323. NEW YORK ON FOOT. Riverdale, The Bronx. “New York City is primarily composed of sediments that were metamorphosed during the Taconica nd Acadian orogenies roughly 500 – 400 million years ago. Garnets can be found in the rocks of the Hartland Formation and Manhattan Schist. It is in these hard rocks that the city skyscrapers have their foundations.” -Wiki. An Engineer in Queens once told me that you can track the location of hard rock by looking at the Manhattan skyline.