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 Commis- sioners 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 ma- terially 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 . The core wall in the centre of the embankment was to be four feet higher than the crest of the spillway , six feet wide at the top and increasing to 18 feet at a depth of 136 feet , then same width to the base .