HUTTON'S GEOLOGY OF OTAGO.

Mount Ida Chronicle, Volume VI, Issue 357, 7 January 1876, Page 3

HUTTON'S GEOLOGY OF OTAGO.

VALLEY OF THE SHAG RIVER. This valley, although not very large, is very ofd, and is particularly interesting as we ; can fix the date of its formation to have; been in the eocene period. The proof of; this is that the upper cretaceous rocks of; the horse ranges (Waipara formation) form one side of its valley, while lower rriiocene rocks (Oamaru formation) are found lying horizontally in.the centre. The valley, therefore, must have been scooped out between the dates of the Waipara and Oamaru formations. At this time it extended into the Kyeburn, as proved by the marine tertiary rocks found near the coal mine, and it is probable took its rise in Mount Ida. The eocene Shag River was therefore considerably greater than the-present-one. Subsequent, during the upper eocene and iiiiocene periods',' The riWj was submerged and filled up with marine and volcanic rocks, which were afterwards partly eroded out again during the older pliocene upheaval; while at the same time the upper part of the valley, as far as the hard basaltic rocks of the Houndburn, was worn away by the Upper ' Taieri glacier, and formed into part of the basin that was ultimately to become the Maniototo plains.

OLD LAKE BASINS OF THE INTERIOR

That the Maniototo plains the Ida-burn valley, and the Manuherikia Plains, were once lakes that have been filled up, is proved by the nature of the rocks that fill them, and the occurrence of fossil fresh water mussels at Oiyde, and Maniototo Plains; and I have already given my reasons for assuming that they were, hollowed out by ice. The general configuration ot surrounding mountains shows that the Maniototo Lake was chiefly formed by the Upper Taieri glacier, and Ida-burn valley by the Pool-burn glacier, both these glaciers flow- , ing north. Also the rounded form of the low narrow ridge of schist which separates the two where the Rough Ridge joins the spurs of Mount Ida, makes it appear probable that the Maniototo glacier joined that of the Ida-burn at that point, and that both together flowed round Blackstone Hill into the Manukerikia glacier. There is also confirmatory evidence on this point. In the neighborhood of. Naseby laro-e. quantities of boulders of a white or pinkish highly metamorphosed quartz conglomerate occur, the blocks of the rock are also common in the Marurawhenua on the other side of the Kakanui mountains, and are occassionally found as far down as the Awamoko. I have not been able to find this rock in situ but from the disposition of the boulders I think that it will probably be found somewhere between Mount Ida and Kyeburn Peak Now boulders of this conglomerate are found all down the valley of the Manukerikia as far as Clyde, and as the streams do not run in that direction now, it follows that they must have been taken into their positions by a different drainage system to the present one. [The boulders containing tertiary fossils found at Alexandra may also have come from the upper Shag Valley.] If this view is correct, the terminal moraine of all these glaciers would have been at Clyde and Alexandra, and the river running from it would have flowed down the valley of the Clutha. I have already said that I saw no trace - of a moraine in this positio 1, but it must be remarked that the mountains from which these glaciers would have been fed are round-backed and tolerably equal in altitude; consequently they would be almost entirely covered with snow Also the Clutha has had plenty of time to remove any moraine that might once ha\e existed there.

We have now to try to account for the change in the system of drainage that took place after the disappearance of the glaciers, and which caused the Mamototo JLake to overflow into Strath-Taieri, and the Ida-burn Lake to break through the Haggedy Range near Black's. When the land b gan to sink and the glaciers to retire, those of the Maniototo and Jda-burn would retreat much faster than that i,;' the Aliirjuhur'rkia, as t'.ivy were fed by Hie k;i')\v* on i0.v..: ir.j./cs " f,f rri'Mi'itaiin which an, of nearly eqiiu! : !' ; u!^'; unit'.i:-: liiti'eiXMcc wo'..'!l k- i 1, .; [vo-.ter

if, as is probable, the Manuherikia gtacier was a continuation of the Ohau glacier. If now we assume, as we reasonably may, that the Maniototo and Ida-burn glaciers had been greatly diminished, or altogether disappeared, while the Manuherikia glacier was still continued as far as Blackskaie Hill, lakes would be formed in the Idaburn and Maniototo valley, separated from one another by the Rough Ridge , and if the ice at Hills Creek was sufficiently high, these lakes would be dammed back until they overflowed at the lowest place along their margins, which in the case of the Maniototo, would be into' Srath Taieri, and in. the case of the Idaburn, into Manuherikia, above Blacks. Gorges would then be cut down in these plains which, by the time that the Manuherikia glacier hacft'etreated to Hawkdun Mountains, might be sufficiently low to continue the drainage.