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How did the erosion happen? At what time period was that much water there?

https://en.wikipedia.org/wiki/African_humid_period

https://en.wikipedia.org/wiki/Sahara_pump_theory

You know sand and wind erode as well, right?

Different ways of erosion leave different kinds of traces. The ones in the Sphinx enclosure are indicative of water (more specifically - rainfall) erosion.

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