Abstract: An apparatus and method for providing a seal between first and second relatively rotatable parts of machines which operate in an environment comprising dust, grit or other finely divided solid material entrained in air, water or other fluid. The seal includes a rotor and a stator between which there is an annular passage with substantial clearance between the interfacing surfaces defining the passage. The solid material entrained in the fluid which enters the annular passage is deposited in layers on the interfacing surfaces when the mechanism is in use. The layers substantially reduced the cross sectional size of the annular passage and restrict flow of the fluid there through. The annular passage may be rendered tortuous by providing the rotor with annular fins which in use project into annular recesses in the stator, and vice versa.