The broad structure of the Trans-Antarctic Mountains has been interpreted to be a horst, a monocline, or an anticline. Gravity measurements show that a negative Bouguer gravity anomaly of 130mgal follows the mountains and a maximum gravity gradient of 7 mgal/km is found along the west side of McMurdo Sound. Surface faults cannot explain the high gravity gradient, and a deep fault or step in the Moho causes a gradient that is too small. The gravity anomaly is satisfied by a shallowing of the Moho from 40 km under the Trans-Antarctic Mountains to 28 km under McMurdo Sound, and by a shallow, dense 8-km-thick slab under McMurdo Sound. This suggests that the crust in West Antarctica is thinner and more dense than in East Antarctica. The Antarctic continent may have formed by fusing together of two continental plates along an early Paleozoic subduction zone.