Those who say the World Trade Center site is changing at a glacial pace have no idea how right they are.

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, leaving profound gouges in the earth and rocks from the Palisades, the Ramapo Mountains and an area of northern New Jersey known as the Newark Basin.

Plumbing these glacial features and souvenirs has been critical in preparing the foundation for Tower 4 of the new World Trade Center, being built by Silverstein Properties. The concrete footings from which its columns rise must rest on firm bedrock. Engineers need a clear understanding of the rock’s contours.