swamps, deserts and ice fields are among the clearest examples of natural barriers. Rivers are a more ambiguous example, as they may obstruct large-scale movement across them (especially by armies) but may facilitate smaller-scale movement along them in boats, once some of the people in the region have developed the relevant technologies. Seas have likewise been an obstacle at first, then a convenient medium for transport along coastlines, and finally a medium for intercontinental transpor
