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The article implies it was sort-of landfill:

This literal trash foundation is unfortunately how big parts of numerous American cities have been built.

Not only American cities. If you talk to any archeologist, they will attest that building cities on trash is common all over the world.

Some of the oldest settlements in the world are literally trash heaps. There are tells all over the Middle East that are tens of meters tall, representing generations of trash piling up and built over.

To my knowledge those accumulations of material are composed of eroded debris of adobe, mud or stone bricks from preexisting buildings or towns after centuries of erosion.

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