babyheads

Rounded, cobble-sized rocks that litter a trail or riverbed, just large enough to turn an ankle. They wobble underfoot like they're not quite committed to the ground. Unfriendly to feet and mountain bike tires. Not conducive for rock skipping.

Etymology

Climber and backcountry slang. The name comes from their size and shape — roughly that of an infant's skull, smooth and unstable.