Allan Hills Near Western Icefield
The Allan Hills Near Western Icefield (76° 44'S 158° 48'E) lies approximately 12 kilometers west of the Allan Hills Main Icefield (ALH-DAV-Figure 1- 46 KB JPEG). This northwest trending icefield is nearly 14 kilometers long and up to 4 kilometers wide. No outcrop or surficial moraines are present. All areas of exposed ice have been systematically searched and outliers and peripheries visited on a reconnaissance basis. ALH2-Figure 1 (37 KB JPEG) shows the area covered by the Near Western Icefield map and the location of the two slope profiles . The two slope profiles diagrammed in ALH2-Figure 2 (17 KB JPEG) are given to show the general topographic trend across the icefield.
During the 1977-1978 season, 21 meteorites were recovered from the Near Western Icefield. The location map published by Yanai (1984) gives the names and indicates the locations of these specimens. Subsequently, 162 meteorites were recovered from the Near Western Icefield during the 1981-1982, 1983-1984, 1984-1985, and 1988-1989 seasons.
During the 1997-98 field season, a small team of glaciologists were studying ice dynamics at the Near Western Icefield. They recovered 12 meteorite specimens in the course of traversing the bare ice. No meteorite location information was gathered for the specimens, however they were all found near their camp in the vicinity of ALH 84146. A total of 4 additional specimens were recovered in the 1999-2000 season during the continuing ice motion studies. GPS positions were recorded for these specimens.
ALH2- Table 1 gives a tabulation of meteorite types from the Allan Hills Near Western Icefield collection.
Crude location methods were used to relatively determine the geographic location of meteorites found during the 1981-1982 season. Most of the specimens found in subsequent seasons were surveyed using the more precise theodolite/EDM system from poorly constrained base stations. More recently, GPS locations have been made available for meteorite finds.
A preliminary location map was generated using survey data from the 1983-84 season. The data from the 1981-82 season were then "fitted" to a preliminary map and UTM coordinates scaled off, entered into the database, and a new location map was generated. Therefore, the map depicts accurately the relative locations of meteorites found in the same season, but somewhat less accurately the locations of specimens from different seasons. The geographic positions used for base stations were poorly constrained. Therefore the meteorite positions determined by GPS could not be related to the older data set. To solve the problem, two base stations at the Near Western Icefield were surveyed using differential GPS methods, initially in 1999 then again in 2003. An estimated of the rate of ice motion could then be determined. (It was found that the ice motion rate for Station 171 was on the order of 0.13 meters per year). The positions of the base station in the year they were occupied could then be back-calculated. These positions were input into ARCGIS and a spatial adjustment was performed to bring all the other meteorite locations into common. These new coordinates are used in the current database.
Acknowledgments: We thank Ghislaine Crozaz, Robert Fudali, Ursula Marvin, and John Schutt (1981-1982); Bill Cassidy, Robert Fudali, A.C. Hitch, Kunihiko Nishiizumi, Paul Pellas, Ludolf Schultz, John Schutt, and Paul Sipiera (1983-1984); Catherine King-Frazier, Scott Sandford, Roberta Score, Carl Thompson, John Schutt, and Robert Walker (1984-1985) for their contributions to the Allan Hills Near Western Icefield Meteorite Location Map. We appreciate the continuity of effort by John Annexstad, Georg Delisle, Ludolf Schultz, and Joachim Sievers in determining the positions of the meteorites recovered by the GANOVEX expedition. Ludolf Schultz provided location data for meteorites recovered in the 1988-1989 season. Ted Dettmar, Blue Spikes, and Eric Venteris contributed the specimens from the 1997-98 season. Blue Spikes and Leigh Sterns contributed the specimens found during the 1999-2000 seasons activities.
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