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A fantastic landscape in Lower Manhattan — plummeting holes, steep cliffsides and soft billows of steel-gray bedrock, punctuated by thousands of beach-smooth cobblestones in a muted rainbow of reds and purples and greens — has basked in sunlight this summer for the first time in millennia.

This monumental carving was the work of glaciers, which made their last retreat from these parts about 20,000 years ago

“As the ice passed over New Jersey,” Ms. Moss explained, “it picked up local rocks such as red shale and sandstone and gray basalt from the Palisades. As ice melted from the advancing glacier, raging streams of water flowed in front of it. The strong currents picked up the sand, gravel and boulders and carried them downstream across the World Trade Center site.

“As these rocks bounced across the bedrock, essentially sandblasting the surface, the softer layers started to erode out and the harder rock left behind became polished. In places, the water swirled in whirlpools of varying sizes, carving out deep potholes and larger basins.”

Along the east side of the pothole, the rock layers run vertically — not horizontally. The result, where the surface has been carved away in a concave form, is an abstract canvas of swirling, concentric rings; not unlike a gouge in a wall that reveals many layers of old paint.

This speaks of a period far more ancient than the glaciers, about 500 million years ago, when the edges of the colliding North American and African continental plates got shuffled together.