Speight. — Geologiad Features of (.'/ir/.'<f church Artesian Area. 423 



tlio plains have been built up on a land-surface by the aggrading action 

 of the rivers issuing from a mountain tract, where they are plentifully 

 supplied with sediment and flowing on a high gradient, and then depositing 

 their load at the base of the mountains, where their transporting-power is 

 less. Such plains occur under somewhat similar conditions at the base 

 of the mountains in central Asia and central United States, where marine 

 action cannot be called in to explain their approximately level character. 

 The present contours of the plains, as shown by the detailed surveys of 

 Haast and Doyne, emphasize their fluviatile origin, since they are just those 

 which would have been exhibited had the plains been built up by the coal- 

 escence of the fans of low gradient formed by the large rivers as they issued 

 from the mountains. 



Formation of the Eastern Part of the Plains. 



There are certain factors which must be allowed for when considering 

 the formation of the eastern portion of the plain. The rivers have been 

 bringing down for a long period vast quantities of the detritus. On the 

 Ninety-mile Beach this is exposed to a strong northerly drift up the coast, 

 which sweeps it northward and piles it up against the southern coast of 

 Banks Peninsula. When the supply of shingle fails, the sea encroaches on 

 the land, as, for instance, near Oamaru, where works are necessary for the 

 protection of the town, and the neighbouring coast northward to the Waitaki 

 River is being rapidly attacked. This river brings down an enormous 

 amount of material, and in consequence the coast immediately north of 

 its mouth is advancing in spite of a probably downward movement of the 

 land, evidenced by the valleys in the dolerite plateau near Timaru, which 

 have the outlets depressed below sea-level, and also by the presence of sub- 

 merged forests on the coast near the mouth of the Pareora River. The gradual 

 pushing-forward of the shore is evident right up to Timaru. The railway- 

 line is placed for the greater part of its length on the level beach which 

 h.as accumulated at the base of the cHffs, now removed some distance from 

 the sea. Some idea of the importance of the drift along shore can be seen 

 at Timaru itself, where several acres of land have been reclaimed by this 

 agency on the weather side of the breakwater, which has checked the 

 northerly movement of shingle at that point. This check can only be 

 temporary, as no possible human works can obstruct for long the action of 

 an ocean-current. Further north, towards the southern end of the Ninety- 

 mile Beach, the sea seems to be getting the best of the struggle, and the 

 fringe of the old fan of the Rangitata and Ashburton Rivers is now being 

 cut away. But further north still, when the Ashburton and, above all, the 

 Rakaia Rivers have poured in their contributions of shingle, accumulation is 

 rapid, as is proved by the presence of the great shingle-bank between Tau- 

 mutu and Birdling's Flat, dividing the shallow waters of Lake Ellesmere from 

 the deep sea. The shore-current has followed here a direct course across 

 the mouth of a deep indentation of shallow water straight to the solid mass 

 of Banks Peninsula. By it the northerly current is turned eastward into 

 deeper water, and is thus rendered incapable of carrying its load of coa]:ge 

 material, so that this is piled by wave-action across the mouths of the bays 

 facing south, which have now become the valleys known as Price's Valley 

 and Little River. The fine detrital material is swept off into deep water, 

 a.nd some may find its way round the eastern side of the peninsula, and 

 be dropped in the somewhat sheltered area on its northern side. The rate 



