Marshall Grattan—emphasis on the second syllables of both his first and last name—has agreed to talk bouldering. Remarkable, because the svelte French boulderer is publicity-shy. But the opportunity to speak on his greatest enthusiasm has overmastered his reserve.

“Coarse and gritty,” Grattan is saying. “Like if you were to rub your hand on sandpaper. Something like sandstone or like quartz conglomerate, which is a mix of sandstone and quartz. For the opposite, imagine rubbing glass.”

Grattan is describing friction, an essential quality for bouldering. Without it, all those boulderers you see in the videos hanging off the rock by their fingers and toes like some species of rock-climbing insect—they’d fall right off.

“You can’t free climb outside in the rain,” Grattan notes. “It’s just w

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