Ah, simple mistake.
The Civ model really doesn't allow for the vague and specific exploration, or even "forgetting" territory (because the player will remember even if your people haven't been there in 2000 years).
Maybe they should add a second layer to the fog of war? All units can go exploring, and you'll see the general lay of the land (borders, forests, water, plains, etc...). But to be able to discover resources, you need your units or scouts to actually sit on a square for a turn (to discover the terrain in the visible fog). This would take a turn to do for scouts, but explorers wouldn't have to (thus making explorers useful units).
Or even something like each tile has certain "exploration points" associated with it. So, something like a tile being visible by a unit gives it 1 point. A unit moving over the tile gives 2. A unit ending its turn on a tile gives 4. Scouts give double points. Explorers give 4X. You see the layout of each tile normally, but each resource has a certain number of points required (maybe even only after you discover the tech for the resource, since it doesn't make sense that your scout sitting in the desert for 5 turns in 3000BC will magically be able to tell you that there's oil on a square when you get to that point).
The problem with this is that it requires much more randomness (ie. getting lucky to settle a city by the copper), but it does mean that you would need to do a lot more exploration to actually map out the best sights.