Optically stimulated luminescence dating method
arroyo - A steep-sided and flat-bottomed gully in an arid region that is occupied by a stream only intermittently, after rains.
artesian well - A well that penetrates an impermeable layer of rock to reach an aquifer containing water under pressure.
alkali metal - A strongly basic metal like potassium or sodium.archean, archaean - An eon of geologic time extending from about 3800 to 2500 million years ago. The Archean eon is divided into four eras: Eoarchean, Paleoarchean, Mesoarchean, and Neoarchean.