As of about a month and a half ago work began on reconstruction of the Crystal Springs Dam roadway. The original roadbed was demolished, taking the two benchmark disks in the process.
I attempted to contact the agency overseeing the project in the hopes of making a physical recovery of the two disks in order to return to the NGS. Was greeted warmly but was not allowed to recover the two disks. I was informed that the agency would be recovering the disks and that it may "re-install" the two disks after the new roadbed was completed.
I informed a contact at the NGS regarding this situation and we agreed that without following a specific recovery process, that the marks, while they may be physically restored to their original locations, would be worthless to a subsequent survey. I made one more attempt to make a physical recovery, spoke with the agency's field rep regarding specific steps that would need to be followed in order to properly restore the two benchmark disks after reconstruction work had been completed but was again informed that "it would be taken care of".
Sadly, this rare "twin" disk monument is destroyed.