298 



There could scarcely be a more striking illustration of rain erosion than is 

 seen in these lofty and bare cliffs, set far back from the sea, the waves of which 

 have never washed their base, yet they are wasting so rapidly that no living 

 plant can establish a footing on their slopes. They do not weather in a straight 

 line, but the rain sculptures the face of the cliffs into regular scallops, nicked by 

 crevices and varied with coves and buttes. At the southern end the cliffs are 

 suddenly truncated facing to the outlet of Morphett Vale Creek, where a pic- 

 turesque butte (see pi. xxiii., fig. 2) has been isolated and is surrounded by many 

 acres of perfectly bare clay, undergoing waste, which, on a small scale, recalls 

 to mind the "bad lands" of the Colorado. 



The outlet of the Morphett Vale Creek is largely choked by sandhills which 

 form the banks on either side. On the southern side of the creek, and for some 

 distance in the same direction, there are usually thick layers of a black and 

 ruby-coloured sand piled up against the base of the cliffs, giving the sands an 

 appearance of being covered with coal dust. 



A little south of the Morphett Vale Creek the upper clay beds of Recent 

 age begin to show themselves at about high-water mark and form a cliff about 

 10 feet in height. As these beds are washed by high tides the cliff is rapidly 

 retreating. These clays continue to form the only feature of the sea-cliff for 

 about a mile in length, the latter slowly increasing in height as it goes southward. 

 At about half-distance between the Morphett Vale Creek and Witton Bluff the 

 underlying mottled clays and sands begin to show themselves in the cliff section. 

 The contrast between the two beds of clay is here very marked. The lower 

 bed is a compact, greenish and reddish mottled sandy clay, carrying numerous 

 angular, or more or less rounded, stones sporadically distributed through the mass 

 and lying at all angles. The bed is sufficiently tenacious to resist for some time 

 the action of the waves, as large patches, a foot or two in height, occupy the 

 beach between tides. Harder portions that have become strengthened by cal- 

 careous or siliceous infiltrations stand up as ridges intersecting the beach after 

 the softer portions have been washed away. The bed, in its upper limits, 

 becomes grey and red, in mottled patches, but preserves its essential features in 

 its compactness and the sporadic manner in which the included stones are dis- 

 tributed. In both cases the stones consist mainly of quartz and quartzite, and 

 in addition to these there are irregular concretions such as are commonly found 

 in fluviatile sediments. A precisely similar clay bed makes a low cliff a little 

 south of the mouth of the Aldinga Creek. 



The relationship which the newer and less compact clays bear to the older 

 mottled clays can be well studied on this part of the coast. A very distinct 

 unconformity can sometimes be recognized between the respective beds — the 

 overlying reddish clays and water-sorted gravels 'occupy gutters of erosion in 

 the underlying compact clay. 



With the exception of a large conical sandhill, situated a little south of the 

 Morphett Vale Creek, no important sandhills exist at prseent between that 

 creek and Witton Bluff, the wind having swept the low cliffs entriely bare of 

 their sand, and the exposed floor consists of a continuous sheet of nodular 

 travertine. 



From the jetty at Port Noarlunga to the mouth of the Onkaparinga — 

 about a mile of coast — there are uniform features. The whole distance is 

 covered by sand dunes which rise to a height of 50 feet or 60 feet. As soon as 

 the tidal flats are reached, at the mouth of the river, the older marine Tertiary 

 beds appear in the form of low tabular exposures situated between tide marks. 



