The boundary of the south polar region is marked by a pattern of parallel, Y- and V-shaped ridges and valleys. The shape, orientation, and location of these features suggest they are caused by changes in the overall shape of Enceladus. As of 2006 there were two theories for what could cause such a shift in shape: the orbit of Enceladus may have migrated inward, leading to an increase in Enceladus's rotation rate. Such a shift would lead to a more oblate shape; or a rising mass of warm, low-density material in Enceladus's interior may have led to a shift in the position of the current south polar terrain from Enceladus's southern mid-latitudes to its south pole. Consequently, the moon's ellipsoid shape would have adjusted to match the new orientation. One problem of the polar flattening hypothesis is that both polar regions should have similar tectonic deformation histories. However, the north polar region is densely cratered, and has a much older surface age than the south pole. Thickness variations in Enceladus's lithosphere is one explanation for this discrepancy. Variations in lithospheric thickness are supported by the correlation between the Y-shaped discontinuities and the V-shaped cusps along the south polar terrain margin and the relative surface age of the adjacent non-south polar terrain regions. The Y-shaped discontinuities, and the north-south trending tension fractures into which they lead, are correlated with younger terrain with presumably thinner lithospheres. The V-shaped cusps are adjacent to older, more heavily cratered terrains.