New York City’s Little Black Book is free today through the 19th. This is an example of what’s covered:

Most people assume the Manhattan skyline is the way it is because of money, real estate, or zoning laws. The real answer is 300 million years old.

When the supercontinent Pangaea existed, the land that would become New York sat near its center. Over millions of years, massive mountains — taller than Everest — built up over the region, and the weight buried the rock beneath under eight miles of pressure. That pressure created Manhattan schist: some of the hardest, most stable bedrock on earth.

Here’s where it gets interesting. That schist doesn’t sit at a uniform depth. It rises close to the surface in two places: midtown and lower Manhattan. Those are exactly the two places where you see skyscrapers clustered. Builders could anchor massive foundations directly into solid rock.

In between — roughly in the 30s and Village area — the bedrock dips far below the surface. Sinking foundations deep enough for skyscrapers becomes enormously expensive. That’s why the mid-section of Manhattan stayed lower for so long.

The skyline isn’t an accident of capitalism. It’s a map of what lies beneath — drawn by geology 300 million years before the first steel beam was bolted into place.

New York is full of secrets hiding in plain sight. This one is just bigger than most.

— From New York City’s Little Black Book by Bob Mayer (www.bobmayer.com)