To: AUS-NSW-PENRITH-L@rootsweb.com WONDERING WHY CASTLEREAGH-PENRITH-EMU PLAINS EXISTS by John Byrnes Castlereagh Study Group http://www.webspawner.com/users/castlereagh/index.html Hello Penrith listers, Here's a few thoughts of mine about what formed the sandy plain we now call Penrith, Emu Plains and Castlereagh (really one continuous geological formation historically divided for naming-convenience by the river channel). The Castlereagh river flats land and the Emu Plains are all the same thing just divided by an east-west bit of the river, namely one big expanse of alluvium .. All this abundance of sand which attracted quarrying to the area from very early days (including as a source for concrete for Warragamba Dam), from about 1880 I think (The Department of Mines kept rather meagre of records of what went on around Penrith in quarrying the alluvium, and sand extraction has never attracted anything like the geological attention that metals mining gets). Why is that big flat wide area of river deposits there anyway? I'm uncertain, as probably you are too(?). One obvious aim of the Castlereagh study group which I hope to promote is to very comprehensively collect ALL references to everything written on Castlereagh (and make this easily available ~ for if such a bibliography already existed it could be emailed or downloaded by anybody with ease - such is the computer age). The fact is I don't have a bibliography (just the aim to see one grow) and merely append here a meagre few references. So, for all one knows, there maybe is already in the literature some publications where somebody has already explained why the sandy plain exists? If so, please forgive my ignorance of it. Equally I know there have been some significant prehistoric finds too (stone artefacts) but I don't have the references to those either (,,,, and would appreciate getting such from anybody who does). One suspects that the alluvial plain was built up somehow in connection with the observed fact that the Nepean River at Penrith diverges out eastwards from its general northwards course along the Lapstone Monocline or foot of the Blue Mountains. Downstream of its confluence with the Warragamba the Nepean River falls into alignment with a lineament named the Kooree Creek Lineament (which extends northeasterly towards Maroota). I don't know where Kooree Creek is (somewhere to the NE?) and this lineament was named by Maugher et al. (1984) as part of a bigger study of fracture patterns. This lineament might be a zone of jointing which allowed deeper weathering and softening of the strata and was where the river could more easily erode away the stone. Thus the river was diverted to the northeast but its present course does not continue all that far to the northeast before swinging back to return to the Lapstone Monocline, and in the process give us features like Birds Eye Bend. What made it divert back to the Lapstone Monocline; why didn't it just continue going northeast directly towards Windsor once it had fallen under the influence of the Koree Creek Lineament? It did likely go a bit further northeast along the lineament in the past. Nowadays the river begins to turn in its swing back westwards at the Penrith Weir but in the past it probably continued northeasterly as far as Cranebrook Road and then swung back towards the west, passing along west of the foot of the terrace (Londonderry Terrace). A previous channel of the Nepean River can easily be recognised there to have run along the foot of this terrace. There is still a mild depression there, filled by about 2m of pale grey sand overlain by 3m of black clay. This clay band has been dated at 34,200 BP (Nanson and Young, 1985). It is also noted that the Willan (1925) map also shows a line of gravels along this orientation as well (whatever that means?). Whatever it did, and why ever it did it, this lateral mobility of the river around Penrith is what formed the alluvial plain. The details of it all would be quite complex I feel, and the sediments below the Castlereagh flats should bear a rich historic record of the last thirty or forth thousand years or more (I'd take a punt at sixty thousand years myself). I wonder how much of this history will get recorded, or deciphered .. as so much of it is destined to be soon lost forever. I first had some interest in this sedimentary sequence because of the large silcrete boulders that have been hit in the gravel pits (e.g. in the pit just north of Farrells Lane and east of Cranebrook Creek, GR 855678) and