The sandstone consists of thick- to thin-bedded, well sorted, clean quartzose and glauconitic sand with thin interbedded red and green fissile shales occurring near the top. Seven samples of the interbedded shales have been dated by the Rb-Sr whole rock method with a resultant age of 555 + or - 18 m.y. and initial Sr (87/86) of 0.7183 + or - 0.001 as determined by least squares analysis. A single Rb-Sr age on presumed authigenic glauconite yields an age of 542 m.y. using an assumed initial (sea water) Sr (87/86) value of 0.707. Both the shale and the glauconite ages are consistent with a Middle Cambrian age to which the Flathead Sandstone is assigned on geologic evidence. Four samples of shales taken near an andesitic sill have evidently been metamorphosed in such a way as to destroy the original (depositional) Rb-Sr balance.

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