The atlas, backwards

You do not meet a rock where it was made.

You meet it in your hand, on a bar, already broken off from somewhere and carried a distance you cannot see. A map points forward: here is a place, go to it. This tool points the other way.

Every mark below sits at its true distance to the sea, measured along the water. The vertical gap between connected points is the transport distance, so a material's survival range becomes a length you can hold against the picture. Nephrite can survive about 450 km; serpentinite usually cannot survive more than 30 km.

Divides are absolute, lakes and dams stop bedload, and every material wears down at a different rate. The model rules impossible sources out before it ranks plausible ones.

The wall at river-kilometre 100.5

At the downstream tip of Yaalstrick Bar, under Sumas Mountain, the Fraser's bed changes from roughly 80 per cent gravel to entirely sand in under a kilometre. That transition bounds the collectable river: the gravel reach from Laidlaw to rkm 100.5, followed by a diffuse tail of gravel patches over the sand bed.

Run it forwards

These are materials the model says could reach a vetted atlas bar from an unimpounded source inside their survival range. They are predictions, not recorded finds.

Read this honestly. An absent find is not an absent rock. The source register bounds the model, every survival distance is inferred, and glacial transport ignores drainage divides. A prediction is a reason to study a public bar—not permission to enter a claim, closure, park or private property. The access status in the public atlas still governs.