Some Small Natural Bridges in Eastern Wyoming

Abstract

Natural bridges have been described at length by Mr. Herdman F. Cleland2 in a paper read before the Geological Society of America, December 29, 1909, and less extensively from time to time by Walcott, Cummings, Pogue, and others.3 Small bridges observed by D. E. Winchester and the writer, in Weston and Converse counties, Wyoming, seem to justify a brief description, as they belong to a little different class from any discussed by these writers. Of the different types enumerated the bridges in Wyoming more nearly resemble the one formed by a petrified log spanning a ravine in the petrified forest of Arizona, described by Mr. Cleland,4 but differ from it in that they owe their existence to indurated masses or concretions instead of to a petrified log as in the case cited by Mr. Cleland. The beds of the Fort Union and Lance formations of this district (eastern Wyoming) contain a predominance of soft sandstone and sandy shale. Almost everywhere in the sandstone are inclusions of concretions and indurated masses. They present a great variety of forms. Some are nearly perfect spheres from the size of a marble to eighteen or twenty inches in diameter; others are more irregular and vary from the shape of a log to very irregular masses sometimes several hundred feet in length. These larger