Sydney Silcrete

PREFACE: The information on this page is being added to progressively (updated by JB)

and is intended for use primarily of LachlanHunter Associates for private study and

correspondence. It is considered to be of "working notes" standard and reliability.

Silcrete is basically 'self-cemented sand' - sand which has become cemented by exactly the same substance as the grains themselves are composed of, silica.

Although silcrete is a distinct and clearly defined rock type, there are still some people who do not know it or may confuse it with other things. Silcrete-recognition classes are available for anyone who thinks they may need such, starting at $5 p.h. (for pensioners or the unemployed) but mostly $10 p.h.

Silcrete is a hard fine to medium-grained sedimentary rock (occasionally pebbly or coarse-grained) formed mainly of silica, and sometimes with a conchoidal fracture. Various other names are met with for bodies of silcrete: grey billy or billy, ganister, sarsen stones, grey wethers and siliceous duricrust.

Etymology: Latin silex = flint + (con)crete < concrescere = united or connected by growth>.

Definition: Any size of detrital clasts bound together by siliceous material to form concretionary rock, such as breccia, conglomerate, or silicic sandstones.

Historical Connection: Any element or mineral in solution capable of being a cement for unconsolidated detritus can be used in prefix to make a specialized type of concretion, ferricrete and calcrete for example.

Cited Usage: Silex is the name of a genus of semi-pellucid stones [1753 Chambers' Cyclopaedia Supplement].

The name silcrete was popularised for surficial siliceous material of this sort by Lamplugh (1902, 1907).

A very good book for those interested in silcrete is Trevor Langford-Smith's (ed.) "Silcrete in Austalia" which is currently available at used/antique booksellers (e.g. currently for EUR 18.00, ca. $22 US from Holland; or $50 AUS from Sydney).

Thin sections of silcrete, some shown below, reveal the large content of detrital sand grains, usually dominated by quartz grains. If lithic grains were originally present these may have been altered or dissolved. Between the detrital grains is a very fine-grained nearly amorphous siliceous cement that is thought to have been deposited from solution. Silcrete which is bimodal, like T40955 below, often shows illuvial features and may have formed from sandy soils rather than fluvial sand deposits. The opposite extreme to the bimodal or "terrazo" type fabric is that zero matrix or "quartzitic" or "saccaroidal" type fabric seen in the thin section (below) of silcrete from German Hill. Clean well sorted stream sand may favour such textural development. All gradations appear to occur, similar as described for English sarsen silcretes (below). The large silcrete bodies at Upper Castlereagh, however, are formed in typical stream deposit sand, as is shown from the sedimentary structures preserved (which are somewhat similar to those seen at German Hill in sarsen blocks) and the texture of some Maroota silcrete suggests it is similar. For a time it was thought in New South Wales that there were two different genetic modes of silcrete, that formed beneath basalt being different to that formed as duricrust, and that the sub-basaltic type might be diagnosed by preponderance of the quartzitic type texture. This distinction, however, has grown dimmer over time and whether there is any distinctively differ process of sub-basaltic silcrete formation must be doubted.

Silcrete is very common in the arid regions of Australia, and the northwestern part of New South Wales, often forming prominent landscape features like resistant caps and breakaways. The term siliceous duricrust is also used for it in these regions.

The best known and most prominent silcretes, at least in Australia consists of Tertiary sediments. However, older occurrences of silcretes, often as much thinner horizons, also occur in Mezozoic and Palaeozoic sequences. The older and often thinner formations of silcrete are often known as ganister (Jackson 1997). These sometimes occur as coal seam seat earths. From many detailed studies of "ganisters" (e.g. Gibling and Rust 1992, Perciveil 1982 and 1983, Retallack 1977 and 2001) the typical ganister is regarded as a silicified near-surface horizons, e.g. E horizon of a palaeosol. Their silica-cementation is attributed largely to dissolution of plant opal and its redeposition as silica cement. The formation of ganisters within modern soils have been observed by McCarthy and Ellery (1995) and Rettallack (2001). Various silicified weathering surfaces or unconformities have been recognised in Early Palaeozoic sequences for a long time, from as early as Leith (1925).

