I think its really up to the luck of the draw when your world is generated.
A few friends of mine joined a friends world and as we were exploring around. We finally found a swamp. And as we noticed a few crypts within view of eachother. We began marking them and on our maps. Fast forward 10 minutes and we had no joke, marked 32 crypts in a small area. When we set upon clearing them, We ended up with a cart easily over 5k in weight and we had only gone through 6 of them. That much weight in a cart makes even the ever slightest inclines a damn mountain. Needless too say, Iron wasn't a rare resource for us.