New York City Petrography
The bedrock beneath New York City, in thin section.
The rocks that underlie the city — schist, gneiss, marble, amphibolite and granofels — were carried to depths of 25 to 40 kilometres during two episodes of mountain building, recrystallised under high pressure and temperature, and later exhumed. What is now bedrock at street level formed roughly fifteen to twenty-five miles down.
Thin sections make that history legible. Mineral assemblages and textures record the pressures and temperatures the rock passed through, and the sequence of deformation that shaped it — the evidence behind the geological mapping of the city.
