The slope of this pit extended southerly along the line of the wall for a distance of 150 feet , thus the core wall at its highest end was not built in a narrow trench below the surface of the ground , as is usual in ordinary cases . The bed rock on the north side of the valley is gniess and the structure is founded upon it for a length of 1,200 feet . The character of the rock then changes abruptly from gniess to lime stone and this extends southerly beyond the end of the structure .

Although the contract drawings , upon which the contract was based , indicated " hard rock " at a depth of 75 feet below the bed of the river , yet it was found necessary to excavate the foundation pit to a depth of 131 feet before finding a firm foundation of hard rock . The character of the stone removed varied greatly ; in places it was compact , then diversified by masses of stone broken up by open , eroded and mud - filled seams , and in places there were belts of disintegrated lime stone .

During the progress of the excavation water flowed freely through the seams and several open caves were found in the rock . In consequence of this great additional depth , it was also necessary to greatly increase the width of the base of the dam . Thus again the cost was materially increased because of the additional quantity of excavation and masonry .

Before the writer assumed the responsibility of this work , the foundation of the stone dam had been completed to the surface of the ground and the core wall had been completed , excepting the stretch between the stone dam and the gate house , which lacked about 60 feet of its height .