Figure 3. Precambrian geology of the south-central United States (from Van Schmus et al., 1996). G—Grenville Front, GIPB—Green Island plutonic belt, MCR—Midcontinent Rift, MGL—Missouri gravity low, RR—Reelfoot rift, SFM—St. Francois Mountains. Dashed line extending from Texas to Ontario represents inferred eastern limit of pre–1600 Ma continental crust. Solid circles are drill-hole and outcrop samples for which depleted mantle age (T_DM) 1.55 Ga; open circles are drill-hole and outcrop samples for which T_DM ≤ 1.55 Ga. Black cross in northwestern Ohio is drill-hole OHSN-001; solid square in southeastern Michigan is drill-hole MIHI-001.