The method of building the dam was simple enough even though it sounds complicated in the telling. When Congress acquiesced in the minority report of the Board of International Engineers, approved by the President and recommending a lock type canal, it meant that instead of simply digging a ditch across the Isthmus we would create a great artificial lake 85 feet above sea level, confined by dams at either ends, with locks and two short canals to give communication with the oceans. To create this lake it was determined to impound the waters of the Chagres, and a site near the village of Gatun, through which the old French canal passed, was selected for this purpose. Conditions of topography of course determined this site. The Chagres valley here is 7,920 feet wide, but the determining fact was that about the center of the valley was a hill of rock which afforded solid foundation for a concrete dam for the spillway. Geologists assert that at one time the floor of the valley was 300 feet higher than now, and that in the ages the Chagres River cut away the shallow gorges on either side of the rocky hill. These, it was determined, could readily be obstructed by a broad earth dam of the type determined upon, but for the spillway with its powerhouse and flood gates a rock foundation was essential and this was furnished by the island. [Illustration: _Photo by Underwood & Underwood_ CONSTRUCTION WORK ON GATUN DAM The space between two rock walls has been filled with mud, which having hardened, supports dirt trains bringing spoil from Culebra Cut to build up the dam to required dimensions] [Illustration: _Photo by Underwood & Underwood_ PUMPING MUD INTO THE CORE OF GATUN DAM]