Geochronologic results are reported for 12 localities where deep drilling has penetrated Precambrian basement in Iowa, Illinois, Indiana, Michigan, and Kentucky. For the northern Illinois deep drill holes UPH 1 and UPH 3, a zircon U-Pb age of 1463¿10 m.y. was obtained. Rb-Sr data for these samples indicate mild disturbance of this system but are not interpreted in terms of any specific younger event. U-Pb ages for other localities fall in the interval 1450-1510 m.y and indicate that this was a major period of igneous activity throughout the southern Great Lakes region. Rb-Sr results indicate that alteration was ubiquitous and that most ages in the range 1100-1400 m.y. probably represent disturbed systems. The results obtained so far tend to support interpretations in which Kentucky, Indiana, and Illinois are primarily underlain by a rhyolite and epizonal granite terrance that is 1450-1500 m.y. old; older Proterozoic rocks may be absent or more deeply buried.