The 1960s witnessed a striking change in geology. Since at least the seventeenth century, one of the central problems of the subject had been the origin of the major irregularities of the surface of the globe—continents and oceans, mountain chains and ocean islands—irregularities that were not anticipated by most physical theories. Traditionally these features had usually been explained either as residual traces of events occurring during the very early history of the globe, or as the result of vertical movements of (...) the earth’s crust, caused, for example, by changes in the heat budget. The last two decades have seen an end to all this. The vast majority of geologists now believe that these irregularities largely result from the lateral movement of thin rigid plates covering the earth, a theory now known as “plate tectonics”, but a theory which also has obvious parallels with the hypothesis of continental drift, in which it was postulated that continents can move laterally. (shrink)