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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 zones are found. These sandstone units exhibit a definite geometric pattern of distribution. Sandstone beds of the Point Lookout formation were deposited as a shoreline phase of a sea regressing northeastward. Sandstone zones of the Cliff House formation represent the shoreline of a sea transgressing southward 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 pl ces where the shoreline remained stationary for the longer periods of time.
The successive positions of the various Cliff House and Point Lookout shorelines have been established both vertically on cross sections and laterally on 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 with the positions where the shoreline stabilized have been divided into a series of "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 "steps" exposed at the surface in the northwest part of the basin exhibit the same strandline trend and in general are correlative with the "steps" found in the subsurface.
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