Speight. — Shingle-spit in Lake Coleridge. 335 



the lake. The shape of current-formed spits, or those in which current- 

 action is predominant, will naturally be determined by the direction of the 

 current. This will always tend to sag shorewards as it passes from point to 

 point — the usual position of a spit — and therefore spits formed in this 

 way will naturally present a concavity to the open water. 



The end of the inner spit in Lake Coleridge is covered with a small clump 

 of manuka, which seems to have established itself on the raised rounded 

 termination of the loop, a feature which strongly marks the Lake Heron 

 spits. This slight elevation is due to the swing-round of the waves before 

 breaking, owing to differential retardation, and thus there can be little onward 

 movement of the detrital matter in the direction of the spit's length. 

 The beach is therefore raised slightly by the direct piling-up action of the 

 waves coming from nearly all directions. This raised knob at the end of 

 spits I have noticed in numerous illustrations, and it undoubtedly occurs 

 at the end of the Nelson Boulder-bank. 



The inner spit is not so perfect in form as those in Lake Heron, a feature 

 no doubt due to the deep water off shore allowing of little differential re- 

 tardation, and the spit is chiefly built up by the send of the seas past the 

 point where the rata-tree is growing. However, it approximates to the 

 shape which I should expect. The ideal is more nearly reached by the 

 outer spit, which, according to Mr. Kissel, has increased appreciably from 

 the spoil thrown out from the tunnel- works. Its inner margin is, however, 

 tolerably even, but the outer edge is distinctly serrated, three marked 

 jags no doubt indicating the work of successive storms. The end is dis- 

 tinctly incurved, and approximates closely to the ideal form. 



Art. XLIL — Reddiff Gully, Rakaia River. 



By E. Speight, M.Sc, F.G.S. 



[Read before the Philosophical Institute of Canterbury, 4th December, 1912.'] 



In Redcliff Gully, on the south bank of the Upper Rakaia River, there is 

 a small outlier of pink-coloured limestone, which derives some importance 

 from the fact that Captain Hutton, in his paper on the " Origin of the New 

 Zealand Fauna and Flora,"* cited the occurrence as a proof of his con- 

 tention that the Rakaia Valley during early Tertiary times was eroded to 

 a deeper level than at present, and that there could have been no plateau 

 to the south of the Rakaia, as demanded by Haast in his explanation of the 

 former extension of our glaciers. McKay also mentiousf that the Redcliff 

 limestone was placed on or near the line of one of the great structural faults 

 which he postulated as being responsible for the preservation of several of th<^ 

 small outliers of Tertiary limestone and other beds which lie in the remote re- 

 cesses of several of our alpine valleys. He suggested that they were remnants 

 of a wide extent of Tertiary beds which had been faulted down, and so 

 escaped to some extent the erosive action of the great Pleistocene glaciers. 

 Seeing that the occurrence was of somewhat varied interest, and that no 

 accurate account of the locality was available, the present author examined 



* "Annals and Magazine of Natural History," 1884 and 1885. 



t " Rep )rt of tho New Zealand Geological Survey for 1890-91." p. 1.5. 