Silcrete from Schofields, thin section T35620. (Photo: Tessa Corkill)

Silcrete from Maroota, thin section T41528. (Photo: Tessa Corkill)

Silcrete from Newtown, thin section T40955. (Photo: Tessa Corkill)

Silcrete of German Hill, Central Otago, New Zealand. The silica cement is nearly all in the form of crystalline quartz

overgrowths that are syntaxial with their host detrital quartz sand grains, and has filled all of the available pore

space to render the rock extremely hard with tightly interlocking grain boundaries. There is some minor

suturing of grain margins. (Photo: Youngson et al. 2005)

A mesa capped by silcrete developed in sands of the Mirackina Palaeochannel sands, described

by McNally and Wilson 1966 (Photo: Greg McNally)

In the flat areas of 'inland' Australia, silcrete is very widespead as detritus and in situ is often best oberved associated with mesas or scarps at tableland edges. The above example is one of many mesas in the Arkaringa region of northern South Australia. These mesas are "reversed topography" of Tertiary silicified palaeochannel sands, along the Mirackina Paleaochannel described by McNally and Wilson (1966). The palaeochannel remants now exist 30-40m above the surrounding plains. The thick pallid deep weathering zone is seen below the Tertiary level, locally obscured by a thin veneer of ferruginous material spread down from above.

"SICRETE TRAINS" - Silcrete 'trains' doesn't mean trains full of silcrete, like coal trains. Rather the term has come into use more and more for certain highly elongate or string-like distribution patterns of silcrete bodies which appear to reflect preferential silcrete development along old drainage lines. As further discussed below, the term 'silcrete train' has been used for occurrences in England, and at German Hill in Central Ontago, New Zealand (Youngson et al. 2005). Nash, Shaw and Ullyot (1998), as well as Shaw and Nash (1998) have described drainage-line silcretes of the Middle Kalahari and the Okavango Delta area as possible analogues for the silcrete/sarsen trains observed in England and elsewhere. Shaw and Nash suggested that surface silcretes have developed by silica accumulation in seasonal pools remaining after the annual Okavango flood, whilst sub-surface horizons appear to have formed under conditions of varying pH associated with fluctuating groundwater levels beneath the channel floor.

The Mirackina Paleaochannel, hundreds of kilometres long, is a good example of such linear concentration of silcrete related to a former stream flow line:

( Source: McNally and Wilson 1966 )

WIDE INTEREST IN SILCRETE

Silcrete is of general interest to archaeologists, geologists, geomorphologists and occasionally others (e.g. on account of past industrial uses as a refractory material).

Why silcrete is of interest to archaeology is because it has been extremely widely used in stone tool manufacture, and also because the large "sarsen" stones (a.k.a. grey whethers because they were fancied to look like a flock of sheep) used to build megalithic monuments in southern England and in Europe. At Stonehence and Avebury and many other sites this material has been used. The sarsen stones are in general regarded as silicified remains of a cap of Tertiary sandstone which once covered much of southern England. It is a dense hard rock, consisting like all sicrete of detrital sand grains bound by a silica cement.

Sarsen field at Fyfield Down, England.

Pitting of uncertain origin on a transported sarsen stone on a beach of Langstone Harbour,

west side of Hayling Island, Wessex coast of England. (Photo: West, 2004)

Sarsen stone used as a garden feature at The Lee near Chesham station, Hertfordshire.

(Photo: Open University Geological Society, Walton Hall Branch)

Summerfield (1979) wrote that no detailed explanation has yet been provided for the origin of the sarsens which are widely distributed across southern England. He noted that analogous materials are recorded from every continent except Antarctica. Many sarsens have been transported and occur as erratics but many others are thought to be more or less where they formed, such as the Fyfield Down ones shown above. The preservation on the surface of some sarsen stones of pockets of very weakly cemented sand, does not concur with the idea of long-distance transportation, but rather suggests local derivation from an original Cainozoic formation. Summerfield (1979) recognised that they are typical silcrete remnants and from that drew broad conclusions about their genesis and palaeoenvironmaental significance. From macromorphological, micromorphological and chemical comparisons with low latitude silcretes, primarily from southern Africa, he concluded that sarsens represent remnants of surface and near-surface silicification developed on stable land surfaces of minimal local relief. Most sarsens seem to have formed under a semi-arid or arid climate, although there is evidence of development in a relatively humid environment for some occurrences.

