The Gueydan Formation in outcrop is pastel-colored tuffaceous clay, sandstone, conglomerate, bentonite, and minor ash. The clay, bentonite, and ash are composed predominantly of silicic pyroclastic debris or the alteration products of such debris. Sandstone and conglomerate are composed largely of debris from volcanic and hypabyssal rocks plus minor detritus from sedimentary rocks and reworked Gueydan deposits.

Bailey (1926) reviewed the problem of the source of the igneous material and, although the evidence was inconclusive, favored a source near Duval and McMullen Counties. The writers' study suggests that sand and gravel debris of igneous rocks was derived from the Big Bend region of Texas and adjacent parts of northern Mexico; the fine pyroclastic material may have come from the same area or farther west.

Cross-bed data show that streams flowed east-southeast to the Gulf during the time of Gueydan deposition; hence, detritus was derived from terrane in an updip direction. Pebbles of soda-rich trachyte and trachyandesite (latite), rhyolite porphyry, and welded tuff in the Gueydan are similar in texture and composition to rocks exposed in the Big Bend region and adjacent Mexico. Although the Gueydan lacks detritus from some rocks exposed there, the rocks which are not represented as pebbles are mafic types that weather rapidly. Boulder-size clasts in the Gueydan were transported as stream bed-load and possibly for short distances by mudflows; they are not proof of a local source. Cretaceous foraminifers and rock fragments in the Gueydan show that pre-Tertiary bedrock was exposed locally cross the drainage area.

Upper level winds today and presumably throughout the Tertiary moved eastward across Texas; hence, the source volcanoes were toward the west. Although ash may have travelled to the site of deposition by wind, much was reworked by streams after initial deposition and subsequently modified by soil-forming processes.

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