Si'EiOHT. — Geological Featurex of Chrhtchurch Arfesio/i Area. 431 



Series No. (>. — Ellesmere North to Addington and Gehhie's Valley. 



(Plate XIV.) 



This series gives a selection of wells from the district round Lake Elles- 

 mere. Its special features are the marked preponderance of gravel in the 

 upper beds in the southern parts of the area and the comparatively poor 

 flows of water that are met with even in deep wells. The requisite arrange- 

 ment of beds seems to be absent or but slightly developed, a condition 

 which occurs in other cases where the distance from Banks Peninsula 

 increases. 



A very interesting part of this area is that lying along the base of the 

 hills in the neighbourhood of Gebbie's Valley. The beds here are almost 

 wholly of sand and sandy clays, as is shown by the sections numbered 15, 

 16, and 17, which may be taken as typical ; they also contain a fair pro- 

 portion of angular volcanic matter, and thus resemble those on the north 

 side of the peninsula. 



At Teddington, just over the pass at the head of the valley, and within 

 the basin of Lyttelton Harbour, there is another system completely cut off 

 from that on the outside of the hills by the slates and volcanic rocks which 

 form the western part of the old crater-ring of the Lyttelton volcano. It 

 has been pointed out by Page and Prideaux in a paper entitled " Notes on 

 an Artesian System at the Base of the Port Hills " (Trans. N.Z. Inst., vol. 33, 

 p. 335, 1901) that these wells, and also those at Gebbie's Valley, present some 

 decided differences from those of the plains, in that they contain a much 

 higher proportion of chemical salts, notably chlorides, as well as exhibit 

 a higher temperature than those of the Christchurch area. 



It is very probable indeed, as pointed out in the paper just referred to, 

 that the Gebbie's Valley wells belong to a different system from that on the 

 plains, and the conditions governing the flow of water will be in that case 

 very similar to those which occur at Teddington. In both the structure 

 of the beds does not seem to be at all favourable to artesian flow, but con- 

 ditions of texture appear to be the controlling factor. 



A well which shows a relation to these is the fourteenth on the series, 

 sunk near the foot of the Port Hills, at Hoonhay. This gives a flow of only 

 4^ gallons per minute, and the water-bearing stratum is a layer of scoria 

 lying on solid rock. The water has in all probability followed the surface 

 of this rock down from higher levels on the Port Hills, and the well is quite 

 distinct from the Christchurch artesian system. 



Level of the Water in the Wells. 



The height to which the water rose from the various levels in each well 

 at the time of sinking is given in a small figure placed near the vertical 

 sections at the depth at which the water was encountered. The surface of 

 the ground is taken as the datum-line, and the height above this is reckoned 

 as positive and below it as negative ; the latter applies to non-flowing wells 

 and to beds which do not yield a flow at the surface. These records furnish 

 a basis for determining the amount of fall in levels which takes place after 

 a well has been flowing for some time, a fall due partly to the fact that all 

 wells go down slightly after a short period of time, even if they are not 

 interfered with by others, and also to the fact that wells affect each other 

 materially if they happen to be sunk so as to cause overlapj)ing of the cones 

 of depression which surround each well. The fall in level of the wells has 

 been very rapid since they were first sunk in the district. 



