore, forms a low pass over the Hum- 

 boldt Range. The pass is only a quarter of a mile wide, and then 

 descends precipitously to the valley of the Hidden Falls Stream, 

 whose bed is here 1,000 ft. lower than that of the Rock Burn, 

 the barometer giving a reading of 2,560 ft. above sea-level. 



There is no bush in the Rock Burn Valley at a greater height 

 than 3,000 ft., but on the east side of the Hidden Falls Valley it 

 rises to 4,000 ft. The valley of the Hidden Falls is less U-shaped 

 than that of the Rock Burn. This is mainly due to the huge 

 slips and scree slopes that have broken away from the steep 

 faces and litter the sides of the valley. 



The Hidden Falls Stream obtains most of its water from 



