The MSSTS-1 drill hole in western McMurdo Sound provides the earliest physical record thus far for Antarctic Cenozoic glaciation. The hole penetrated 227 m of muddy marine glacial sediment containing scattered pebble- and cobble-size clasts, including seven diamictite beds deposited from grounded ice during glacial advances. A diatom-based chronology, refined by paleomagnetic stratigraphy, for the interval below 116-m subbottom indicates sediment accumulation from 29 Ma to about 24 Ma. Sediment texture and paleoecology of diatoms and foraminifers indicate nearshore sedimentation on a wave-dominated shelf. The oldest strata record a significant shallowing of water depth, culminating in ice grounding at around 28 Ma and a deepening at 25 Ma that may be glacioeustatic.