The clearing of this great basin for the reservoir , twen- ty miles in length , was in itself an extraordinary under- taking , embracing the clearing of all timber , the removal of three villages , numberless buildings , farm houses and cemeteries and the abandonment and reconstruction of railroad tracks , telegraph and telephone lines , highways and bridges . The work of constructing the new Croton dam com- menced October 1st , 1892. It calls for a very large quantity of masonry ; in fact , there will be no less than 800,000 cubic yards . In this connection it may be of in- terest to note that it will be the largest block of masonry in the world , excepting the Egyptian pyramids . There are three distinct features of the dam , the spill- way at the north end ; the main stone dam crossing the valley and the embankment at the south end . The spill- way wall is built on the side of the valley , curving up stream from the stone dam ; it is 1,000 feet long , to feet ; high at the end and 150 feet high at its junction with the dam . It is rubble masonry , faced on the water side with cut stone in regular courses and the lower side is in steps of large blocks of stone . 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 .