The role of deformation in changing the physical properties of rocks has been quantitatively determined: more specifically, an attempt has been made to assess the possible damage by deformation to aquifers. The near subsurface of perhaps 25% of the area of the United States consists of moderate to severely deformed rocks. Moreover, much of the dry southwest is underlain by deformed strata, and lacking surface water, the location of groundwater reserves is of considerable economic importance. The investigation was under-taken in the disturbed belt adjacent to the Sawtooth Range in Montana. The disturbed belt is a transition zone from flat-lying underformed strata of the plains to severely deformed rocks of the fold and thrust belt in the Sawtooth Range.