AS LEWIS AND CLARK’S Corps of Discovery made its way across the continent to Oregon, the men (and woman) of the party probably weren’t thinking much about their place in history. So they weren’t taking any particular pains to document their every movement.

There were, however, some particular pains they were experiencing, as a result of a relentlessly low-fiber diet: Everyone was constipated, all the time.

Luckily, they had something that helped with that — a lot. The Corps of Discovery left on its journey with a trove of 600 giant pills that the men called “thunder-clappers,” which the soldiers and travelers used to jump-start things when they got bound up. And everyone used them pretty regularly.

And, strange as it seems, that fact is why we know several of their campsites along the way. The main active ingredient in “thunder-clappers” was a mercury salt, which is a pretty stable compound. Archaeologists simply have to search for dimples in the ground — which is what old latrine pits often end up looking like, hundreds of years later, after Nature has partly filled them in — and take samples of the dirt in them.

If it comes up with an off-the-charts reading for mercury, well, that’s a Corps of Discovery pit toilet — and the layout of the rest of the campsite can be extrapolated with considerable precision by consulting the military manuals they used to lay out their camps.

(Astoria, Clatsop County; 1800s) (For text and pictures, see https://offbeatoregon.com/2501d1006d_biliousPills-686.077.html)