Geology Along the Lewis & Clark Trail

Geologic Points of Interest

Pompey's Tower

Pompey's Tower, or Pillar, is an outlier of Cretaceous sandstone that was formed when the Yellowstone River first meandered around a protruding headland of rock (A). Continued erosion by the river cut off the headland to form an isolated erosional remnant known as an outlier (B). The river now passes between the headland bluffs in the distance and the outlier (C). It was here on July 25, 1806, that Clark engraved his name on the cross-bedded sandstones. The fossil cross-beds from the ancient river channel are visible in the date (D). (Clark, 7/25/1806)

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outlier

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