270 
The Palæozoic rocks, which constitute the gorge, continue 
in outcrop along the east and south sides of the “horseshoe,” 
until they reach the centre of the convex bend, when they 
abruptly give place to marine Miocene beds. The older 
rocks form the basal beds of the narrow neck of land in the 
bend, and are exposed for about a quarter of a mile along 
the right bank of the river, as it leaves the township to the 
north-west. On the north side of Noarlunga the Palæozoic 
rocks run for a mile or more parallel with the Adelaide road, 
on its eastern side, whilst the whole of the country on the 
western side, as far as Ше coast, consists of older and newer 
tertiary beds, most of which has been brought under cultiva- 
tion. Several road cuttings give sections of these beds, and 
in the case of one, situated a little north of Hackham, marine 
fossils, of Miocene age, are found in a series of sands and 
fine gravels. About a quarter of a mile down the river 
from Noarlunga, a white, marly clay, similar to Witton 
Bluff, on the coast, makes a cliff twenty feet in height, and 
passes under water level. The bed contains crinoid stems, 
spines of echini, Turritella aldinge, brachiopods, etc. These 
fossiliferous clays rest on coarse, sandy beds, which show a 
dip of 25? west; but, as the latter abut against a nearly ver- 
tical face of the older rocks which formed coastal cliffs of the 
tertiary sea, it is probable that the apparent dip of the beds 
arises from the deposits having been laid down on a shelving 
beach. The tertiary beds, on the south side of the river, 
are bounded by the Palæozoic outerops, which gradually 
trend towards the coast. 
The beds exposed in the gorge of the Onkaparinga corres- 
pond with those already described as occurring in the valley 
of the Field River, but the former have been subjected to 
much greater disturbance by faulting. The less cover on the 
highlands bordering the Onkaparinga, and the deep tribu- 
tary gorges of the valley, permit a more complete study of 
the plan of the beds than is possible in the Field River and 
Brighton districts. 
Purple Slates.—In consequence of the westerly trend of 
the coast the outcrop of purple slates, which at Marino, near 
Brighton, is only half a mile wide, increases to a mile on 
the Field River; and on the Onkaparinga, when measured in 
a direct line from the coast to the eastern limits of the faulted 
sections, is nearly four miles wide. 
On the north side of Noarlunga these slates cross the main 
road and are exposed at the base of the cliffs, on the right 
bank of the river, for a quarter of a mile, capped by tertiary 
beds and old river gravels. Оп their western limits the 
purple slates end abruptly in a steep cliff facinz the west, 
