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Field Division of Education
The Geology of Devils Tower National Monument
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At the edge of the main talus slope on the west-southwest side of the tower is a small, elliptical, grassy hill encircled by talus. It is about 150 yards long, trends west-southwest and consists of agglomerate which does not appear elsewhere in the vicinity, although it may underlie some of the talus. A trench was excavated in the hill in order to obtain specimens of all the fragments and of the matrix, the last appearing to be a decomposed porphyry. The most conspicuous fragments of the agglomerate are granite, in either rounded or angular forms, varying in size from small pebbles to boulders one or two feet in diameter. The rounded fragments have a somewhat faceted character, unlike the smooth polish of stream-rolled material. A limestone boulder of characteristic carboniferous habit, containing spirifers and other fossils, about a foot in diameter, was found to be encased in a shell half an inch thick, which could be broken away, parting smoothly from the rounded surface beneath as if the mass had been subjected to calcination by heat. Except for the greater variety of contained fragments, this agglomerate is essentially like the one found in the Little Missouri Buttes. The Devils Tower agglomerate contains in abundance two varieties of carbonaceous shales, which, according to Jaggar, are of a type not known to occur below the Lower Cretaceous and Benton terranes, which are stratigraphically higher than the present location of the agglomerate. This is believed by Jaggar to be strong evidence in favor of the Devils Tower intrusion being an offshoot from the Little Missouri Buttes, for the magmas giving rise to the Little Missouri Buttes are known to have intruded the Benton carbonaceous shales. Darton (1909), however, discredits this evidence, stating that such carbonaceous shales occur at intervals in the formations as old as the Minnelusa (Carboniferous).

It seems most natural to suppose this agglomerate to represent some of the first material to be injected in the sedimentary beds to form the igneous mass of Devils Tower. The magma, ascending through the underlying formations, picked up and carried along pieces of these formations, the heat partially fusing and altering the outer margins; but finally, with the cooling of the igneous material, the fragments were left as inclusions in the magma.

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