覆盖层下的埋藏土。它基本上不受现代地表成土环境作用，类似地层中保留的生物化石一样，原形态大致可辨，但性质已有一定变化。
较古老的冰川物质层之间是发育良好的土壤，它含有（生活在）温暖气候中的植物的化石。
Between the older layers of glacial material were well-developed soils containing fossils of warm-climate plants.
—通过化石周围的火山土壤层测定它的年份是440万年前。
Volcanic layers around the fossil were used to date it from 4.4million years ago.
—洛斯阿拉莫斯国家实验室的地质学者沃德·盖布·瑞尔采集化石挖掘地点上层和下层的火山土壤层分析测定其年代在440万年前。
Geologist Giday WoldeGabriel of Los Alamos National Laboratory was able to use volcanic layers above and below the fossil to date it to 4.4 million years ago.
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