Speighe, — Shingle-spit in Lake Coleridge. 333 



spit by a strip of shallow water. The water is comparatively deep within 

 the loop of the spit, and between its end and the shore is as much as 20 ft. 

 Lake Coleridge is a long narrow lake, some eleven miles in length, and 

 a mile and a half wide at its widest part, narrowing to three-quarters of a 

 mile about half-way up, where a peninsula juts out from the eastern side. 

 It is very deep, with steep under-surface slopes. Its length lies directly in 

 the line of the northerly winds which sweep down from the Southern Alps, 

 and at times raise a nasty and dangerous sea. The spit is placed at a 

 spot where these waves would in all probability reach their maximum 

 height and have a maximum effect. No other wind is deserving of serious 

 consideration as far as this spit is concerned. There can be no current in 

 the lake in the vicuiity of the spit, since what small discharge there is takes 

 place from the other end of the lake, eleven miles away. 



Origin of the Material of which the Spit is built. 



The spit is formed entirely of the greywacke which is the principal 

 rock occurring in all the moimtain district of Canterbury. The pebbles 

 attain a diameter of about 3 in., but included among them is a large 

 quantity of small material. Finer detritus mixed with peaty matter has 

 formed a fringe of swamp along the inner margin of the hook, especially 

 towards its proximal end, where there is a considerable area of boggy ground. 

 The great bulk of the material of which the spit is formed has come from 

 farther up the lake, where there is a supply of pebbles of all sizes in the cliffs 

 of glacial boulder-clay fringing its shore. These cliffs commence about 10 

 chains away from the point on which the rata-tree grows, and extend with 

 intermissions of solid rock for nearly three miles up the western side of the 

 lake. It is remarkable, however, that pebbles can be transported along a 

 shore which deepens so rapidly. I have been able to secure a tracing of 

 the minute and accurate survey made by Mr. F. T. Kissel, the Resident 

 Engineer for the power scheme, and I find that in places along this 

 stretch of shore the 60 ft. subsurface contour-line is at times within 2 chains 

 of the margin of the lake, although in others it is as much as 6 chains. It 

 would seem that even these stretches of steep slope are not thoroughly 

 efficient in preventing the transport of beach-shingle. That it does so travel 

 is certain, for at the intake of the tunnel for the power-supply, which is 

 about a mile and a half from the spit, a peculiar form of hard slaty grey- 

 wacke was encoimtered. The spoil from the tunnel is thrown into the 

 lake, and this is carried down and forms small beaches between it and the 

 spit, and is, indeed, transported across the water, 20 ft. in depth, which 

 separates the distal end of the spit from a beach at the southern end of 

 the lake. This beach is formed of fine shingle, some of which is undoubtedly 

 derived from the tunnel, and its presence serves to emphasize the import- 

 ance of waves as transporting agents even when there are no appreciable 

 tides or currents to aid them. Mr. Kissel has pointed out to me that the 

 strongest waves occurring in the lake tend to destroy the small beaches 

 round its shore, because the loose material is carried outwards to greater 

 depths by the stronger undertow and dropped down the steep subsurface 

 slopes into deep water, and is thus removed from the belt along shore where 

 waves exert influence. Moderate waves are, therefore, in this case more 

 potent beach-formers, because they do not move the shingle so far out. 

 This factor is perhaps of relatively small importance in the neighbourhood 

 of the spit, since the water deepens off shore gradually, and shingle is not 

 likely to be carried out into deep water by the undertow. No doubt this 



