Lunar grabens are bounded by ~60¿, inward dipping shear fractures. Although both pure bending and simple extension mechanical models for the formation of grabens are derived, geometric constraints rule out a bending origin for lunar grabens. The model used assumes simple extension of the lunar crust and includes a correction for the mass wasting of grabens walls. Using this model, the crustal extension associated with each graben and the thickness of the faulted layer can be determined. The base of the faulted layer, defined as the depth of intersection of the faults bounding a graben, represents a mechanical discontinuity in the lunar crust. All major lunar grabens are used to map the thickness of the faulted layer, and results show excellent agreement with the findings of previous workers for the thickness of total ejecta. Hence the mechanical discontinuity at the base of the faulted layer probably corresponds with the base of the megaregolith. If moon-wide expansion caused the formation of lunar grabens, then an increase in radius of only ~18 m is necessary. This small an increase implies that the expansion needed for graben formation could be accounted for by local basin movements.