That sediment is special, indeed. Desert sand from, say, the Sahara is no good for making concrete because it’s too rounded and uniform. Over millennia, winds push those grains around, polishing them. If you make concrete out of such sand, “it’s almost like building with marbles,” says Bendixen. “You want particles that are more angular in shape, not rounded. And that type of material is exactly what you get from rivers, for example, or material that has been deposited by glaciers.”
As Greenland’s ice sheet—which covers 700,000 square miles and is up to 10,000 feet thick—rubs against the land, it grinds up
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