This article is edited and drawn from:
Davies, B.J., 2022. Dating Glacial Landforms II: Radiometric Techniques, in: Haritashya, U. (Ed.), Treatise in Geomorphology (Second edition). Cryospheric Geomorphology. Elsevier, pp. 249-280. (link)
Shielding from mountains
Exposure-age samples will be shielded by any nearby topography (Balco, 2014; Balco et al., 2008; Codilean, 2006; Darvill, 2013; Gosse and Phillips, 2001; Nishiizumi et al., 1989). Exposure age samples must therefore be corrected with a non-dimensional shielding factor (0 to 1). Samples with a shielding factor of 1.0 have no topographic shielding.
Cosmogenic shielding calculators
The shielding factor used in cosmogenic calculators such as CRONUS-Calc is derived from measurements of the elevation to the skyline and the dip and dip direction of the rock surface sampled. This is discussed in more detail under “Sampling Methodologies”. The horizon will impede the cosmogenic ray flux, if the obstructions are more than several effective attenuation lengths for spallogenic production (i.e., several metres thick) (Balco et al., 2008).
The effect of common topographic shielding is relatively minor; a flat surface at the bottom of a pit with 45° sloping walls would still receive 80% of incoming radiation (Gosse and Phillips, 2001). Horizon shielding of less than 5° is of negligible importance. Shielding will be accentuated if the sampled surface is sloped towards a major topographic obstruction, such as a valley side. This is because, as most incoming radiation is close to vertical, a significant slope on the surface will reduce the radiation flux (ibid). The rock surface above the sample will also act to obstruct radiation. Attenuation lengths will be shortened on dipping surfaces, because particles will entre at oblique angles.
Shielding from snow, water, soil, peat
Winter snow, water, sand, peat or soil can also act to shield rock surfaces (Gosse and Phillips, 2001). The attenuation lengths for these common materials are given in Table 5. In general, if thicknesses of these relatively low-density materials is small, then shielding is limited.
Snow survey data can be used to estimate average snow depth and snow water content, and these data can be used to correct for shielding (Gosse and Phillips, 2001). However, snow cover can be an important and is often a neglected uncertainty in exposure- age calculations (Delunel et al., 2014; Schildgen et al., 2005). Jones et al. (2019) account for snow cover and cover by other materials in the IceTEA calculator.