96 
The N.Z. Journal of Science and Technology. [April 
has actively eroded its bed, at the present time it is aggrading its lower 
course, and this enables the gorge to be passed with ease. The filling of 
the channel of this stream with a load of debris is a good illustration of a 
process which is very evident in many parts of the mountain region of 
Canterbury — viz., the aggradation of the channels of the rivers with an 
overload of waste. Good illustrations can be seen in the bed of the 
Percival River in the Hanmer region, in the upper course of the Thomas 
River in the Castle Hill region, and in Forest Creek in the Rangitata region. 
This suggests that we are approaching a period when the rivers will be 
more heavily charged with waste, if indeed this period has not already 
commenced, and the more rapid silting-up of their lower courses becomes 
more probable, so that dangers are threatened to the low-lying land on 
the coastal margin. This unfortunate effect is usually attributed to the 
destruction of forest and other plant covering of the mountain region by 
human agencies, but it is probable that this is only one of the causes 
promoting alteration in the behaviour of the streams, and that the 
aggradation depends primarily on some secular change of wider incidence. 
Other small yet active torrents of more recent origin have cut gorge¬ 
like channels, which become specially deep where they enter the river, the 
most noteworthy being Alum Creek and Bushy Creek, which join the main 
river from the south on the eastern border of the area, and Rocky Creek, 
which runs along the northern boundary for some distance and then turns 
to the south and enters Broken River through a precipitous gorge just 
opposite the inflow of Alum Creek. 
An important scenic feature of the area is a bold sandstone bluff which 
rises sheer in places, nearly 500 ft. above the river. Although this adds 
considerably to the interest in the landscape, both the precipitous nature 
of the cliff and the proximity of the river render it almost impossible to 
make a close examination of a most important geological feature of the 
region. 
C. STRATIGRAPHY. 
(See Fig. 2.) 
1. Maitai Series. 
This forms the basement on which the coal-measures rest. It consists 
of typical greywackes, sometimes highly feldspathic and liable to rapid 
decomposition, but usually siliceous and resistant to weathering-agents. 
Sometimes they are dark and slaty in structure. Near the mouth of the 
gorge below the area there is a mass of jasperoid slate, reddish in tinge, 
and associated with it a green cherty rock. Other occurrences of the red 
rock are in Sloven’s Creek and near the mouth of Alum Creek on Broken 
River. The average direction of the strike is about north-north-east, but 
frequently it is north-east, and again at times in beds in close proximity 
the strike changes to west of north. On the saddle between Alum Creek 
and Bushy Creek it is west-north-west, with a south-south-west dip of 55°. 
Under the coal-measures at Alum Creek, and in its vicinity, the strike is 
north and the beds are vertical. This high angle of dip is general, and 
in places the beds are much crushed and with slickensided surfaces. As 
far as could be ascertained the general direction of the faults causing this 
crushing is about north-north-east—that is, parallel to the strike. At the 
mouth of Sloven’s Creek the strike is north-east and the dip vertical. 
Although the beds were diligently searched no trace of the Mount 
Torlesse annelid (Torlessia mackayi) or other organism was found, even in 
beds whose general lithological character resembles that in which such 
fossils are found elsewhere. 
