The Rio Grande is an example of a physical boundary, other examples exist where boundaries correspond with any rivers, mountains, deserts, or really any defining body of water. A cultural boundary would be the borders between countries or regions that also corresponds to differences in ethnicity, language, and cultural norms, a geometric boundary would be the border between countries without regard to physical boundaries, such as the US-Canada border, and religious boundaries would be state boundaries that also correspond to religious differences, such as between primarily Hindu India and primarily Muslim Pakistan.
