The fracture pattern of the Northern Piceance Creek Basin, in Rio Blanco and Garfield Counties of Northwestern Colorado, evolved during at least four periods of brittle failure in Eocene rocks of the Green River and overlying Uinta Formations. Fractures in these rocks of are interest to hydrologists because matrix permeabilities in both formations are low, due either to poor sorting and interstitial calcite cement (Uinta sandstones) or to low pore volume and growth of authigenic minerals (Green River oil shales). Ground water at shallow to intermediate depths thus circulates mostly through secondary openings such as fractures and through voids created by the dissolution of nahcolite and halite. Fracture-induced permeabilities probably dominate most at shallow depths, where fractures are most abundant, apertures of fracture walls are greates, and solution openings are least common. Shallow, fracture-dominated aquifers are strongly anisotropic. At deeper levels, in leached zones of the ''saline facies''of the lower part of the Green River Formation, solution openings contribute greatly to fluid flow and permeabilities probably are less direction dependent.