178 Dating a Prehistoric Site

Key Idea: A wide array of techniques can be used for dating artefacts, some of which show a high degree of reliability. The use of several appropriate techniques to date material improves the reliability of the date determined. The diagram below shows a rock shelter typical of those found in the Dordogne Valley of Southwest France. Such shelters have yielded a rich source of Neanderthal and modern human remains. It illustrates the way human activity is revealed at archaeological excavations. Occupation sites included shallow caves or rocky overhangs of limestone. The floors of these caves accumulated the debris of natural rockfalls, together with the detritus of human occupation at various layers, called occupation horizons. Accurately dating these finds is important to understanding their significance.

Rock shelter used by early humans

Enlarged below

Dating method Dating range (years ago) Datable materials

Radiocarbon (14C) 1000 - 50,000+ Bone, shell, charcoal Potassium-argon (K/Ar) 10,000 - 100 million Volcanic rocks and minerals Uranium series decay less than 1 million Marine carbonate, coral, shell Thermoluminescence less than 200,000 Ceramics (burnt clay) Fission track 1000 - 100 million Volcanic rock, glass, pottery Electron spin resonance 2000 - 500,000 Bone, teeth, loess, burnt flint

Limestone cave formations can be dated using uranium series decay measurements. This method can be used to date calcite deposits up to the age of 300,000 years.

Rock fall from the roof of the overhanging shelter.

B A

Pottery

Pottery bowl dated at 7000 ± 350 years old.

Bones

Skull of an early human but unable to directly determine its age.

1. What is the significance of occupation horizons?

Charcoal

Hearth

The remains of an ancient fireplace was dated at 18,500 ± 1000 years old. Occupation horizon A, with evidence of an ancient hearth in its uppermost layer.

Occupation horizon B, with evidence of a human burial.

Zone without any evidence of human occupation.

Tooth

A bison's tooth was dated at 45,000 ± 2500 years old.

2. Determine the approximate date range for the items below (Hint: take into account layers/artefacts with known dates):

(a) The skull at point B:

(b) Occupation horizon A:

Human skull Charcoal fragments (possible evidence of fire use and excellent for radiocarbon dating).

Bones from a large mammal with evidence of butchering (cut and scrape marks from stone tools). These provide information on the past ecology and environment of the hominins in question.

Photo: RA

istock Stone tools Excavation through rock strata (layers). The individual layers can be dated using both chronometric (absolute) and relative dating methods.

Searching for ancient human remains, including the evidence of culture, is the work of palaeoanthropologists. Organic materials, such as bones and teeth, are examined and analysed by physical anthropologists, while cultural materials, such as tools, weapons, shelters, and artworks, are examined by archaeologists. Both these disciplines, palaeoanthropology and archaeology, are closely associated with other scientific disciplines, including geochemistry (for chronometric dates), geology (for reconstructions of past physical landscapes), and palaeontology (for knowledge of the past species assemblages).

The reconstruction of a dig site, pictured above, illustrates some of the features that may be present at a site of hominin activity. Naturally, the type of information recovered from a site will depend on several factors, including the original nature of the site and its contents, the past and recent site environment, and earlier disturbance by people or animals. During its period of occupation, a site represents an interplay between additive and subtractive processes; building vs destruction, growth vs decay. Organic matter decays, and other features of the site, such as tools, can be disarranged, weathered, or broken down. The archaeologists goal is to maximise the recovery of information, and recent trends have been to excavate and process artifacts immediately, and sometimes to leave part of the site intact so that future work, perhaps involving better methodologies, is still possible.

3. Which dating method or methods could have been used to date each of the following, at the site on the previous page:

(a) Pottery bowl: (c) Hearth:

(b) Skull: (d) Tooth:

4. Explain why palaeoanthropologists date and interpret all of the remains at a particular site of interest (e.g. animal bones, pollen, and vegetation, as well as hominin remains):

5. Outline the importance of involving several scientific disciplines when interpreting a site of hominin activity: