260 Transactions. 



(3.) High-level Terraces South of the Little Totara River and East of the 



Limestone Cuesta near Charleston. 



The " 500-foot " terraces described lose their distinctive character south 

 of the Little Totara River ; some hummocky country of slightly greater 

 elevation, consisting of dissected " Blue Bottom," intervenes for a short 

 space, and then commences a great belt of level gravel-capped " Blue 

 Bottom" terraces, Avith an altitude of about 600ft., occupying a depres- 

 sion between the edge of the gneiss mountains and the coastward limestone 

 ridge, which slopes gently inland along the dip of the beds from the crest 

 of its high escarpment. Against the mountains the " Blue Bottom " beds 

 are steeply tilted by the main fault, and there attain an elevation of over 

 1,000 ft. Even at this altitude traces of terraces, cut by the streams now 

 flowing 400 ft. or 500 ft. below in deeply cut gorges, are plainly delineated. 



The depression to which attention has just been drawn continues in a 

 south-south-west direction many miles to the south of the area shown in 

 the locality map illustrating this paper ; its drainage is effected by mode- 

 rately large streams flowing in a general north-west direction from the 

 Paparoa Range and cutting in deep gorges through the coastal limestone 

 ridge to which they are antecedent. 



Two such streams near Charleston are the Waitakere and Four-mile 

 Rivers. There is evidence in a wind-gap south-east of Trig. Station U in 

 the limestone cuesta, of another large stream having formerly flowed north- 

 west across this ridge. Probably the gneiss floor of the Miocene beds has 

 intervened at a higher altitude here than in the case of the existing streams, 

 and, owing to failure to corrade the resistant rock sufficiently rapidly, the 

 drainage of this valley inland of the cuesta was captured by the Waitakere 

 and Four-mile Rivers, and thus reversed. 



McKay* draws attention to the fact that despite vigorous prospecting 

 no auriferous black-sand or other leads have been discovered in the gravels 

 of these widespread ten-aces. He noted the inland depression, and, specu- 

 lating upon the absence there of beach leads, considered that most probably 

 it was due to these having been removed by denudation, although he 

 states at the same time that this absence may possibly have been caused 

 by " inequality of level affecting the areas east and west of the limestone." 

 The writer believes that the limestone has been an effective barrier to wave- 

 encroachment during and since the period of sculpture of the highest wave- 

 formed terraces or platforms of the district, and that the depression, with 

 its gravel cap, is the work of the streams now transecting and draining 

 it at a period corresponding to that when the " 500-foot " terraces were 

 formed, since there was then considerable standstill. Prominent elevated 

 more-recent flood-plains of the Four-mile River, cut in the weak Miocene 

 sandstones beyond the eastward limit of outcrop of limestone, are largely 

 dependent in origin on the control of stream-grade by the local base-level 

 of erosion caused by a resistant gneissic barrier at the Brighton Road 

 bridge across the Four-mile River. At this place, which, though only one 

 mile from the coast, is at an altitude of approximately 350 ft., the stream 

 is slowly lowering its bed below the lip of the low gorge already cut. 



Further Details of the Charleston District. 



At Charleston, where everywhere is semi-obliterated record of the 

 remarkable industry of the early gold-diggers, auriferous beach leads were 



* Geology of the South-west Part of Nelson and the Northern Part of the Wcstland 

 District," Parliamentary paper C.-13, 1895, p. 23. 



