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They are fixed on a bedding plane, and so provide more reliable evidence of exactly when humans left them.

They could be dense, too; in one bedding plane, there were hundreds of filaments per square meter.

“I have rarely felt as far from the human realm,” Macfarlane writes, “as when only 10 metres below it, held in the shining jaws of a limestone bedding plane first formed on the floor of a warm Cretaceous sea.”

I have rarely felt as far from the human realm as when only 10 metres below it, held in the shining jaws of a limestone bedding plane first formed on the floor of a warm Cretaceous sea.

The strings are usually straight, unbranched, and remain within a single bedding plane.