Dr Stewart Ullyot in the Geography/Environment department at Brighton University has research interests centred on the development of silicified Cainozoic sediments in southern Britain with particular emphasis on the sarsen stones, as well as silicified near surface sediments (silcretes) in Europe and elsewhere in the world.

Stewart J. Ullyot and David Nash developed a course: "Unravelling the diagenetic history of groundwater silcretes in. the eastern South Downs, Sussex, UK" and published a paper on the silcretes of eastern South Downs (Ullyott et al., 2004). Silcrete distribution in the eastern South Downs is localised and discontinuous, with occurrence principally as dislocated boulders on the Chalk. Many occur at higher levels in the landscape, on or near interfluves. Boulders commonly have a-axis dimensions of 0·5-2 m and thicknesses of 0·3-0·6 m. Silcretes in higher positions typically exhibit angular tabular or prismatic shapes whilst those in derived settings are more rounded, suggesting weathering and erosion during transport from a localized high level silcrete lens (or lenses). Three varieties of silcrete have been identified. Pale grey saccharoid sarsens are most widespread, and exhibit a simple grain-supported fabric with predominantly sand-sized quartzose sediment cemented by optically continuous quartz overgrowths and minor micro- and cryptocrystalline silica. Brown hard sarsens also exhibit a grain supported fabric but with more fine sediment and a greater variety of cement types. Deposition of the host sediment appears to have been followed by illuviation of clay-rich material via the primary fabric and along cracks to form geopetal caps, drapes and vein-like structures. During silicification this finer material has been replaced by less well-ordered silica whilst optically continuous quartz overgrowths characterize purer areas.

Sarsen field at German Hill, Central Otago, New Zealand

Large sarsen stone at German Hill, Central Otago, New Zealand. At German Hill the sarsen stones are slab-like, with clear horizontal bedding and vertical jointing, and may show cross-bedding and conglomeratic bases of channels. Such features are also detectable for the large silcrete

bodies at Castlereagh west of Sydney, Australia.

(Photo: Youngson et al. 2005)

Large sarsen stone silcrete remnants up to 5m thick are preserved relatively undisturbed at German Hill in Central Ontago, New Zealand (Youngson et al. 2005). Some of the stone are regarded as still genuinely in situ and they occur as a field of blocks ca. 7 km long and about 300 m wide. This then is overall a very elongate "train" of sarsen blocks.

Silcrete blocks regarded as still in situ where the silicification took place at German Hill are underlain by soft sandy sediments. On the south coast of New South Wales, at Bendalong (Red Head), the 1-2m thick silcrete deposit has also been described in terms of it being in situ, e.g. grading into weak sandstone and clay-rich sands (Callender, 1978).

In New South Wales those who have been interested in silcrete include Glenn Atkinson. Glenn currently lives in Wauchope and works as Senior Natural Resource Analyst at the Department of Natural Resources in Kempsey. He formerly resided in Sydney and has long been interested in Castlereagh where the largest bodies of the silcrete have been found. Glenn graduated in Geology at UNSW in 1974 and then tutored there in Geography until joining the Soil Conservation Service in Sydney in 1980. There he developed the soil-landscape mapping program that is still used and published a map of the Soils of NSW for the Bicentenial Atlas of NSW. One interest he developed was in the Quaternary stratigraphy of the alluvial terraces along the Nepean River. When doing soil landscape mapping for the Sydney region he considered that a good understanding of paleogeography was essential to understanding soil distribution. He did a soil survey and consideration of erosion control measures for the Penrith Lakes Scheme (Atkinson 1982) and completed a Masters of Science thesis in Physical Geography at the University of NSW researching the landscape history of the River Terraces of the Nepean River in the Penrith-Windsor area (Atkinson 1983). When he was working at Castlereagh a very big silcrete boulder was dug up (presumably the one that is the largest ever found and which is currently situated at the Murru Mittigar museum at the Lakes). Thus began his long interest in silcrete "erratics", puzzling because they just didn't seem to fit most models of landscape evolution. He considered some earlier ideas in a review of soil and geological maps of the Nepean River terraces (Atkinson 1987). Glen has also been interested in Aboriginal stone utilisation. He has found artefacts at a number of sites at Richmond, Agnes Banks and and Castlereagh and has made photo and location records of such, as well as later on finding some remarkable silcrete artefact factory sites north of Inverell that were undocumented. He collected large alluvial silcrete boulders from Barden Rd at Menai (Barden Ridge), in laterite overlying Wianamatta Group shale, and that site is discussed below.