which have now been placed outside the aboriginal display centre (Long's old house?) at Castlereagh ~ becoming something like Castlereagh's answer to Stonehenge I always think. At that particular spot just cited, the gravel excavator also brought up large lumps of fresh Ashfield Shale when I visited (many years ago). Several of these silcrete boulders have been recovered, apparently always at the base of the gravels and in this respect it resembles the record of such large silcrete clasts in river deposits elsewhere in the world, in that these large clasts tend to work their way down to the bottom of the bedload. Also it is typical of them to be scarred with zillions of little crescentic percussion marks that record the strikes of cobbles against them during fast flow periods. The original source of these large clasts may have been a silicified sand horizon formed anything up to 20m higher than the present river flats. Earlier on (Byrnes 1982) I'd thought that the Nepean River was likely entrenched to a stratigraphic level about 15m above the base of the Ashfield Shale. This was based upon recovery of a piece of a distinctive rock type (shale spotted with small phosphatic concretions) which had been excavated at gravel workings "downstream of Penrith Weir". As that weir is further east than the river at Castlereagh, the river bed at Castlereagh could be incised even closer to the base of the Ashfield Shale. Now a map by Willan (1925) of the Sydney District interestingly shows the "Sea Level" structure contour on the base of the Ashfield Shale as running north between Castlereagh Road and the river, and crossing the river both near Jacksons Ford and Yarramundi. The river is at about 10m about sea level just above Jacksons Ford. Very roughly then, below the Castlereagh flats the base of the Ashfield Shale (and underlying Hawkesbury Sandstone) might be down about 22m (which is the elevation of the flats above sea level). Doubtless the quarry geologists would have worked all that out long ago more precisely, but I'm not aware of any references. The prominent escarpment which forms the eastern boundary of the fertile Castlereagh river flats is cut in a geomorphic terrace of older river gravels which Walker and Hawkins (1957) named the Londonderry Terrace. The river in relatively recent times became entrenched through a narrow cutting at Castlereagh, which Walker and Hawkins called the "Castlereagh neck" (and also this is where the river from its most divergent phase re-joined the monocline). It is also apparently a prominent nickpoint in the stream channel gradient. That may correspond, some postulate, to the point of maximum upstream extent of Pleistocene low sea level influence. The gravels underlying the Castlereagh flats have been found to contain logs and radiocarbon dates on these range back to 32,000 BP and the oldest sediments might have been deposited around 42,000 BP (Nanson and Young, 1985). At the surface of the flats the unit east of Cranebrook Creek, called the McCarthy's Lane unit by Atkinson (1982) is older than the unit west of Cranebrook Creek which is called the Castlereagh Road unit. This can be thought to reflect the migration of the plains building back towards the west after the phase of maximum divergence along the Koree Creek lineament. REFERENCES Atkinson, G., 1982. Soil survey and erosion control measures for the Penrith Lakes Scheme. Soil Conservation Service of New South Wales. 74 pp. (unpubl.). Byrnes, J.G. and Ferguson, A., 1982. Lithological evidence that the Nepean River channel lies within basal Ashfield Shale at Emu Plains. Geological Survey of New South Wales. Pet Rep No 82/51. Mauger, A.J., Creasey, J.W., and Huntington, J.F., 1984. Extracts and notes on the Penrith 1:100,000 sheet, IN: The Use of Pre-development Data for Mine Design: Sydney Basin Fracture Pattern Analysis. CSIRO Division of Mineral Physics - Report for National Energy Research Development and Demonstration Program Project 81/1357. Nanson, G.C. and Young, R.W., 1985. Late Quaternary history of the Nepean River near Penrith, New South Wales. Geological Society of Australia. Abstracts 13, 25-27. Willan, T.L., 1925. Geological map of the Sydney District. Prepared under the direction of E. C. Andrews, B.A. Government Geologist, Department of Mines, New South Wales, 1925. Walker, P.H. and Hawkins, C.A., 1957. A study of river terraces and soil development on the Nepean River, New South Wales. Royal Society of New South Wales, Journal and Proceedings, 91, 67-84.