First the excessive height , narrow base and unstable foundation of the embankment ; sec- ond , the great height of the core wall , and , third , the double means afforded the water to reach the core wall . To take up the first , the embankment : It was to be 150 feet high , and only 650 feet thick at the base . This section , would be not only about 40 per cent . higher than any heretofore built , but in comparison with other high embankments its base was narrow for its height . As an example , the Amawalk dam , which forms one of the upper Croton reservoirs , while only about half the height , 85 feet , yet has a base even wider than that of this embankment of unprecedented height . And further , this embankment was hazardous because of the unsta- ble nature of its foundation . It was founded over a great refilled pit , which was 360 feet wide at the top , 170 feet at the base and 70 feet deep ; this pit was a necessary ex- cavation for the foundation of the end of the stone dam , which was 164 feet wide at the base as before stated . It would be impossible to refill this pit as compactly as orig- inal ground , hence the safety of the reservoir was de- pendent not only on an embankment of a problematic section , but this problematic section , rested upon an un- staple foundation . The second of the objections , the core wall of this embankment was 200 feet high and with no lateral pro- tection or support from original ground whatsoever , as the artificially placed earth on each side of the wall had the height of the wall itself , 200 feet .