The Harold D. Roberts Tunnel is a 23.3-mile dog-leg diversion tunnel designed to transport water from Dillon to Grant, Colorado. The tunnel passes beneath the Continental Divide at the crest of the Colorado Front Range and intersects numerous rock types and complex geologic structures. From the west portal to Station 468 + 49 the tunnel encountered the following lithologic and structural units: (1) from the west portal (Station 9 + 46) to Station 180+ 00, folded and faulted Jurassic to Upper Cretaceous continental and marine sedimentary rocks; (2) from Station 180 + 00 to Station 291 + 60, intensely sheared and altered Precambrian igneous and metamorphic rocks above the Williams Range thrust fault; (3) from stations 291 + 60 to 343 + 00, contact-metamorphosed shaly and sandy sedimentary Cretaceous rocks; (4) from stations 343 + 00 to 468 + 49, quartz monzonite and quartz monzonite porphyry of the Montezuma stock of Laramide age.
Structural analysis of a complex pattern of faults and joints indicates that the Williams Range fault in the vicinity of the Roberts Tunnel is a gravity-slip structure that formed in response to sharp uplift of a crustal block along a nearly vertical shear zone essentially parallel to the present north-north-west-trending surface trace of the fault. The Montezuma stock is later than the Williams Range fault and was emplaced at or near the intersection of a north-northwest shear zone and closely fractured rocks in the northeast-trending Front Range mineral belt. Dislocations along north-northwest fractures and along the mineral belt continued after emplacement of the stock and produced in it fracture patterns similar to patterns in rocks outside the stock. The fractures in the stock localized aplite dikes and veins that are accompanied by pervasive hydrothermal alteration.