The inscription reads:

About 1 mile N.E. of here lies Mono Diggins, the first extensive placer mining excitement east of the Sierra. Cord Norst is generally credited with being the discoverer on July 4, 1859. A town, Monoville, boasted a transient population of 500 – 1,000. A post office established December 12, 1859, was closed by April 2, 1862, as the prospectors moved to Aurora.

One of the most ambitious hydraulic water projects of the time was the transportation of water from Virginia Creek to the Diggins by open ditches known as “the Mono Ditch”. The remnants of the system may be seen by looking easterly from the highway summit or northerly from the bottom of the grade.

Source: https://www.hmdb.org/marker.asp?marker=49929