- Sleep (1971) has presented some very interesting observations on the exponential decrease of the subsidence rate in coastal basins. He shows that the time constant of this exponential decrease is of the same order of magnitude that the thermal time constant of a lithospheric plate. He then makes calculations with a physical mode1 combining erosion and thermal contraction. The purpose of these comments is to show that his physical assumptions are invalid and that consequently his calculations are grossly in error. Starting with an initial elevation E,, at time t = O, Sleep defines the law of variation of the elevation E by dE kE + di = - aEo exp (-al) where aE, exp (-at) is the thermal contraction term, a-' is the thermal time constant, -kE is the erosion term and k-' is the erosion time constant.