Unconformity, or UNCONFORMABILITY, in Geology, is a structure which implies an interruption in sequence. When strata occur in regular sequence—each successive bed resting regularly upon the surface of the bed subjacent to it—they are said to be conformable. But when one set of beds extends over the denuded surface of another series we have what is called unconformity. The structure is shown in the accompanying section, where we have a discordant junction between two sets of strata, the upper series (a) resting unconformably on the upturned and denuded edges of the lower series. Such an unconformity usually points to the lapse of a long period of time. In the section the lower series (a) must first have been deposited, and subsequently folded and subjected to much denudation, which removed the tops of the anticlinal arches so as to expose the truncated ends of the beds. In many cases this denudation has been in great part subaërial—the rocks have formed part of a land-surface for some protracted period. Afterwards the denuded land-surface was submerged, so that newer deposits (b) were accumulated unconformably upon the older series. A well-marked unconformity thus usually indicates the following succession of changes: (1) a movement of elevation, followed by (2) terrestrial conditions and more or less excessive denudation; and thereafter (3) subsidence accompanied by deposition of sediment over a gradually increasing area.