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Article Excerpt
Abstract
Remote sensing methods--ground penetrating radar (GPR) and cesium magnetometer--were employed to investigate the internal structure of the Pulemelei mound, a large earth oven (umu ti) and a smaller stone and earth structure to the north of the large mound. Results suggest that Pulemelei does not contain a burial vault like those built in Tonga, and GPR indicates at least two platform construction events, as well as a small mound-shaped feature at the base of the Pulemelei mound. The use of geophysical techniques on these structures at the Pulemelei site in Samoa indicate they can be applied successfully to other examples of monumental architecture in the Pacific.
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Mounds of earth and stone dating to the last 1000 years are a common feature of the Samoan landscape, and while most can be interpreted as the remains of domestic house foundations, a relatively small number with monumental dimensions are an enigmatic component of the prehistoric settlement pattern. Earth mounds with a volume greater than ca. 2500 [m.sup.3] are found mainly on Upolu, whereas on Savai'i large mounds were generally built of stone, due to the quantity of volcanic rock from extensive Holocene lava flows. Exceptions are found on both islands depending on the local availability of materials (Buist 1969; Davidson 1974:226), but in the small and precipitous volcanic islands of American Samoa no mounds have been recorded that in size rival the largest structures built on nearby Samoa (Green 2002).
A Tongan origin for large Samoan mounds has been asserted by local informants, inviting a tentative comparison with the tiered, coral-slab faced burial mounds (langi) of the Tui Tonga lineage (Golson 1969:14; Davidson 1974:231-2).
No intact stone mounds with monumental dimensions have been excavated, and the Pulemelei mound has been interpreted as a foundation for a god house (Scott 1969; Kirch and Green 2001:251), an elite residence structure (Sutton et al. 2003:235; Asaua 2005) that might have been constructed in a single phase (Davidson 1974:226, but see Scott 1969:81), and a ceremonial venue that might contain burials (Scott 1969:90; Tamasese 2003, 2004). Physical investigation of the mound's interior that could shed light on its genesis and purpose was not logistically feasible in the current study and could have diminished the heritage values of the structure, which the local community, land owners and archaeologists wished to preserve. Two non-invasive and non-destructive geophysical techniques were used to examine the volume of the Pulemelei mound below the top platform as well as two associated structures--the smaller 'North mound' and a large underground oven (umu ti). This study was the first to use remote sensing techniques intensively on monumental architecture in the Central Pacific (see Sand 1998), and had both specific and exploratory aims as follows:
1. Was there geophysical evidence for cavities or structures in the mound that might represent a burial or tomb, as was known for Tongan langi?
2. Did the composition of the mound have an internal structure suggesting single or multiphase construction?
3. How effective were remote sensing methods for investigating typical Samoan archaeological remains such as mounds and ovens?
Petrological and soil environment
Previous archaeological excavations had shown that the petrology, sedimentology and soil characteristics of the area containing the Pulemelei mound could, for geophysical purposes, be divided into eight types of material. The physical properties and distribution of different material types were expected to contrast significantly, and account for most of the variation recorded in remote sensing results. The loose stones found in material types 3 and 5 (see below) were pebbles and boulders of vesicular basalt derived from local bedrock, as was the silty-clay. The sediments were generally damp or wet, except for the non-weathered bedrock and loose stones on the ground surface.
Petrological and pedological materials
1. Silt-clay humic topsoil.
2. Natural silt-clay soil without stone.
3. Natural soil composed of silt-clay and loose stone.
4. Anthropic soil of silt-clay grain size.
5. Anthropic soil made of mixed silt-clay and loose stone.
6. Anthropic rock pile, grain-supported with a predominant air fill.
7. Weathered bedrock (wet/dry).
8. Non-weathered bedrock.
Instrument choice
The resistivity method was not used because of the probability of poor electrode coupling. The gravity method was rejected also due to slowness of data acquisition, and seismic techniques were inappropriate to use on a culturally sensitive site. Electromagnetic methods (to measure conductivity and susceptibility) were considered appropriate for the types of material concerned, but the dimensions of the target features led us to chose ground penetrating radar (GPR) and the magnetic method (termed here 'magnetometry').
Ground penetrating radar
Description of the GPR method is given in Conyers and Goodman (1997), Davis and Annan (1989) and de Vote (1990). In brief, a transmitting antenna sends out a short pulse of high frequency electromagnetic energy, which is recorded by a receiving antenna after passing through the subsurface and encountering materials with different electric and dielectric properties--generally described as reflectors, surface interfaces and object discontinuities. When the emitted energy wave encounters materials with different electromagnetic characteristics, part of the wave is reflected back to the surface where receiver antennas record the return energy, and part of the wave is transmitted downwards (Davis and Annan 1989).
Since its early use in the 1970s, GPR has been increasingly applied to archaeological sites with various degrees of success. For instance, it has been used to image structures within platforms, mounds, pyramids and burials...
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