Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Other than the transfer sometime after 50,000 years ago of the Yamuna River from the Indus to the Ganges (Clift et al. 2012), the timing of specific capture events cannot be reconstructed from the terrestrial evidence alone and in many cases remains unclear or imprecise (Clift and Blusztajn 2005).
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