Abstract

Columnar joints in cross-bedded sandstone of the Miocene Browns Park Formation adjacent to sills and dikes are similar in most aspects to columnar jointing described in igneous rocks. The occurrence is located in secs. 20 and 21, T. 12 N., R. 89 W., Moffat County, Colorado, in the northern part of the Elkhead Mountains volcanic field. The columnar joints are oriented normal to the igneous contacts, but at random to the bedding and cross-lamination. The columns vary from 3 in. to 5 in. in diameter and are up to 3 ft long. Some columns are curved but no "ball and socket" joints were observed. The sandstone was cemented by introduction of opaline silica prior to cooling. Jointing is due to the same processes as those reported to cause columnar joints in the igneous rock masses.

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