When a .series of strata is deposited in a basin with sloping sides, or one sloping side, each bed will extend farther than the one upon which it lies, and thus in a thick mass of strata, if the shelving bottom be gently inclined, the upper beds will extend far beyond the lower ones, or overlap them (see Fig. 203). Overlap also occurs where the sea is advancing or transgressing slowly across a subsiding land surface, the rate of depression not much exceeding the rate of deposition. Here also each stratum extends farther across the old land surface than the one beneath it, and conceals the edges of the latter. The relation of overlap is between the successive layers of a conformable series.

Unconformity without change of dip, and overlap.

Fig. 203. - Unconformity without change of dip, and overlap.

Overlap may be a structure of much economic importance, if one of the lower strata, say a coal-bed, is mined. It is not safe to assume that wherever the upper beds of such a series are found, the lower will be found directly beneath them, an assumption which may result in costly failure.