Burr , who had con- curred in the condemnation of the wall , reported to Mayor Low as to the foundation as follows : " It is most fortunate that the core wall has been removed so as to dis- close the actual situation and thus remove what would have been a source of gravest danger to the completed embankment structure as originally planned . " Prof. J. E. Kemp , of Columbia University and Prof. J. J. Stevenson , of New York University , both geologists , were retained to investigate the locality affected . They re- ported that the conditions were deserving of very serious considerations and that in certain locations the material was for nd to be so soft that in their own words it " runs with water like sand . " Under their advice this loose and disintegrated rock has been removed to the depth of 75 feet below the base of the core wall , for it is only at this great depth that a suitable foundation for extending the dam has been found . The removal of this soft rock and the replacing it with masonry has again materially in- creased the cost of the structure . The work of building this masonry is now in progress . Another important change of plan is that of the gate- house , above referred to . This gatehouse is located in the embankment outside of the core wall . It is well up on the slope of the valley , its base being about 100 feet higher than the bed of the river . The substructure of this gatehouse is described as follows : It has four water chambers , 74 feet deep and about 12 feet square . Two on the southerly side of the structure are to control the flow of the water in the old Croton aqueduct which passes through this gatehouse .