Department of the Interior. Bureau of Reclamation. Columbia Basin Project--Washington. Columbia River Dam Calyx Drill Holes Location up to 1938
Seven 36-inch diameter "Calyx" holes were drilled in the dam foundation rock in 1937, two of which were in block 41, to serve as "sumps. There was a total of 19 holes put in by the Government for inspection of the rock, the other fourteen being drilled in 1935 and 1936.