THE CANTERBURY PLAINS.

New Zealand Herald, Volume XLVIII, Issue 14819, 24 October 1911, Page 4

THE CANTERBURY PLAINS.

THEORIES ON BANKS PENINSULA. A POPULAR lecture upon " The Canterbury Plains and Banks Peninsula" was delivered by Mr. E. K. Mulgan, M.A., chief inspector of schools, in St. Andrew's Hall, Lower Symonds-street. last evening, under Hie auspices of the Auckland Institute. The president of the institute (Air. J. H. Upton) presided, and there was a large attendance.

The lecturer, who illustrated his remarks by a very fine series of limelight views, explained that the plains were built up by the rivers carrying down material from the Alps, or, in other words, that the material was of fluviatile and not of marine origin. At present, however, tho rivers were not to any great extent adding to the material already deposited, except along the shore-line, which in places was being pushed further out into the sea. Well-sinking operations had revealed the existence of peat at depths varying from 400 ft to 700 ft beneath the surface, which indicated that the land at one time stood at least 600 ft above its present level. An elevation of 600 ft (it was probably considerably more than this) would have a marked effect on the climate of the country; it would largely increase the area above the snow-line, and cause a great extension of the glaciers; it would expose a much larger surface than the present one to the action of frost: and it would extend the area unprotected by plant growth, and so enable the weathering agents to work with greater rapidity. The jointed character and age of the rock masses of which the mountains are largely composed brought about widespread disintegration, so that immense quantities of material were available for transport by river and glacier. In short, the conditions following from ' elevation accounted for a supply of waste which an area of waterborne material such as that occupied by the Canterbury Plains demanded. The sounding-line now touched bottom at a depth ot 100 fathoms at a distance of about 40 miles to the east of the present shoreline, so that an elevation of 600 ft would further imply an eastward extension of land for this distance. •

Terrace development, ho pointed out, occurred on a large scale along the upper portions of the rivers, just after they left the gorges and entered the plains. The three main factors controlling the corrosive power of a river were (1) the gradient, (2) the volume, and (3) the load. In the earlier ages, when the plains were being formed, the rivers were running on a steeper gradient, and their volume was undoubtedly greater than at present. But the terraces are of recent growth, or at least were formed subsequent to the deposition of the material composing the plains. From this it appealed that the predominating factor in determining whether or not the river would be able to terrace the plains was the load. With a heavy load the river's energy was wholly occupied in transporting material, whereas if the load were diminished there would be an excess of energy left over for terracing. The rivers at first were fully occupied in carrying down and spreading out their loads of material in the form of large shingle fans, which gradually overlapped and formed the plains. Subsequently when tho supply of waste diminished the energy thus set free found an outlet in terracing the banks. After referring to the Christchurch water supply, derived from artesian wells, Mr. Mnlgan dealt at length with the formation of Banks Peninsula, which, ho said, was a mass of volcanic rock, varying considerably in character, and extruded* mainly from the Lyttelton and Akaroa craters, the sites of which were now occupied by tho sea and formed the two well-known harbours on the peninsula. To judge from the state of preservation of the crater ring, Akaroa was the younger of the two centres, but neither had been in activity for a very long period. The history of Lyttelton crater revealed several periods of activity, the final outburst breaking through the old crater ring and forming Mount Herbert and other high hills in the neighbourhood. The loess which covered the land in many places on the peninsula, and added so much to its general fertility, was undoubtedly a windborne deposit driven into its present position when the whole country stood at a higher level. The lecturer received a. most attentive hearing, and his remarks were frequently punctuated with applause. On the motion of Mr. Upton a hearty vote of thanks was accorded to Mr. Mulgan for his address.