Archaeologia Islandica - 01.01.1998, Side 130

Archaeologia Islandica - 01.01.1998, Side 130
I. A. Simpson, K.B. Milek & Garðar Guðmundsson Thin section micromorphology Thin section micromorphology, in- troduced to pedology by Kubiena (1938), represents an extension of sedi- ment profile observations made in the field. Undisturbed blocks of sediment are removed from an exposed section profile, and in the laboratory water is removed from the sample by acetone exchange to prevent structural modifi- cation of the sample during the drying process. Samples are then impregnated under vacuum with crystic polyester resin and hardened through the addition of a catalyst and an accel- erator. Once cured, blocks are sliced, bonded to glass and precision lapped to 30 (im, with coverslipping complet- ing the preparation of the thin section. Thin sections are described using a petrological microscope and by fol- lowing the internationally accepted procedures outlined in the Handbook for Soil Thin Section Description (Bullock et al. 1985). This allows sys- tematic description of microstructure, basic mineral components, organic and anthropogenic inclusions and features resulting from soil formation processes (pedofeatures). A range of magni- fications (from x2-x400) and light sources (plane polarized, cross polar- ized, circular polarized, reflected and UV light) can be used to obtain detailed, semi-quantitative descrip- tions. Further quantification of key features will be obtained through the analysis of digitized images captured by a video camera attached to the microscope. Interpretation of features in thin section rests upon the accumulated evidence of a number of workers, notably Courty et al. (1989) and FitzPatrick (1993), on research undertaken by one of the authors in the North Atlantic context (Simpson and Barrett 1996; Dockrill and Simp- son 1994) and on the comparison of sedimentary properties observed in different areas of the Hofstaðir site. It will also be aided by reference to micromorphology samples taken from known contexts in Þverá, a recently abandoned turf house that is only I4.25km away from Hofstaðir in the same river valley (Laxádalur). 1996 Observations: emerging hypotheses Two stratigraphic sections were ex- amined during the 1996 season. Sec- tion 1 was on the west face of the structure designated as D-1 and Sec- tion 2 was located in area G. Context 4, which was thought to be potentially useful as a horizon marker, was also examined at various locations on the site. Section 1 (Area D) Sediments from Section 1 can be considered in four stratigraphic units. The lowest unit is comprised of sequences of windblown silt (7.5YR 3/3 and 7.5YR 3/4) interspersed with prehistoric tephra layers, including Hekla-3 (2,900 BP), Hverfjall (2,800 BP), unidentified tephra (5YR 2.5/1), and the Landnám tephra (AD 871 ±2) (Magnús Sigurgeirsson, this volume). There is also evidence of standstill phases during which there was limited 130
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