If cosmic rays slowly build up beryllium-10 inside quartz grains, but only while those grains sit in roughly the top metre of soil and rock, why does river sand that carries very little beryllium-10 tell geologists that the whole basin upstream is eroding quickly?

Answer: Erosion pulled the grains out of the shallow zone before much beryllium-10 could build up

Rain washed the beryllium-10 out of the grains while the river carried them downstream — Rain cannot wash this beryllium-10 away, because it sits inside the quartz crystal itself, not on the surface of the grain. Cosmic rays create it by knocking apart oxygen and silicon atoms deep in the crystal structure, and quartz is so chemically tough that those atoms stay locked in through weathering and a long trip downstream. A second kind of beryllium-10 does form in the atmosphere and arrive with rain, but it only coats the outside of grains, and labs dissolve that coating away before measuring. What remains is a record of time spent near the surface, not of weather.

Earthquakes on nearby faults shook the grains and reset their beryllium-10 to zero — Earthquakes do not reset the count: nothing a fault does can remove beryllium-10 atoms that already sit inside a quartz grain. The only thing that stops new atoms forming is burial deeper than cosmic rays reach, and even then the old atoms linger, fading only by radioactive decay with a half-life of about 1.4 million years. Faults do matter, but indirectly: they steepen slopes and speed up erosion, which shortens every grain's stay near the surface. The sand records that faster erosion, not the shaking itself.

Erosion pulled the grains out of the shallow zone before much beryllium-10 could build up ✓ — Very little beryllium-10 means the grains spent only a short time in the shallow zone where cosmic rays can reach them. On a fast-eroding hillside, rock is broken up and carried off within centuries to a few thousand years, so each grain collects only a small dose before a stream sweeps it away; on a slow-eroding one, grains linger for tens or hundreds of thousands of years and build up far more. Because a river gathers sand from every slope it drains, one scoop from the channel averages those exposure times across the whole basin, giving an erosion rate for the entire landscape upstream rather than for a single hillside.