Australia: The Land Where Time Began
Nauwalabila I Arnhem Land
This rock shelter is formed by a massive boulder that toppled off the nearby escarpment. This sloping sandstone slab covers a black, sandy floor on which were found stone tools and spear points. It is situated in Deaf Adder Gorge in the southern part of Kakadu National Park, in an area called Nauwalabila by the local Aboriginal People. The site is now known as Nauwalabila I. It had previously been called the Linder Site after a local explorer.
Excavations found stone tools to a depth of 3 m below present surface. Charcoal was found at a depth between 1.9 and 1.7 m that was radiocarbon dated to about 20,000 years. The upper 2.4 m was of fine sand that accumulated over a period of at least 25,000 to 30,000 years. A layer of weathered sandstone rubble about 40 cm thick was found under the sand layer. Below this rubble was a a layer of large rocks and red sand.
Stone tools were found in the basal parts of the rubble - heavy weathered quartzite flakes, that reveal a thick chemically-weathered skin on the outer surface if they are snapped.
At this site dolerite flakes were found showing signs of edge grinding, the ground facets of some having use-wear striations on the ground surface. These used flakes were found throughout the top 1.4 m of the deposit. The lowest example of these flakes coming from ground-edge axes is in the 14,000 year old level. Below this, pieces of volcanic rock (dolerite), the typical material for ground-edge artefacts because of its hardness, but at this site they have been heavily chemically weathered making it difficult to be sure of their original form.
Several lenticular objects made from volcanic rock were found in layers from about 25,000 - 30,000 years ago. They are exactly the same shape as Pleistocene hatchet heads from other sites, but were too weathered for positive identification. Also found in this layer are slabs of sandstone showing evidence of grinding and hard, heavy pieces of haematite, a high-grade ferric iron ore, with grinding facets on their surfaces, indicating they had been used as a pigment. The presence of high grade haematite at this site indicates long distance exchange or transport, as the nearest known haematite deposits are many kilometres from the site (Jones & Johnson, 1985b;Jones and Negerevich, 1985; Chaloupka, 1993).
Many of the Pleistocene sites have many colours of ochre in the lower levels, and there is also evidence that the ochre was ground and pulverised. In many Top End rock shelters there are small circular holes on flat rock shelves. Similar circular structures are found in excavated sites below sand deposits dated to more than 18 000 years, at Malangangerr and Nawamoyn.
The lowest levels of Nauwalabila I have been dated by optical (OSL) techniques. These levels contain the earliest stone artefacts, securely stratified within the basal rubble layer that dates to between 60 000 and 55 000 years old. Optical dating has been shown to be consistent with radiocarbon dating and with depth below the present surface, as it is at Malakunanja.
This site also contains evidence of a change in the local landscape. At the time of the arrival of hunters at this site the surface was 3 m below the present level. Since then the extensive sand sheet around the site, and the floor of the site, has risen by 3 m. So there was a sudden sand build-up in the valley, coincident with the arrival of humans, the rate of sediment accumulation increasing a thousandfold.
This sudden increase in sedimentation rate seems to have resulted directly from the impact of humans on the environment. When the first migrants to the Top End arrived they used fire extensively, for hunting and to increase the food source of their favourite prey, as well as for cooking. Modern Aboriginal People still use fire in this way. In the Kakadu environment this caused massive slope instability leading to erosion - with the consequent accumulation of sediment on the valley floors, as happened at Deaf Adder Gorge. The dramatic change that came to Kakadu during the Ice Age, most likely because of the one new factor, humans wielding fire-sticks.
Many pieces of ochre, many with use/grinding facets, have been found from a period of time between 30,000 BP and 20,000 BP. (Schrire, 1982; Jones & Johnson, 1985b).
The2 extreme weathering that is characteristic of monsoonal climates in semi-arid areas will decrease the potential for preservation of artefacts and bias a record in a sedimentary sequence in favour of younger material. An example comes from excavations in the Jinmium Rock Shelter where in the uppermost sediments seeds were noticeably weathered and below 40 cm the number declined, which indicated that this record was influenced more by factors of preservation than cultural processes (Atchison et al., in prep.). The lack of radiocarbon ages at depths greater than 150 cm in the Keep River region most likely is a reflection of in situ organic preservation resulting changes in the level of the water table.
It has also been observed at Nauwalabila (Fifield et al., 2001) radiocarbon ages were not reliable beyond this depth, which was coincident with the appearance of pisoliths (aka a pisoid, a concentric sedimentary grain >2 cm diameter formed as a concretion) of a prior water table that was fluctuating. There may therefore be environmental factors, past or present, limiting the age-depth range of radiocarbon dates, and in some cases this may be avoided by choosing sites at higher elevations.
A Nauwalabila I in Arnhem Land, a similar site, it was also noted (Hope et al., 1995) that there was a progressive increase in the accumulation of sediment, from <1 cm/1,000 years in the Late Pleistocene-Early Holocene, prior to the past 2,000 years when it ceased completely. Exposure time and increased potential for preservation may be reduced by high rates of sediment accumulation, which may explain the observed increased artefact rates of accumulation per unit time (see also Ferrand, 2001; Ward & Larcombe, 2003).
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