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:The Andean uplift began in the late Oligocene to early Miocene (about 23 Ma), but the most intense peak of Andean mountain building occurred during the late middle Miocene (about 12 Ma) and early Pliocene (about 4.5 Ma) (Hoorn et al. 2010; Folguera et al. 2011).
Category: