We investigated the geology, stratigraphy, morphology, and topography of Syrtis Major, one of the large Hesperian-aged volcanic provinces on Mars. New data from the Mars Global Surveyor (MGS) and Odyssey spacecraft allowed us to study Syrtis Major in unprecedented detail. On the basis of Mars Orbiter Laser Altimeter (MOLA) data we estimated the thickness of Syrtis Major lavas to be on the order of ∼0.5–1.0 km, their volume being ∼1.6–3.2 × 10⁵ km³. MOLA data also show that the calderas of Nili and Meroe Patera are located within a large N-S elongated central depression and are on the order of 2 km deep. MOLA data further indicate that the slopes of the flanks of the shield are ≪1°. These very gentle slopes are similar to other Martian highland paterae such as Amphitrites, Tyrrhena, and Syria Planum but are much smaller than slopes measured on the flanks of large Martian shield volcanoes such as Olympus, Elysium, Ascraeus, and Arsia Mons. At kilometer-scale baselines, the surface of the Syrtis Major Formation is smoother than the surrounding highland plains but rougher than the floor of Isidis. We propose that the differences in surface roughness and wrinkle ridge patterns between Isidis Planitia and Syrtis Major are probably caused by additional sedimentation in the impact basin. Both the wrinkle ridge pattern and topography of Syrtis Major are interpreted to be consistent with a gravitational collapse of the southeastern sector of Syrtis Major. Recently a model has been proposed to explain the low elevation of the Isidis rim in the Syrtis Major region [e.g., Tanaka et al., 2002 ]. This model calls on catastrophic erosion of the rim that is triggered by the emplacement of sills, which interacted with a volatile-rich substrate. We used MOLA data from nearby terrains of the Isidis rim to investigate the Isidis rim in locations where it has supposedly been eroded. On the basis of our investigation we conclude that the smooth morphology and relatively low topography of the Isidis rim in the Syrtis Major region can be best explained by initial heterogeneities in topography and flooding with lavas, together with some erosion and/or tectonic modification of the original rim. New crater counts show that Syrtis Major is of Early Hesperian age (N(5) = 154), older than in the geologic map of Greeley and Guest [1987] . On the basis of inspection of all available Mars Orbiter Camera (MOC) images we identified several terrain types and developed a stratigraphic sequence for Syrtis Major. Large-scale erosional modification (e.g., deflation) of the Syrtis Major lavas and/or tephra deposits and redistribution of eolian material are evident in numerous MOC images.