A map, recording the thickness of a sequence of rocks between surfaces that were once flat or relatively flat, records the structural movements that occurred between the development of the limiting surfaces. Such an isopachous map is essentially a structure map of the first surface at the time of the second. The accuracy with which such maps reveal the structural movements that took place during the interval depends on how closely the limiting surfaces approached a plane. They are most accurate where the confining surfaces were depositional. Beveled erosional surfaces are useful where

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the topographic relief was low with respect to the regional deformation but, in some cases, may reveal the structural movements only over broad areas. The relation of the sequence to underlying and overlying formations is important in evaluating the structural significance of the isopachs. Isopachous maps that include formations whose thickness was affected by conflicting patterns of folding express a composite of both movements and reveal neither.

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