Silcrete has been a material favoured worldwide for manufacture of stone tools. Stone Age tools made of silcrete are abundant in Africa, Europe, etc.; and in Australia silcrete is perhaps the most common material used by Aboriginal people to manufacture flaked stone artefacts.

Since 1991, excavations at Cuddie Springs, an ephemeral lake near Carinda in NSW, have exposed numerous bones of the now extinct animals that constituted the Australian megafauna. These animals included a giant flightless bird called Genyornis newtoni, the large wombat-like creature called Diprotodon and a kangaroo called Sthenurus. Cuddie Springs had been excavated in the past (late 19th and early 20th century) but in the more recent work a silcrete utilised flake found beneath a Genyornis limb bone has been interpreted as having traces of blood preserved near the used edges and a silcrete hammerstone with bone residue was found with Diprotodon and Genyornis. Cuddie Springs is now thought to have been a butchering site and, according to Field (2006:13) it is "the only site on the Australian continent that has yielded a record of humans and megafauna that co-occurs over an extended time period and through sequential stratigraphic units". The ages of levels where bones and stones are found together has been determined by radiocarbon dating to be between approximately 36,000 and 30,000 years ago (Field 2006:12-13).

A silcrete hammerstone found with Diprotodon and Genyornis bones. Arrows
indicate location of bone residues. (Furby 1996)

Corkill (1999a) did research to identify the distribution of potential sources of flaked stone found in Aboriginal archaeological sites in the Sydney region, and to find methods suitable for directly relating artefacts and source materials. She found that all stone raw materials in Sydney archaeological assemblages are available in the Sydney region, mainly from Tertiary and Quaternary gravel beds, and that these are widely scattered. In most analysed assemblages, the highest percentage of flaked stone, numerically, is silcrete or quartz, depending on site category and location - quartz predominates in rockshelters in the deeply incised sandstone areas, silcrete is dominant in open "campsites" on the Cumberland Plains. Evidence suggests that, where a number of other knappable rock types were also available, silcrete may have been preferentially selected.

Geologists also have been interested in the silcrete associated with the coast-flowing rivers. The main interest is that this silcrete remains mysterious as to its source. Unlike silcrete further inland, nothing has been found of the surface or surfaces where the silcrete now found on river terraces or in fluviatile deposits likely formed. All the silcrete in the Sydney region occurs as transported clasts, some of them very big. Silcrete clasts in the Nepean River gravels at Upper Castlereagh range up to be larger than a small car.

Besides the silcrete that is well known from the deposits of the former Nepean and Georges Rivers, silcrete clasts also occur in the Hunter River deposits (Esteves, 1999), and along the Hastings River as shown in the below photo by Glenn Aitkinson.

Photo by Glenn Aitkinson: Large silcrete boulder at the subdivision "Riverbreeze Estate"

Source: Glenn Aitkinson. The red dot marks the site of "Riverbreeze Estate" near Wauchope where the above silcrete boulder was photographed. This is on a string of Tertiary deposit residuals marking the paleo Hastings River course. The deposits are along a terrace of about 50 m elevation.

Many large silcrete bodies like these, generally well smoothed by abrasion, are laid to rest around the Penrith Lakes set

of quarries at Upper Castlereagh near Penrith. They have been excavated or dredged from within the layer of old

Nepean River gravel. These ones were photographed near the Penrith Lakes corporation buildings in 1995.

The river lies out of view, behind the foreground rise, and the hills in the background are the face of the

Blue Mountains plateau (Lapstone Monocline structural zone). (Photo:Tessa Corkill).

Much larger silcrete clasts than this are present at Castlereagh (photos yet to be added).

Crescentic gouges, shallow grooves and striations on Sioux Quartzite, southwestern Minnesota.

