Speight. — The Geology of Banks Peninsula. 387 



developed beach with shells occurs at the foot of a wave-cut cliff, now over 

 a chain from the sea at its maximum distance. This evidence is supported 

 by that obtained from the neighbourhood of Sumner, where the marine 

 cliffs are now removed a considerable distance from the present shore-line. 

 A part of this effect may be- attributed to the progression of the shore by 

 the deposit of sand, which has developed in places a dune system ; but this 

 is hardly sufficient of itself to account for all the phenomena. With this 

 movement is connected in all probability the system of low terraces which 

 fringe the Avon and Heathcote Rivers, immediately north of the peninsula. 

 These streams have reached a second base-level below an original one, 

 which was disturbed by this slight elevation of the land. Pronounced 

 elevatory movements are in evidence on the coast-line from thirty miles 

 farther north, near Amberley, Motunau, Amuri Bluff, and Kaikoura, and 

 it may be that the peninsula is on the outskirts of the area affected, since 

 the upward movement in its neighbourhood is very slight. 



(b.) Results of Erosion and the Formation of Calderas. 



A consideration of the form of the harbours strongly supports the con- 

 tention that stream erosion combined with wave action in cutting back 

 the cliffs is responsible for their principal landscape features. The im- 

 portance of stream erosion is very evident when we consider the depth to 

 which valleys have been eroded on the outer slopes of the volcano. In 

 numerous instances they have cut back till a very narrow ridge separates 

 the inner and the outer slopes. This is well exemplified at Heathcote 

 Valley in the dividing ridge above the tunnel, and at the head of the valley 

 leading to Dyer's Pass ; and in two places- — viz., at the entrance and at 

 Gebbie's Pass — the wall has been completely broken down. It has been 

 mentioned previously that the heads of the valleys generally lead to the 

 lowest parts of the crater-ring, and that this suggests that a former stream 

 system has had the upper parts of the valley removed either by explosion 

 or by collapse. It is possible, however, that it merely means the approach 

 of the heads of two valleys from opposite sides of a ridge, and that the 

 combined attack on the crest has resulted in its being lowered at that spot ; 

 and, further, the form of the valleys is a modern development, whereas 

 the formation of the caldera by explosion belongs to some date in Tertiary 

 geological time, and therefore such a feature is not likely to exhibit such a 

 perfect arrangement now. The extent of denudation and its marked effect 

 on the form of the mountains, reducing their height, is emphasized by the 

 facts that the radiating ridges are frequently higher some distance away 

 from the crater-walls, and that these higher portions are frequently capped 

 by rock of more resistant nature, and that when its slope is prolonged crater- 

 wards it reaches a much higher level than even the highest parts of the 

 crater-ring in the vicinity. This is markedly true of the Akaroa volcano, " 

 for there we have eminences reaching well over 2,000 ft. on the spurs which 

 divide adjacent radial valleys. Instances of this are Pigeon Bay Peak and 

 View Hill, on the northern flanks, and there are as well numerous unnamed 

 elevations on the ridges to the south. 



While a volcano is in process of construction streams will establish 

 themselves on its outer slopes, with radial orientation, their heads cutting 

 farther and farther back towards the crater, so that the valleys will be more 

 bunched together towards the centre of the cone than on its margins, and 

 a struggle for existence will occur among them, those reaching farther back 



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