Cotton. — Block Mountains and a " Fossil " Denudation Plaiii. 75 



of the Slate Range, the greater part making its way out eastward as 

 the Big River, and the remainder westward as the Saxon River, which 

 joins the Big River before reaching the sea. These east-and-west stream- 

 courses are obviously consequent. The eastward-tiowing reach of the 

 Big River is of great interest, as the stream meanders in full maturity 

 upon a wide flood-plain cut but little below the surface of the denuda- 

 tion plain of the " downs," indicating that the cycle initiated by the 

 great differential movements is still current. Rejuvenation of the valley 

 of the Big River, due to the later movements of regional uplift which 

 have affected this part of New Zealand, has not yet ijroceeded so far 

 up-stream as to modify the form of the Gouland Downs. 



A number of streams which cross the western " downs" in a north- 

 ward and north-eastward direction to join the streams in the fault-angle 

 are obviously superposed consequents as they follow the general slope 

 of the surface. They cross obli(iuely the outcrops of the strata of the 

 oldermass which have a uniform north-north-west strike. The streams 

 are roughly graded, though still in narrow, steep-walled gorges, and 

 hence are sunk most deeply beneath the plateau in their middle courses. 

 Plate Y, fig. 2, illustrates tlie type of features thus produced. The 

 sharp contrast between the steep walls of the gorges and the level plateau 

 above is very striking. As the floors of the gorges are occupied to their 

 full width by the streams, the latter rise rapidly, and become impassable 

 after a shower of rain. 



Farther to the east the streams flowing towards the Big River cross 

 the " downs " in a north-north-westerly direction — a direction more 

 northerly than that of the general slope of the surface. Tlie}^ have 

 perhaps been guided by irregularities of the initial surface, but it is 

 noticeable that they are parallel with the strike of the strata of the 

 oldermass. Dissection is here more advanced than it is farthest west, 

 and the numerous longitudinal gullies are separated by rounded quartzite 

 ridges suggesting a subsequent origin. 



The eastward-flowing consequent reach of the Big River is connected 

 witli a westward-flowing consequent reach farther down-stream (see fig. 3) 

 by a northward-flowing reach, where the stream makes its wa}' in a 

 gorge around the eastern end of tlie Slate Range. Tliere can be little 

 doubt that this is of (superposed) consequent origin also. The south- 

 western rim of the " downs " basin is high, and the probable former 

 extension of the surface to the south-westward would be still higher. 

 Thxis the present outlet of the Big River may well mark the position of 

 the lowest gap in the rim of the basin-shaped initial surface of the 

 covering strata, some hundreds of feet above the site of the Gouland 

 DoAvns. 



Ihe outlet gorge has been cut to a depth of many hundreds of feet in 

 the extremely resistant rocks of the oldermass. During the process of 

 gorge-cutting the local base-levels on the Gouland Doavus have been 

 very sloAvly lowered, and tlius perfect conditions have been afforded for 

 the stripping of the cover from the plateau. 



