High-resolution (1:500,000-scale) geologic mapping of a 300-km-wide and 1500-km-long strip, or geotraverse, across the transition from the Xanthe Terra cratered highlands to the Chryse Planitia lowlands encompasses an area on the margin of the Chryse Basin unaffected by outflow channels. It thus characterizes the transition where it has been relatively unmodified by erosion and the stratigraphy of pre-outflow units is well-preserved. Two types of highland-lowland boundary are identified: (1) a topographic boundary and (2) a geologic/morphologic boundary. The topographic boundary occurs well within the cratered highlands and separates a lower-standing surface with characteristics transitional to cratered highlands from the true highland surface of Xanthe Terra. The lower transitional highland surface, or transition zone, is partially inundated by plains-forming materials, interpreted to be plains type lava flows of late Noachian-early Hesperian age similar to later basin-fill material forming Hesperian-aged lowland ridged plains. The geologic boundary between this transitional surface and ridged lowland plains surface is identified with the traditional highland-lowland boundary as mapped in previous studies. Mapping in this study identifies a moat-like region between this geologic boundary and the basin interior is filled with an easily eroded material. Erosion of the moat-filling deposit along the outer margins of the Chryse basin accounts for the streamlined tables formed during outflow channeling throughout the margins of Chryse Planitia. The geology of the central basin interior, where the Viking 1 Lander is located, is consistent with the presence of fluvial sediments derived from both the adjacent highlands and remobilized from the marginal moat-filling unit, and deposited near the termini of the Maja and Kasei Valles outflow channels. Because the plains-forming lavas overlie much of the proximal portions of the highlands and moat-filling sediments ring the margins of the basin, materials eroded from the transition zone and deposited toward the basin interior by later outflow channels may be similar to interior plains lavas and marginal moat-filling deposits. If the transitional highland surface is typical of the basin elsewhere, true highland rocks are likely to be a small component of any outflow channel sediments, even in settings near the basin margin. Rock populations and sediments at the proposed Mars Pathfinder site, located near the geologic boundary further east, could contain a significant portion of materials typical of lowlands in addition to some highland materials.¿ 1997 American Geophysical Union