For comparison with large bounders found in the Nepean at Castlereagh, such sizeable clasts can be moved by floods on even smaller rivers than the Nepean. Following extreme rainfall in the Bay of Plenty region of New Zealand a debris rich flood swept through the town of Matata on May 18, 2005. This devastated parts of the town as it was swamped with boulders, mud, sand and transported logs. The carriage of large boulders was a feature of this event, and older deposits show that similar or even larger flows occurred in the past. Transported boulders are up to 7m across. The May 18 flow moved very fast and ferociously, like a flash flood, and one eyewitness reported seeing trees and boulders fired out of the moving mass "like canonballs" (Mr Maloney, NZ Herald 18 June 2005). Nothing near Castlereagh was ever likely to have been closely analogous to the mode of transport at Matata, yet this provides examples of large transported boulders.

Bill Hutchinson besides a large boulder carried in the 2005 Matata flood deposit.

Another large clast in the 2005 Marata flood deposit. One complete house was also

Silcrete is found at many places in the Sydney region. It has been of some interest to geologists, geographers, soil scientists and archaeologists. Tessa Corkill has recorded most of the sites noted herein. Menai occurrence information comes from Glenn Atkinson.

Further inland from the Sydney Basin the typical Australian siliceous duricrust is extensively associated with (overlies) deep weathering profiles early regarded as having been formed by Tertiary weathering under climatic conditions resembling existing subtropical types ( Langford-Smith and Dury, 1955 ).

In the past various authors, e.g. Val Gobert (Val Smith) in the Penrith 1:100,000 sheet geological notes, have thought that around Sydney silcrete had in some places formed in situ. However every reported occurrence of in situ formed silcrete near Sydney which has been visited to date has not been substantiated. Gobert made short mention of silicification within the Tertiary sediments west of Sydney,

e.g. that the St Marys Formation is "well consolidated and silicified", and referred to silicified matrix in the Rickabys Creek Gravel.

Gobert and others have thought that thought such "silicification" was part of a lateritisation process. The present writer (JGB), however, is yet to find any silicification of these sediments - but is yet to visit all of the sites discussed, such as at GR 935702 in the Londonderry Clay where Gobert noted a "highly siliceous" hard white layer which she thought might be a lateritic profile pallid layer (Penrith notes, p. 36 - Jones and Clark, 1991). Although this writer (JGB) has not yet seen any such silicification in Sydney's Cainozoic sediments, Glenn

Atkinson and others (pers. comm.) have spoken about seeing groundwarer silicification of more porous units, especially sands, in Tertiary deposits and even in modern soils where the terrain and silica supply is right.

Example of relatively young hardpan formation. In Australia some relatively recent soil hardpans may be siliceous to some

extent but do not approach such degree of silicification as the supposed Okavango flood water silcretes. The above

hardpan, up to 1m thick is in a railway cutting at Glenrowan, Victoria. It rests upon alluvium-colluvium derived

from the Glenrowan granite. The surface half metre or so of that granite's saprolite has also been indurated

Silcrete of neither hardpan nor laterite weathering profile origin has definitely been observed in the Sydney region.

Indeed, no in situ silcrete whatsoever has been found. Previously reported "in situ" silcrete and "outcrops of silcrete" have been revisited and discounted. No silcrete is seen other than material which has been transported from wherever it could have formed. Silcrete clasts are found in pale clayey sand and sandy clay deposits regarded as "Tertiary" in age, although sometimes only on meagre evidence. These host sediments very often show signs of ferruginisation or laterite development at or near the present land surface. A notable exceptions is at Castlereagh (location 6 on map below) where large rounded bodies of silcrete have been reworked into younger gravels of the Nepean River.

The only known case of silcrete, or any silcrete-like rock, now being found where it may have formed is at Marsden Park diatreme. Here some silcrete-like 1-2m diameter boulders were found bulldozed aside at the breccia quarry. From thin section examination it was concluded (Larry Barron, pers. comm.) that this rock was originally an uppermost facies of the diatreme, and consisted of silicified reconstituted sandstone (i.e. made of quartz sand grains originating from disaggregated Hawkesbury Sandstone brought up by the diatreme). At the Marsden Park diatreme the entire original exposure, except for one small patch of volcanic breccia, was of this 'reconstituted sandstone', Larry thought.

The above illustration (with location numbers added) is taken from a 2004 conference poster, in turn from

Corkill 1999a. These occurrences have been studied in detail (ongoing) by Tessa Corkill, and a few

also described by John Byrnes. A further occurrence, at Maroota, is just north of the map view.

