Art XXVIII.—The Rate of Erosion of Hooker and Mueller Glaciers, New Zealand.
Read before the Otago Institute, 5th December, 1916; received by Editors, 30th December, 1916; issued separately, 30th November, 1917.]
In a paper on the rate of erosion of the Hooker and Mueller Glaciers, presented to the Otago Institute in August, 1912, Dr. Marshall* records the results of thirteen determinations of the amount of suspended matter carried by the Hooker River, which drains the Hooker and Mueller Glaciers. The first sample was taken on the 17th November, 1911, and the last on the 25th March, 1912. The lowest result gave 1 part of suspended matter in 885 parts of water, and the highest 1 in 39,141. He concluded that the whole of the suspended material was the product of glacier erosion due to the slow movement of the ice over the rocky bed. From his meagre data he calculated that the rock bed over the whole névé and glacier area of the Hooker and Mueller Glaciers was being removed at the rate of 1/63 in. annually, equal to 1 ft. in 756 years.
In the discussion which followed the reading of the paper I pointed out that both the Hooker and Mueller Glaciers were heavily dirt-laden, and that in all probability a large proportion of the suspended matter ascribed by Dr Marshall to glacial erosion was in reality released from the melting névé.
Since that date I have again examined the Mueller and Hooker Glaciers. The Mueller Glacier, especially in its lower portion, is covered with an extraordinary quantity of transported material, ranging from large angular flaggy slabs of argillite and greywacke to small angular grit mixed with fine particles of fresh unoxidized rock. Ten pannings of the finer material, taken at different points near the terminal end of the névé, contained from 0–08 to 3–60 per cent. of material sufficiently fine to form suspended matter when placed in water moving at the rate of 3 ft. per second.
The Hooker Glacier also carries a large load of frost-shattered rocky material, and in many places is interstratified with thousands of thin bands of fine material that alternate with thin bands of almost clear névé. These dirt bands descend through the body of the névé to an unknown depth. When the névé melts they discharge their load into the river draining the glacier.
Experimental tests of the material forming the dirt bands showed the presence of from 0–16 to 2–34 per cent, of material sufficiently fine to form suspended matter in slowly moving water The lowest observed velocity of the water at the Hooker bridge was 5–6 ft. per second.
The névé is constantly melting at the terminal face and on the walls and roof of the ice-tunnel under the glacier. Moreover, the glacier river flows in the ice-tunnel with a great velocity, as may be seen at various places where the sides have collapsed. The wear-and-tear and pounding of the larger fragments of argillite when they fall into this turbulent stream, in the two miles between the terminal face and Hooker bridge, must be
[Footnote] * P Marshall, Note on the Rate of Erosion of the Hooker and Mueller Glaciers, Trans N Z Inst, vol 45, 1913, pp 342–43
considerable — probably sufficient to form a certain amount of fine suspended matter.
How much of the suspended matter in the Hooker River is liberated from the melting névé, or how much originates from the attrition of the fragments that are carried along by the fast-flowing stream, or from ice-erosion of the glacier rocky bed, is indeterminate. At any rate, Dr. Marshall has neglected to take into account some obvious sources of suspended matter, other than ice-erosion. For that reason I am unable to regard his tests as a trustworthy basis for the computation of the rate of ice-erosion of the Hooker and Mueller Glaciers.