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Within the San Juan basin the sandstone zones that occur at the top and bottom of the Mesaverde group were not deposited as a continuous blanket sand. In some areas thick, relatively clean sandstone units occur. In other areas thin, poorly sorted sandstone beds are found. These sandstone units exhibit a definite geometric pattern of distribution. Sandstone beds of the Point Lookout formation (lower Mesaverde) were deposited as a shoreline phase of a sea regressing northeastward. Sandstone bodies of the Cliff House formation (upper Mesaverde) represent the shoreline deposits of a sea transgressing south-westward at a later date. The shoreline along which these sands were deposited moved rapidly across some areas. In other areas it remained stationary for relatively long periods of time. The thicker sands correspond to places where the shoreline remained stationary, within a narrow belt, for the longer periods of time.

The successive vertical and lateral positions of the various Cliff House and Point Lookout shorelines have been established and are demonstrated on cross sections and maps. Those positions where the shoreline stabilized for relatively long periods of time are apparent in the form of “steps” that can be traced across the central part of the San Juan basin. The relatively thick, well-sorted sandstone units that correspond to the positions where the shoreline stabilized have been divided into a series of sandstone “benches” of varying widths.

Excellent examples of major “steps” in the Cliff House shoreline can be seen in surface exposures in the southeast and northwest parts of the San Juan basin. Those exposed at the surface in the northwest part of the basin exhibit a similar strand-line trend and in general correlate with the “steps” found in the subsurface.

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