In the Sydney region, silcrete was a common stone used for making flaked tools. Earlier researchers were aware of few, if any, sources of suitable flakeable stone in coastal Sydney and it was thought that the silcrete would likely have been brought to coastal sites from the Cumberland Plain up to 40 km away. Subsequently it is known that silcrete sources do occur closer to the coast, as at the Newtown (23) and Newington (22) locations shown on the above figure.

Photograph (also from the abovementioned poster) showing silcrete cobbles and boulders

in a railway cutting at the ADI Site near St Marys. [Source: Attenbrow et al., 2004.]

Another view of ancient (Tertiary) gravel attributed to former presence of the Nepean River. This is at the dip crossing South Creek at the junction of St St Marys and Stony Creek Roads. Silcrete has been recorded from the extensive Rickabys Creek gravel but it is not very common and no locations

Source: Tessa Corkill (Corkill 1999a). Thesis map showing all sites silcrete was collected from.

Data for silcrete sites. A "(T?)" in the 'Geology' column indicates that there is unmapped

Elizabeth Drive at Badgerys Creek - Silcrete in situ in roadway cutting - only one large boulder seen. Several large and small pieces in garden of nearby house. The below photographed spot is nominal "No. 2028" along Elizabeth Drive in Google Earth Street View.

The above two photos are a before-and-after pair. The 'before' one was taken quite some months before the second one, and the silcrete boulder is seen sitting directly in front of the big tree in the centre. The bottom one shows small furrow dug by archaeologst (JE), to make it easier to haul the boulder further up on the bank. This was done (in August 2008) because of some concern the boulder might be lost in further roadside work. Comparison to the first photo shows that the face had been recently 'cleaned up' of graded (probably in early August 2008) .. scraped virtually right where the boulder was sitting in its matrix (which is an ironstone gravel bed of presently uncertain origin at this stage). Also in the ironstone gravel are found clasts of locally derived Bringelly Shale sandstone, some of them quite large.

North of this site is a large landfill site (probably an expansion of a clay pit commenced originally by Lion Tile company?), and in the northeastern corner of that (ca. 2006) there was exposed a palaeochannel of presumably Tertiary age. The channel had not been discerned by the pit owners, as the clayey sediment filling of it was not greatly unlike the surrounding weathered Bringelly Shale clay, except for being appreciably richer in dispersed quartz grains. No silcrete was found in that channel however.

Simple location map showing two stippled areas of which the finely stippled area in the south

is the Castlereagh river flats at Upper Castlereagh, just north of Penrith, and the northern

more coarsely stippled area is the Richmond river flats (or Richmond Lowlands) of similar

aged alluvial deposits (unknown to contain such large silcrete clasts). Relatively young

The Castlereagh, or more precisely "Upper Castlereagh", river flats area shown above has being largely removed by quarrying (leaving a series of voids which become water-filled, and the area is now known as the Penrith Lakes Scheme). During the course of this quarrying many large silcrete boulders have been recovered. However, for lack of more exact details on where any were excavated a map showing their in situ distribution across the area of the river flats is impossible at present.

Fox and Associates (1986a, p. 31) wrote "The Wianamatta shales are also known to have silcrete outcrops north of the study areas at Silverwater and on the Cumberland Plain (Fox & Associates, 1985b)".

Fox and Associates (1986a) actually has no "Fox & Associates, 1985b" in its list of references but it does have a "Fox & Associates, 1986" (Fox & Associates, 1986b herein) in the references list, which might be the source referred to. It is also known that draft reporting for this was done in 1985.

The archaeological component of the Homebush Bay Conservation Study was carried out by Beth Rich. Rich found silcrete cobbles in the Newington Arms Depot and this later led Tessa Corkill to visit the arms depot. Corkill found Tertiary palaeochannel remnants and more silcrete cobbles, and other rocks types including material which looks to be silicified fine-grained sediment with plant remains. Corkill notified the Geological Survey of this occurrence and it was visited by (Iain Patterson). Corkill recorded that at this time some archaeologists thought these palaeochannels were part of the Wianamatta Group, but the phrase "silcrete outcrops" is unexplained and probably an exaggeration as only small cobbles are known to have been found.

