Abstract

Many techniques for relative dating of surficial deposits suffer from (1) the need to make judgments and (2) poor reproducibility of data. Furthermore, many of the techniques generate data that do not permit the use of statistical analysis. In contrast, assessing the etching of hornblende grains yields objectively obtained, reproducible data that can be subjected to statistical analysis. In a study of the deposits of the Bear Gulch basin, Tobacco Root Range, southwestern Montana, the hornblende-etching data proved superior to the clast-weathering data and the soil-profile data in making interpretations of age relationships. The mean etching of hornblende grains decreases as a logarithmic function of depth. Hornblende grains from older deposits are more etched, especially near the surfaces of the profiles.

The collective evidence supports a record of four first-order glacial and periglacial events, which have been correlated with (1) the youngest pre-Bull Lake glaciation, about 280,000 yr B.P.; (2) the Bull Lake glaciation, about 140,000 yr B.P.; (3) the Pinedale glaciation, consisting of two second-order advances, the first and most extensive between 26,000 and 20,000 yr B.P. and the second about 12,000 yr B.P.; and (4) a mid Neoglacial episode of periglacial activity. In addition, a major landslide blocked the valley in pre-Bull Lake time, and an episode of flood erosion along the valley floor is believed to have truncated the late Pinedale terminal moraine about 1500 yr B.P.

In a study area for which numerical ages of the deposits are available, hornblende etching can be expressed mathematically as a function of the time since deposition, with the resulting relationship used to date deposits wherein ages are unknown.

First Page Preview

First page PDF preview
You do not currently have access to this article.