The Chesapeake Bay impact structure, Virginia , is the largest impact crater in the United States . The important question of how large the impactor was that formed the crater remains unanswered. This is primarily due to peculiarities of the crater structure, revealed by seismic exploration, that complicate comparisons with other terrestrial and extraterrestrial craters. One potential explanation for the unusual structure of the Chesapeake Bay crater is that the cratering process was affected by rheologic variations in the target at the time of impact. Using numerical modeling, we find that for a sufficient contrast in material strength between the sedimentary and crystalline units at the Chesapeake Bay impact site, we can produce a model of the final crater that is consistent with observational constraints, and hence we can infer the energy released during the impact, ~1.75 × 106 Mt. In the absence of target strength variations, the final crater diameter is likely to have been ~40 km, rather than the observed 80–90 km.