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The dating of land surfaces has long posed problems for geomorphologists. Relative methods (stratigraphic, geomorphic, topographic) are sound and convincing. Exhumed forms may complicate identification and relationships, for both epigene and etch forms have been buried, and exhumed, but in tectonically undisturbed areas, the higher surfaces are older than those preserved at lower levels. Also, surfaces have an age range. The relationship of surfaces with volcanic deposits, old shorelines, and genetically related sedimentary sequences provides sound ages, and correlation with dated duricrusts and faults is also useful. There are no temporal limits to relative dating, for the methods are equally applicable to the dating, say, of Proterozoic surfaces as of those of Pleistocene age. The disadvantage of such methods is that the necessary evidence is frequently either not preserved or not exposed. The so-called ‘absolute’ (physical, numerical) methods, and especially those based on exposure age dating with in situ cosmogenic radionuclides, are appealing because they produce direct numerical ages, and appear to be widely applicable, but there are severe temporal limitations, and sampling problems complicate, and may invalidate, interpretation. Absolute age determinations must be consistent with the stratigraphic and geomorphologic settings. The best results are obtained when physicists and earth scientists pool their knowledge and experience. A background in local and regional geology is especially important. |
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