Tessa Corkill collected some large pieces of silcrete from the site when it was still Navy-controlled land. Some of the silcrete pieces were on the surface, but in disturbed contexts. There was also some exposure of what looked like an in-situ cobble deposit visible in a road cutting. The Geological Survey also visited this site in April 1995. General opinion was that it might be a palaeochannel deposit.

The area has since then been densely was built over as the Olympic Village. Up to 2004 however, silcrete was noted as still in evidence (possibly in-situ) in the Newington arms depot section of Olympic Park.

Silcrete locality is 300m east of the western end of Barden Road. The heavier line along Barden Road

(This silcrete locality was first told about to us as being in a quarry ca. 300m east of the western end of Barden Road'. No such quarry now exists. In the above the heavier line alongside Barden Road on the plan signified

new road works intended for the Bangor Bypass, which now exists (as seen below in earlier ex-Google Earth

view of it when under construction. Note that the hill crest along the motorway is just west of the Australia

Road bridge over it. To the east of that bridge the land surface slopes down and Hawkesbury Sandstone

soon outcrops (according to local resident information stone was once cut there but the motorway could

have removed any small sandstone quarry workings which might once have been there [?] ). West of

the bridge the thin light coloured horizontal band that is seen is a narrow drainage bench cut along

( This motorway, the Bango Bypass, was opened in February 2005 (hence the GE image viewed

Conceptually, Barden Road is one of the most important silcrete occurrences in the Sydney region, even though the area has now become highly suburbanised and no silcrete (or laterite) can be seen there today (2008). The occurrence of silcrete at Menai was first learned of by LachlanHunter group in 2006 from Mr G. Atkinson. Silcrete as a large boulder in the pallid zone was found there by Glen Atkinson, then a geologist working as a tutor at University of Sydney Geography Department (later, 2008, Senior Environmental Scientist at Department of Environment and Climate Change). According to Glen (pers. comm. 2008) the silcrete was in a quarry just off Barden Road which was a field site by UNSW geography students in the 1970s. The lecturer was Tony Shepherd. Glen's memory of the site is it that on the eastern side of the quarry there was "Wianamatta Shale with a nice red podzolic soil developed. On the western side was a deep laterite with a full indurated, mottled and pallid zone to a total depth of about 5 m. The point of the field work was to observe the influence of time on soil formation on the same parent material but the existence of the silcrete in the pallid zone proved this wrong. It proved that the laterite was developed on a Tertiary fluvial sediment overlaying the Wianamatta. The size of the boulder also says a lot about the stream power on what is now an elevated and relatively flat residual landscape".

Checking via Google Earth showed that the Barden Ridge Quarry was no longer there, and a visit confirmed that. According to Mr Atkinson in 2006, the former quarry was on Barden Rd about 300 m east of Illawarra Rd. It was in Tertiary sediments overlying the shale although the nature of these sediments was not initially noted due to lateritisation. Tutors in UNSW geography department used to take students there to view the laterite development over shale and at first did not look closely enough to note the presence of alluvial material. The silcrete was found right on the hill crest in the area. One large boulder was collected from this site, approximately 60 cm x 40 cm x 20 cm in size. It was moved to Grays Point when it seemed likely that the quarry would be destroyed or built over.

The big Menai sicrete clast. Fearing (correctly as it turned out) that this large silcrete boulder could be lost

in the changes going on alongside Barden Road, Glen Atkinson relocated it to his front yard at Grays Point.

Close-up of the big silcrete boulder clast. The ferruginous stains on the surface reflect the patchy ferruginous

weathering zone it was taken from. The "Kleenex TO GO" packet is 11 cm long. The vertical face in

front of it is quite smooth to the touch. The top surface of the boulder is much rougher and this is

perhaps because the top was once covered by percussion strikes made by other clasts,

Ashfield Shale is horizontal ruled, Hawkesbury Sandstone obliquely ruled. The shale patches north of "WO" in WORONORA is the Barden Ridge area where the Tertiary alluvium containing silcrete occurs.

Atkinson, G., 1982. Soil survey and erosion control measures for the Penrith Lakes Scheme. Soil Conservation Service of NSW, Sydney.

Atkinson, G., 1983, Soil Stratigraphic Relationships of the Clarendon and Cranebrook Formations, Nepean River, NSW - A Multivariate Analysis. University of NSW. M.Sc. Thesis (unpublished).

