The southern Rocky Mountain region is a particularly interesting area to study continental evolution because it has experienced 1.8 Ga of crustal growth and modification that is relatively well known geologically, but until recently, modern seismic data were sparse. However, the Rocky Mountain Front, DEEP PROBE, and the Continental Dynamics Rocky Mountain Project (CD-ROM) seismic experiments have changed this situation. Analysis of the CD-ROM seismic refraction, deep reflection, and teleseismic data show that the crustal thickness is generally 45-50 km thick along the seismic profiles. Given that the Precambrian rocks exposed at the surface were once at depths of about 10 km in Colorado and about 20 km in northern, New Mexico, an obvious question is how and when did the crust attain its present thickness? The seismic refraction data clearly show a thick, high-velocity (mafic) lower crust. This layer is apparently widespread being observed in many areas of the mid-continent region east of the Rocky Mountains and in the Colorado Plateau. Using the word “underplating” to refer to the complex processes whereby mantle material or its derivatives are added to lowermost portion of the crust, this layer seems best interpreted as representing such material. However, given the complex geologic history of the region the question of the timing of the event(s) that formed it quickly arises. There was considerable magmatism that was in part arc-related during the period of continental assembly (1.7-1.6 Ga), magmatism was especially widespread at ~1.4 Ga, and from the Great Lakes region westward, the period around 1.1 Ga involved considerable extension and localized magmatism. The final event of the Precambrian lithospheric assembly that led to the formation of Rodinia was the Grenville orogeny. Phanerozoic tectonism produced many examples of significant crustal modification but none were widespread enough to produce the thick, mafic crust observed over an extensive area. Although preserving such a layer for over a billion years, is problematic, the best times for regional underplating to produce the high velocity lower crust under the southern Rockies and Great Plains was during the initial formation of the crust and at 1.4 Ga when magmatism was widespread. This implies that the crust in this region has been thick and stable for at least 1.4 Ga.