At the beginning of Laramide orogeny, a blanket of undisturbed Cretaceous and minor older Mesozoic sedimentary rocks 1,500 to 3,000 m thick covered the Southern Rocky Mountain province, and the last of a series of Cretaceous seas was starting to withdraw northeastward across the region. Beneath the blanket was an older and inhomogeneous terrane that in some places consisted of the eroded stumps of late Paleozoic mountain ranges made up of Precambrian rocks, and in other places of piles of sedimentary rocks thousands of meters thick on the sites of late Paleozoic basins. At the onset of Laramide orogeny in Late Cretaceous time, most of the buried mountain ranges were re-elevated, and adjoining Laramide basins, in part inherited from the late Paleozoic basins, began to subside and receive orogenic sediments. In addition, two anticlines of mountain-range proportions—the Sawatch and Uinta—rose from the sites of late Paleozoic basins.
Orogeny began in the southwest part of the province before marine deposition ended in the northeast. The late Paleozoic San Luis highland of southwestern Colorado and northern New Mexico was re-elevated in late Campanian or middle Montana Cretaceous time, as indicated by orogenic sediments in the San Juan basin to the south and the Raton basin to the east. The Sawatch anticline, which diverges northward from the rejuvenated San Luis highland, rose at about the same time, as indicated by the age (72 to 70 m.y.) of fault-controlled porphyries on the flanks. The Front, Park-Sierra Madre, and Medicine Bow Ranges, on the site of the late Paleozoic Front Range highland, rose after the marine Fox Hills Sandstone was deposited over their sites 67.5 m.y. ago. Uplift and erosion were rapid. By 66 to 65 m.y., and before the close of Cretaceous time, 3,000 m of sedimentary rocks had been eroded from at least parts of these ranges, and streams were carrying detritus from Precambrian rocks to bordering basins.
Once started, uplift of mountain units continued through Paleocene and into Eocene time, as indicated by nearly continuous Upper Cretaceous to Eocene sedimentary sequences in the interiors of bordering basins. Uplifts grew laterally as they rose vertically. Consequently, the major uplifts of today are areally larger than those that supplied the first orogenic sediments, and their border structures are younger than those sediments. The crystalline rock body of the interior of the Laramide San Luis highland supplied sediments to adjoining basins in Cretaceous time, but the sedimentary rock flank of the uplift at the site of the Sangre de Cristo Range was not uplifted and deformed until Paleocene and early Eocene time. Similarly, an interior part of the Front Range supplied Precambrian rock detritus to bordering basins very late in Cretaceous time, but the flank structures of the range, which involve the early orogenic sediments, developed in Paleocene and Eocene time. The Laramie Range prong of the northern Front Range probably did not begin to rise until Paleocene time. The White River Plateau, the last major Laramide uplift unit to appear in the province, began to rise in early Eocene time, but it did not attain its present outline and flank structure until late Eocene time.
Laramide volcanism and intrusion were almost entirely confined to a broad northeast-trending belt that cuts diagonally across major tectonic units of the province. The Colorado mineral belt constitutes an inner zone of this igneous belt. Andesitic volcanism occurred at several localities within the igneous belt at an early stage in Larmide orogeny but after the first uplift of mountain units. Intrusion of granodioritic porphyries in stocks and smaller bodies began at about the same time as volcanism but continued longer, at least through Paleocene time. Though magmatism may have been the cause of some structural features within the igneous belt, the belt itself has no evident Laramide structural control and seems to be independent of the tectonic elements it crosses. The only unifying structural feature within the belt is a system of discontinuous and overlapping Precambrian shear zones. This system probably furthered the rise of magma bodies into the upper crust from batholiths at depth, but it had no role in the generation of those batholiths. Concurrent magmatism and tectonism must have shared the same cause, but they proceeded independently in the upper crust.