The predictions of a simple model based on the concept that the lithosphere is a thermal boundary layer, somewhat analogous to a layer of ice on a pond, are in agreement with the relevant data from North America. The mean surface heat flow of tectonic provinces is is approximately inversely proportional to the thickness of the lithosphere. If crustal thickness remains constant, a 40-km increase in lithospheric thickness results in a 1-km decrease in surface elevation, because unlike the ice analogy the lithosphere is denser than its underlying asthenosphere. It is suggested that part of the observed decrease of continental heat flow in the time following a thermal event is caused by cooling and thicknening of the lithosphere.