I. I ntroduction . T he old ‘sea-cliff’ theory of Wealden denudation, which regarded the escarpments as the direct result of marine action, has long disappeared, and no one now doubts that the main physical features of this district have been produced by subaërial agencies. Beyond this, however, there is but little unanimity. Whether the subaërial process has been continuous since early Oligocene times; whether it can be divided into two cycles, with a peneplain between; or whether the removal of the top of the dome has been assisted by marine planation, are questions so undecided that, except on the one point already mentioned, geologists are perhaps in no closer agreement as to the physical history of the Wealden area than they were thirty years ago. In the second section of this paper the older lines of evidence are reviewed, and in places supplemented by new arguments, tending to show that the theory of marine planation is the one which, on the whole best, embodies the observed facts. In the third section the history of the four western rivers (Wey, Mole, Arun, and Adur) is examined, and the same inference drawn. Whether this explanation holds good for other parts of the Wealden area must be left to future investigation. The expression ‘plane of marine denudation’ is said by Foster & Topley (5, p. 473, footnote) to have been first used by Ramsay in 1847; but the words themselves are not to be found in the abstract of his paper to which they refer