Speight, Cockayxk, f/.\iN(i. — Mount Arrnw'Ufu'fJi Dis/rict. 323 



it is partially blocked by Shaggy Hill, the remains of a ridge which in all 

 probability formed part of the original divide, but which has been cut 

 through by glacier and stream action. The floor of the valley is here about 

 a third of a mile in width, but it immediately widens out with a broad 

 flat section which reaches a maximum of about six miles in the neighbour- 

 hood of Lake Heron. About five miles further on it contracts somewhat, 

 but is still four miles in width, and continues so till it reaches Hakatere 

 Station, at the upper end of the Ashburton Gorge. Immense morainic 

 accumulations are found here covering the whole floor and extending up 

 a tributary valley coming in from the north-west and the direction of the 

 Upper Rangitata, which now contains no stream at all commensurate with 

 its size, but which was an outlet for the excess of ice in the Upper Rangitata 

 basin. It is probable that this valley marks the original course of the Potts 

 River before it was deflected through a low saddle which was cut down 

 on its western side by an overflow from the Rangitata Glacier, an effect 

 which was intensified by the overdeepening of the bed of the Rangitata 

 itself by its own powerful ice-stream. From the junction of this tributary 

 with the Lake Heron Valley an extension of the latter goes towards the 

 Lower Rangitata through the Pudding Stone Valley, which is now de- 

 serted by any stream commensurate with its great width and length. 

 From this brief description of the Lake Heron Valley it will be readily 

 inferred that the original drainage-lines are quite distinct from those exist- 

 ing now. 



The principal stream belonging now to this valley is the South Ash- 

 burton, which runs across it and not down it. This river rises in the Ash- 

 burton Glacier, on the south-east side of Mount Arrowsmith, and flows in a 

 characteristically ice-eroded valley for some distance, and then passes, 

 by means of an extremely narrow and almost impassable gorge, through 

 elevated down country till it reaches the Lake Heron Valley. It crosses 

 this in a wide river-bed without any distinct banks and with all the features 

 of an aggrading stream, and afterwards penetrates the outer range of 

 mountains by a somewhat open gorge, and emerges on to the plains near 

 the Mount Somers Railway-station. It is joined on the northern side, about 

 half-way through the gorge, by the Stour, the upper part of whose basin 

 was once invaded by the ice-sheet from the great inland valley across a well- 

 defined saddle near the Clent Hills Station. The North Ashburton does 

 not belong to the area, as it has not cut through the outer range into the 

 Lake Heron basin ; its upper valley probably escaped the modifying in- 

 fluence of ice experienced by the southern branch of the river. 



It seems fairly certain that in pre-glacier times the arrangement of the 

 drainage-lines was as follov.'s : First, a small stream joined the Rakaia 

 where the present Lake Stream comes in. This had no great size, being 

 only about five or six miles in length. Then the present Cameron and 

 all the drainage of the Lake Heron basin flowed down towards Hakatere, 

 and also received the Ashburton and the stream that came from the direc- 

 tion of the present Potts River, the combined streams reaching the Rangi- 

 tata through the Pudding Stone Valley, The lower Ashburton Gorge 

 was not then cut completely through, and the river rose in the hills near 

 Mount Possession. It is certain that the formation of this gorge is of later 

 date than the formation of the Pudding Stone Valley, and it was no doubt 

 opened out during the glacier maximum by the overflow of ice across a low 

 saddle, which was then lowered by its erosive action and now forms the 



11* 



