Although now accepted, the theory of continental drift was rejected for many years, with evidence in its favor considered insufficient. One problem was that a plausible driving force was missing. A second problem was that Wegener's estimate of the speed of continental motion, 250 cm/year, was implausibly high. (The currently accepted rate for the separation of the Americas from Europe and Africa is about 2.5 cm/year). It also did not help that Wegener was not a geologist. Even today, the details of the forces propelling the plates are poorly understood.