A channel for the overflow is excavated in the rock back of the wall leading down to the river below the dam . The main stone dam , according to the original plan , was to be 600 feet long , extending from the spillway to the foot of the slope at the south side of the valley , thence an embankment and core wall 568 feet long was to continue to the end , making a structure 2,168 feet long . The main stone dam is rubble masonry , faced on both sides , above the surface of the ground , with cut stone in regular courses .

It is 297 feet high from the deepest point of the foundation , which is 131 feet below the bed of the river . Its greatest width at the base is 206 feet and the width at the top is 20 feet . At the junction with the core wall according to the original plan it had a height of 230 feet .

For the purpose of reaching a point where the core wall would have a lesser height , the Aqueduct Commissioners on Sept. 16th , 1896 , on the recommendation of the former Chief Engineer , authorized him to extend the main stone dam 110 feet , in substitution of the embankment and core wall , but even at the point thus reached the core wall would have a height of 200 feet . This extension materially increased the cost of the structure .

The embankment was to be 30 feet wide at the top , with sides sloping in the ratio of two horiozntal to one vertical . The lower portion of the inner slope to a height of 16 feet below the crest of the spillway , to be paved with stone , 18 inches thick , laid dry , upon 12 inches of broken stone , and on the upper part of the slope to a height 12 feet above the crest of the spillway , the paving stone was to be two feet thick , upon 18 inches of broken stone .