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Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations

The Waitaki Glacier

The Waitaki Glacier.

The lowest portions of morainic deposits belonging to this glacier, which I was able to trace, are situated on the left bank of the Waitaki, six miles below the junction of the Hakataramea, rising about 600 feet above the valley. From here to the sea the ranges bordering the Waitaki are mostly all capped by alluvium, showing that at one time the bed of that river has been lying at least 600 feet above its present level. This is very conspicuous on the ranges between the lower Waitaki and Waihao. Calculating from, the position of the present névé-fields of the Takapo, lying about ten miles more distant than those of the Tasman glacier, the total length of this post-pliocene glacier was at least 112 miles.*

During the greatest extension of the glaciers under review, four principal and some minor branches came down by the valleys of the Takapo, Pukaki, Ohau, and the Ahuriri, which uniting in the Mackenzie plains formed a trunk glacier of a breadth of about thirty miles. As the accumulating ice masses could not be discharged by the valley of the Waitaki alone, several outlets were formed in the depressions between the mountain range on the eastern side of that plain. These depressions are now known as the Burke's, Mackenzie, and Hakataramea Passes. Of these secondary branches the one page 385descending by the last mentioned pass was the most important, being six to eight miles broad, and uniting again with the main glacier about sis miles above its terminal face. A division or forking of this Hakataramea branch took place where now the upper waters of the Waihao are situated. The Burke's Pass branch was also of considerable size. It received a large addition from the Fox Peak range, coming down by the valley where now the Opuha plains are situated. A broad moraine was thrown across the valley of the Opihi, and a river—with which some miles lower down the Mackenzie glacier outlet united—issuing here from the terminal face formed high and broad alluvial beds. These can be followed for nearly thirty miles in a southern direction as far as the Otaio. The great Waitaki glacier must have had in its middle portion, judging from the terraces and roches moutonnées situated here, a thickness of about 5000 feet. On some of these peculiar isolated ice-worn ranges in the Mackenzie plains blocs perchés are not uncommon. After a time this glacier retreated and then maintained for a considerable period about the same position. This phase may best be described as the Lake period, because during it the rocks forming the bed of the glacier were either excavated so as to form a true rock basin, or the terminal and lateral moraines were heaped up so high, that when the glacier retreated, the outlet flowed into the hollow thus made, forming a lake. The retention of the water between the walls was doubtless assisted by the enormous amount of silt, brought down below the glacier from higher regions, by which the morainic accumulations were made impervious. To this important epoch Lake Takapo, Pukaki, Ohau, and the large swampy region in a similar position in the Ahuriri valley, owe their existence. The sections Nos. 4, 5, and 6 on plate 7 will give the characteristic features of the morainic and other deposits surrounding these lakes. The southern portion of Lake Pukaki lies entirely in the plains. The morainic accumulations rise to 250 feet above the level of the lake, whilst even in its northern or upper portion where the slopes of the Ben Ohau range, either cut into by glacier shelves or covered by lateral moraines, form its western boundary, the eastern is still formed by the moraines. There is a distinct series of old lacustrine beaches along the shores of Lake Pukaki, rising about 200 feet, one above the other, clearly indicating that after the retreat of the glacier the waters forming the lake stood at a much higher level. There were several outlets, which were abandoned when the central one cut its channel deeper into the frontal moraine. page 386They now form natural roads to reach the shores of the lake. The frontal moraines of Lake Pukaki are about three miles broad, forming a series of walls enclosing each other. As we advance from the lake towards their outer edge they gradually get lower, till at last only here and there a chain of small hillocks, the upper portion of the last visible moraine, appears above the alluvium. The same is the case with the lateral moraines, showing a similar arrangement. Already abreast of the central portion of Lake Takapo high ranges rise on both sides, on the slopes of which the glacier has either cut deep shelves or thrown its lateral moraines against them, now rising in steps nearly 2000 feet above the level of the lake. Higher up in the valleys of the Tasman and Grodley rivers these signs of former glacier power can be traced to an altitude of nearly 4000 feet on the mountain sides, accompanied by deposits of former glacier lakes, having stood at a high level. All these phenomena present us with clear evidence of the enormous size and power of the mighty ice-streams once descending here to lower regions. Only in a few localities are there faint traces of frontal moraines having been preserved in both these main valleys. They are more conspicuous in the two main valleys forming the Hopkins river, the principal feeder of Lake Ohau, as well as in the upper Ahuriri.

* When publishing in 1865 some notes on this and number of other post-plioceene glaciers, I could only give the results of my examinations up to that date. Since then, further and more detailed researches have shown the existence of morainic accumulations far in advance of the limits formerly assigued to them.