Thermophilic anaerobic bacteria extracted from Triassic, 230 million years old (Ma), sandstone and shale at 2800 m below the land surface in the Taylorsville Basin, Virginia, appear to be indigenous to the rock strata. The observed maximum growth temperatures of these bacteria, 65-75°C, are compatible with the current formation temperatures. Paleogeothermometers, however, indicate that the strata were exposed to temperatures of 160- 200°C at 200 million years ago. This implies that the bacteria migrated to their current depth after this thermal pulse. A 2D paleofluid flow and heat transport model indicates that during uplift and erosion in the Jurassic, 200-140 Ma, topography drove groundwater deep into the subsurface at a rate of 1-100 mm/year. The estimated minimum time required for water to move from the surface to the microbially sampled horizons during this time interval ranges from 1 to 20 million years. The time required to migrate from a depth compatible with the growth range of microorganisms, approximately 1.3 km, to the microbially sampled horizons would be even less. The present-day groundwater flow rates, however, are 10-100 times less than those during the Jurassic and the minimum time required for water to reach the microbially sampled zones from the surface is 50-180 million years. Colonization of the deep subsurface, therefore, probably occurred during the basin s last major tectonic event in the Jurassic rather than today. For many regions of the earth s crust, the age of deep subsurface microbial communities may be equal to that of the last tectonic upheaval.