This article is taken from:
Davies, B.J., 2022. CRYOSPHERIC GEOMORPHOLOGY: Dating Glacial Landforms I: archival, incremental, relative dating techniques and age-equivalent stratigraphic markers. (link)
Introduction to relative dating
Relative dating techniques aim to order landscape features. It assumes that moraines closer to the ice limit are usually assumed to be younger, and those further are older. This is likely to be applicable in temperate environments, but cold-based polar glaciation may override and preserve geomorphological features.
Morphostratigraphy is a relative age assignment to surface features (Briner, 2011), based on ‘freshness’, surface weathering characteristics, or vegetation cover (Lukas, 2006). It has a long history of use, predating the development of numerical dating technologies through the development of relative stratigraphic frameworks (Lüthgens and Böse, 2012).
The degree of rock or boulder weathering can be quantified using techniques such as Schmidt Hammer dating, indicating the differential passage of time. Finally, degradation in shells can be measured with amino acid racemization, which may be applicable for older glacial units or where glacial sediments are interbedded with raised beaches (Davies et al., 2009).
Relative dating techniques allow practitioners to correlate a small number of landforms with numerical ages to many more landforms over a large spatial area with the same morphometric properties, which is a highly powerful approach to ice mass reconstruction (Briner, 2011), and has been used to interpolate between dated moraines in generating ice-sheet scale reconstructions (e.g. Dalton et al., 2020; Davies et al., 2020).
Moraine degradation can be applied as a relative-dating parameter. Moraines are typically deposited with steep sided slopes that degrade with time. The degree of surface roughness and slope steepness is, therefore, a function of moraine age. Measureable features include slope angle, crest width and degree of gullying. Soil thickness, B-horizon thickness and development, and weathering rind thicknesses can be used to indicate relative age (Briner, 2011).
Younger Dryas landforms in Scotland
In Scotland, this approach has been used to quantify landforms of Younger Dryas or older age (Boston et al., 2015; Boston and Lukas, 2017; Chandler et al., 2019; Lukas, 2006). Here, the number and type of moraines, number of river terraces inside and outside moraine sequences, the thickness of sediments on sediment-covered slopes, maturity of talus slopes and distribution of periglacial landforms have been used to differentiate moraines (Lukas, 2006).
In the Scottish uplands, ‘Type 1’ moraines are characterized by small, densely spaced, sharp-crested moraines with intervening meltwater channels and no well-developed river terraces (Boston et al., 2015). These moraines are found in the upper parts of valleys and were assigned to the Younger Dryas.
‘Type 2’ moraines, in the lower parts of the valley, relate to an older phase of local plateau icefield glaciation or deposition by a larger regional ice mass at the end of the Dimlington Stadial (Greenland Stadial 2; Boston et al., 2015; Lowe et al., 2008). These moraines are characterized by assemblages of large (up to 10 m) sporadically placed moraines with rounded crestlines, surrounded by well-defined river terraces.
Building a dating programme
These relative chronological methods can be applied to guide a more expensive and time-consuming numerical dating programme (Lüthgens and Böse, 2012). If dating programmes are not based upon a rigorous understanding of sediment-landform assemblages and where relative ages are not well understood, numerical ages can conflict with geomorphological interpretation because samples could have been taken out of context (Boston et al., 2015).
Relative morphometric dating techniques such as these can be used to widely correlate moraines across different valleys (Davies et al., 2020), and may provide the only chronological tools where materials suitable for dating are absent (Briner, 2011). However, care should be taken, as topographic controls can result in asynchronous moraine deposition in different valleys (Barr and Lovell, 2014), resulting in glaciers of the same age reaching very different sizes and moraine extents in adjacent valleys (e.g., Davies et al., 2018).
References and further reading
Briner, J.P., 2011. Dating glacial landforms, in: Singh, V.P., Singh, P., Haritashya, U.K. (Eds.), Encyclopedia of snow, ice and glaciers. Springer, pp. 175-186.
Boston, C.M., Lukas, S., Carr, S.J., 2015. A Younger Dryas plateau icefield in the Monadhliath, Scotland, and implications for regional palaeoclimate. Quaternary Science Reviews 108, 139-162.
Davies, B.J., Darvill, C.M., Lovell, H., Bendle, J.M., Dowdeswell, J.A., Fabel, D., Garcia, J.L., Geiger, A., Glasser, N.F., Gheorghiu, D.M., Harrison, S., Hein, A.S., Martin, J.R.V., Mendelová, M., Palmer, A., Pelto, M.S., Rodes, A., Sagredo, E.A., Smedley, R.K., Smellie, J.L., Thorndycraft, V.R., 2020. The evolution of the Patagonian Ice Sheet from 35 ka to the present day (PATICE). Earth-Science Reviews 204, 103152-103152.
Davies, B.J., Thorndycraft, V.R., Fabel, D., Martin, J.R.V., 2018. Asynchronous glacier dynamics during the Antarctic Cold Reversal in central Patagonia. Quaternary Science Reviews 200.
Lukas, S., 2006. Morphostratigraphic principles in glacier reconstruction – a perspective from the British Younger Dryas. Progress in Physical Geography 30, 719-736.
Lüthgens, C., Böse, M., 2012. From morphostratigraphy to geochronology–on the dating of ice marginal positions. Quaternary Science Reviews 44, 26-36.