Unconformability

Unconformability, an interruption of the sequence of stratified rocks, in consequence of which the various parts of the undersurface of an overlying stratum do not rest upon corresponding parts of the underlying stratum. In cases of extreme or marked unconformability it is accompanied by a change in dip, as where, in various parts of England, horizontal Triassic rocks rest upon curved Carboniferous ones. This generally implies a considerable break in time, during which the lower strata have been tilted or folded, planed down by denudation, and redepressed, so as to be covered by the more modern deposits. In other cases, however, there may be no discordance in dip, as where horizontal Thanet Sands rest upon the eroded surface of equally horizontal Chalk. This may mark a lesser break in time.