Taiwan is an example of an orogen involving a doubly vergent critical wedge. Critical-wedge theory predicts asymmetrical tectonic development of the pro- and retro- sides of an orogen. Whereas the consequences for exhumation, peak metamorphism, and erosion have been extensively investigated, the implications for sandstone petrography preserved on either side of an orogen have not been adequately documented. Plio-Pleistocene sandstones from the Western Foothills of Taiwan were deposited in a rapidly subsiding foreland basin recording the collision between a volcanic arc and the Asian passive margin. New data on the evolution of sedimentary petrography and clay minerals preserved in Miocene to Pleistocene sandstone of the western pro-foreland basin record the history of Taiwan's orogenesis. Sandstone petrography of the eastern basin on the retro-side of Taiwan's orogenic wedge had been studied previously. Comparison of the provenance record from the western and eastern basins illustrates the opposing signature of unroofing and recycling, held to be characteristic of an asymmetric orogenic wedge. Sandstone petrography can be used in ancient sedimentary basins to complement other indications of the polarity of subduction.