Attenbrow, V., Doelman, T., Corkill, T. and Hugh Watt, H., 2004. Poster: "Artefact Size and Raw Material Sources in the Sydney Region: A Preliminary Investigation" by Val Attenbrow (Australian Museum), Trudi Doelman (Department of Archaeology, University of Sydney), Tessa Corkill (Australian Museum) and Hugh Watt (Australian Museum)].

Callender, J., 1978. A study of the silcretes near Marulan and Milton, New South Wales. In, Langford-Smith, T. (Ed.). Silcrete in Australia. Department of Geography, University of New England.

Corkill T., 1997. Red, yellow and black: Colour and heat in archaeological stone. Australian Archaeology 45: 54-55.

Corkill T., 1999a. Here and There: Links between Stone Sources and Aboriginal Archaeological Sites in Sydney, Australia. Unpublished MPhil thesis, University of Sydney.

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For Attenbrow et al. 2004, the poster that some of the above information is taken from, may be viewed or downloaded in better detail from:

Area	Code	Easting	Northing	Elev(m)	Geology	Rock Types Collected
ADI Dunheved	ADI/-	294150	6264600	30	Qal (T?)	silcrete
Badgerys Creek		See herein (below)				Found subsequent to Corkill (1999a).
Bells Creek	BC/2	300000	6265000	45	Rwb (T?)	silcrete
Blacktown	BT/2	307040	6259680	45	Rwb & Qal (T?)	silcrete
Blacktown	BT/1	304500	6261100	55	Rwb (T?)	silcrete
Cranebrook	C/-	285500	6267800	5	Q over Rwa	silcrete
Doonside	Do/-	302300	6259900	35	Qal	silcrete
Echo Vale	EV/-	297850	6270450	20	Tl (Tr?)	silcrete, silicified wood, imtc, qzite, igneous
Freemans Reach	FR/2	295400	6285400	35	Rwa (T?)	silcrete
Freemans Reach	FR/1	295700	6284400	45	Rwb (T?)	silcrete
Glenroy	G/A,B,C	294900	6284500	55	Rwa (T?)	silcrete, qzite
Holsworthy (N)	H/2	308500	6240800	20	Ta	silcrete, qzite, quartz, unknown
Holsworthy (S)	H/1	306600	6232100	110	Rwl & Hs (T?)	silcrete, ferricrete, quartz
Maroota	Ma/7	313300	6296700	210	MS	silcrete, qzite, quartz, ferricrete conglom.
Maroota	Ma/9	313000	6296500	200	MS	quartz, q'zite, ?igneous, jasper, silcrete
Marsden Park	MP/-	299150	6270100	30	Ts	silcrete, silicified wood
Marsden Park	MP/1	299100	6269850	25	Ts	silcrete, qzite
Moorebank	Mo/-	309800	6243400	10	Tc	silcrete, quartz
Newtown	N/-	331600	6247600	40	Rwa (T?)	silcrete
Olympic Village	OV/-	320100	6254500	10	Rwa (T?)	silcrete, ?silicified wood/chert/FGS
Plumpton Park	PP/-	299500	6263000	50	Rwb (T?)	silcrete, sandstone, ironstone
Plumpton Ridge	PR/-	301000	6266000	50	Ts	silcrete, silicified wood, semi-consolidated sediment
Riverstone	Rs/-	302400	6272750	40	Rwb (Ts?)	silcrete, quartz, s/wood, s/s conglom.
Second Ponds Creek	SPC/-	306600	6271350	40	Rwa (T?)	silcrete
West St Clair	WSC/1	294050	6257460	40	Rwb (T?)	silcrete
Wilberforce	Wi/3	300300	6285750	40	Tr	silcrete, qzite
Wilberforce	Wi/2	300350	6285500	40	Tr	quartz, qzite, imtc, ?silcrete, conglom/ breccia, igneous*
Wilberforce	Wi/1	298440	6286340	30	Rwa (Tr?)	silcrete, silicified wood, ironstone, sandstone

SILCRETE

With special reference to the Sydney region

( Written in collaboration with Tessa Corkill, and draws extensively

on her work - but any faults and errors are due soley to JGB. )