OPTICAL AND HI OBSERVATIONS OF HIGH-VELOCITY GAS TOWARD HD-203664

High-resolution optical stellar and interstellar observations toward t he halo star HD 203664 and six surrounding early-type stars have been combined with a 21 cm mapping of the region to investigate a 70 km s-1 high-velocity cloud (HVC). The distance of the HVC from the Galactic plane (z) has been constrained to lie between 200 and 1500 pc; upper l imits to temperature and velocity dispersion of 4600 K and 2.2 km s-1 have been estimated; and a column density ratio N(Na II)/N(Ca II) cong ruent-to 0.32 has been deduced. These parameters are consistent with t he material being shocked gas, possibly arising from an old supernova remnant. The N(Na I)/N(H I) and N(Ca II)/N(H I) ratios are anomalously large by factors of approximately 20 and 100, respectively, with resp ect to normal diffuse gas; several scenarios are presented as possible explanations of this phenomenon.