The coat of arms of the United States Army Corps of Engineers, also used as the seal.  The left-hand side of the shield (heraldic "dexter") is the original "Essayons" seal, which was the emblem of the Corps from at least 1812 and to this day appears as the Essayons Button on USACE uniforms.  The right-hand side ("sinister") was the emblem of the Corps of Topographical Engineers, which separated from the regular Corps in 1838.   The Topogs were re-joined with the regular Corps in 1863, after which this combined seal was made dating from 1866.  It was made official on April 6, 1897.  The Corps motto "Essayons", meaning "Let Us Try.", is found on a map from 1806 and may date from much earlier.

For more information, see here, here, this chapter of the Graphics Standard Manual, here, and this publication.  For information on the Corps of Topographical Engineers, see here.
Converted from CGM version available from a USACE Graphics page (direct file URL here).
U.S. Army