Uttley. — Tertiary Geology, Otiake River to Duntroon. 139 



to follow owing to the necessity he was under of finding in the series an 

 unconformity marking the line of division between the Cretaceo-Tertiary 

 and Tertiary systems of the old Geological Survey under Sir James Hector. 



The relationship of the various rocks present in the area is obscured 

 by the thick deposit of post-Tertiary gravels, and exposures are revealed ' 

 only when the streams of the area have entrenched themselves in the 

 gravels and cut through the Tertiary rocks. The outcrops of Tertiary 

 rocks along the fault-lines have been mapped only where they were 

 observed. They are probably continuous beneath the gravels. 



The writer wishes to thank Mr. P. G. Morgan for kindly allowing him 

 to examine the lists of fossils determined by the late Mr. Henry Suter 

 from the collections made by McKay forty years ago. To Dr. J. Allan 

 Thomson the writer is much indebted for assistance in determining the 

 brachiopods, and for permission to incorporate his list of molluscan fossils 

 from the upper beds a,t Otiake in the lists detailed below. Mr. H. Suter 

 named many forms from the same beds for the writer some years ago, and 

 his determinations are included in these lists. 



The paper furnishes geological evidence for the existence of two strong 

 faults, which are shown to be intimately connected with the great fault- 

 system of Central Otago, so ably described by Cotton (1917a, p. 272). 

 The elongated relatively depressed area of the Waitaki Valley, between 

 the mountains of South Canterbury and North Otago, occupied by Tertiary 

 beds, is shown to be a tectonic depression which is partly a graben and 

 partly a fault-angle depression. McKay's statement that the Hutchinson 

 Quarry beds lie above the Otekaike limestone is shown to be supported 

 by the evidence. It is further shown that McKay's " two-limestone 

 theory " is radically different from Park's " two-limestone theory," and 

 that the latter's statement (1918, p. 110) that Hutton, Hector, and McKay 

 considered the limestone of the Waitaki Valley to be of Hutchinsonian 

 age is not warranted. On the evidence of the brachiopod fauna this lime- 

 stone is shown to be Ototaran. 



-• . ' A 



II. General Description op the Area. 



Between the Otiake and the Otekaike Rivers, flowing in a north- 

 easterly direction to join the Waitaki River, the surface rock is a thick 

 deposit of coarse water-worn boulders and silt. The initial gravel table- 

 land has been rather deeply trenched by the streams, and the Otiake and 

 the Otekaike now flow in braided courses to join the trunk stream. 

 Remnants of high-level terraces are to be seen near the school at Otekaike, 

 indicating recent elevation. Before debouching from the mountains on 

 to the gravel-covered plain these rivers flow in steep-sided gorges, and 

 narrow terraces are noticeable on the sides of the streams. The moun- 

 tainous country that flanks this part of the area on the south-west rises 

 fairly rapidly to a height of over 6,000 ft. in the Kurow Mountains, or at 

 least 5,000 ft. above the general level of the gravel tableland. 



From Ben Lomond to Black Hill an even-topped gently sloping ridge 

 extends almost to the main road, and the rise from the Otekaike basin 

 to this ridge is abrupt. The quartz-grits that crop out in the basin of 

 the Waikaura Creek have been deeply dissected and eroded, and in places 

 the greywacke imdermass crops to the surface, particularly in the higher 

 country towards Ben Lomond. T|ie country to the east is an elevated 

 tableland, deeply dissected by the streams that traverse it. The surface 

 rock is formed of heavy river-gravels and silts, and limestone is revealed 



