Linear and nonlinear spectral mixture models applied to Clementine multispectral images of the Moon result in roughly similar spatial distributions of end-member abundances. However, there are important differences in the absolute values of the predicted abundances. The magnitude of these differences and the implications for understanding geological processes are investigated across a geologic contact between mare and highland in the Grimaldi Basin on the western nearside of the Moon. Vertical and lateral mass transport due to impact cratering has redistributed mare and highland materials across the contact, creating a gradient in composition. Solutions to linear and nonlinear spectral mixture models for identical spectral end-members of mature mare, mature highland, and fresh crater materials are compared across this simple geologic contact in the Grimaldi Basin. Formal estimates of uncertainty in the calculated fractions of the end-members are ≈1%. Profiles of mare abundance across the contact are extracted and compared quantitatively. Profiles from the linear mixture models indicate that the geologic contact has an average mare abundance of 60% and the compositional boundary is asymmetric with more mare transported onto the highland side of the contact than highland onto the mare side of the contact. In contrast, the nonlinear abundance profiles indicate that the geologic contact has an average mare abundance of 50% and the compositional boundary is remarkably symmetric. Given the expectation that materials will be intimately mixed on the surface of the Moon and that the asymmetries implied by the linear model are not consistent with our understanding of lunar surface processes, the nonlinear spectral mixture model is preferred and should be applied whenever quantitative abundance information is required. The remarkable symmetry in the compositional gradients across this contact indicates that lateral mass transport dominates over vertical transport at this boundary. ¿ 1998 American Geophysical Union