Age and significance of slope deposits, Black Mountain, Canberra

Abstract

Extensive slope deposits are described from Black Mountain, Canberra, with particular reference to a profile exposed in an excavation at 590 m on the lower eastern slopes of the mountain. Buried organomineral soil materials near the base of the slope deposits contain fragments of carbonized wood with an average age of 27,800 (+2500, - 1900) years B.P. This age is not significantly different from the mean age of 26,790 ± 810 years B.P. for carbonized wood in slope deposits near Lake George, and is close to the mean age of 31,500 ± 1000 years B.P. for slope deposits above 1000 m in the Snowy Mountains. A method for comparing the results of carbon-14 determinations is described. The Snowy Mountains deposits indicated a periglacial climate with mean annual temperatures at least 8-10°C lower than at present. On the basis of their similar appearance and age, it is suggested that the Black Mountain deposits were also formed under cold-climate conditions. If this were so, the synchronous slope deposits near Lake George (and river terraces and slope deposits in an upland tributary of the Shoalhaven River) must also have developed during a colder climate. The climatic and associated soil conditions considered necessary for the formation of periglacial deposits at Black Mountain are discussed. Estimates of the lowering in temperature (compared with present-day temperature) which would have been involved vary from a crude estimate of about 14°C (annual) with relatively high precipitations and prolonged snow cover, to a more conservative estimate of 9°C (winter), 11°C (summer), and 10°C (annual) if the winter precipitations and snow cover were light. Even under the latter conditions, tree cover would have been minimal or absent from the tableland environment.