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Show 168 GEOLOGY OF THE HIGH PLATEAUS. They all lie near the ancient shore line of the great Eocene lake, and cases of unconformity, not only with the underlying series, but among themselves, are not uncommon. Their physical characters are, in general, indicative of an epoch of gradual displacement in the several tracts which they occupy. It would be obviously extremely difficult to correlate such a group with any such formations as those which are found on both flanks of the Uintas, forming the comparatively regular and systematic strata of the Upper Green River series, though general considerations may warrant a provisional reference of these local deposits to that period. The unconformities just spoken of are probably in some cases apparent rather than real. It is easy to see that while deposits are accumulating along the slope of a flexure which is in process of formation, the two going on pari passu, there may result a want of parallelism in successive layers as well as other irregularities which produce collectively the appearance of unconformity. This differs, however, from that type of real unconformity which is usually relied upon as proof of an interval of time between contiguous formations in which the record is interrupted by a blank of unknown duration. Where the exposures are satisfactory the apparent and real occurrences may be distinguished, but in a majority of cases the distinction is not easy to find. The thickness of the formation is highly variable, ranging from 300 to 750 feet. It consists of alternating marls and sandstones, the latter being sometimes coarse-grained, with here and there a patch of conglomerate. |