The Arkose Ridge Formation of the southwestern Talkeetna Mountains adjacent to the lower Matanuska Valley, has been a stratigraphic enigma. The formation consists of arkose, shale, sandstone, and conglomerate, contains abundant plant material, and may be more than 6000 ft. thick. The fossil plants were first thought to be of Eocene Age; new collections indicate an Albian or possibly a Cenomanian Age for the formation. This age determination suggests that the Arkose Ridge Formation is correlative with mollusk-dated marine beds in the lower part of the Matanuska Formation; these marine strata vary throughout the region from a thin section of glauconitic calcareous sandstone, silty claystone, and algal limestone, to thin sections of feldspathic sandstone and siltstone.

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