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We have studied the concentrations of 10Be and 26Al in quartz crystals extracted from glacially polished granitic surfaces from the Sierra Nevada range. These surfaces were identified with the glacial advance during the Tioga period ∼11,000 years ago. Our measurements yield the most accurate estimates to date for the absolute production rates of these nuclides in SiO2 due to cosmic ray nucleons and muons for geomagnetic latitudes 43.8°–44.6°N and altitudes 2.1–3.6 km. The estimates are relatively free from uncertainties in snow cover since we studied a suite of rock surfaces inclined 0°–75° with respect to the horizontal. The principal uncertainty arises due to the lack of a precise date for the glacial retreat event, about ±10%. The 26Al/10Be ratio at production (6.0) is determined more accurately since the exposure age of the samples is 2 orders of magnitude smaller than the mean lives of the two nuclides. Production rates of 10Be and 26Al in quartz at other latitudes and altitudes in the troposphere can be determined from the present measurements by scaling, using the known altitude and latitude dependence of cosmic ray fluxes of nucleons and negative muons. Knowledge of the production rates of these nuclides is a prerequisite for their application in erosion and geomorphological studies.

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The central premises of applications of the in situ cosmogenic dating method for studying specific problems in geomorphology are outlined for simple and complex exposure settings. in the light of these general models, we discuss the information that can be derived about geomorphic processes, utilizing concentrations of in situ produced cosmogenic radionuclides "Be (half-life = 1-5 ma) and 26Al (half-life = 0.7 ma) in a variety of geomorphic contexts: glacial polish and tills; meteorite impact craters; alluvial fans; paleo-beach ridges; marine terraces; sand dunes; and bedrock slopes. We also compare 'oBe-26A1 data with results obtained by other dating methods. We conclude that the technique of measuring in situ cosmic ray produced nuclides holds promise for quantitative studies of processes and time-scales in a wide range of geomorphological problems.

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