MANHATTAN GEOLOGY FROM THE BOTTOM UP

The excavation of rock at depths between 500 and 800 feet below the surface of Manhattan for New York City Water Tunnel #3 has provided a unique opportunity to observe the bedrock geology of New York City. Excavation has been accomplished by drilling and blasting, tunnel boring machine (TBM), and by raised boring techniques (at shaft sites). Rocks encountered include; biotite-muscovite schist, phlogopite schist and schistose gneiss, amphibolites, serpentinites, quartzo-felspathic pegmatite, aplite, quartzite, granitic gneiss and calcitic, dolomitic and siliceous marble. Talc, asbestiform amphibole, and kyanite have also been observed. Some of the metamorphic bedrock is highly garnetiferous. Garnet has also been found in some of the pegmatite, perhaps as a refractory residue of anatexis. Magnetite concentrations as high as 23% by weight, have been observed in some of the schist. Pyrite rich zones, perhaps a hydrothermal replacement, are seen, in places, in the marble. The metamorphic rock is not homogenous. This may reflect variability in the protolith. Solution voids in the marble have been detected at depths as great as 200 feet below the surface. Soils include glacially derived materials, saprolites, and anthropogenic fill. Depth to bedrock varies from 0 to over 100 feet. Bedrock is mainly Cambro-Ordovician in age, underlain unconformably by Neo-Proterozoic rocks. All of these rocks were affected by the Taconic and Acadian Orogenies, Neo-Proterozoic rocks by the Grenville Orogeny as well. Overturned folds and fault related structures are easily observed in the tunnel.