Really? I'll assume it's because it has a wheel, but where can I find a reference to that?

I've locked horns with gov agencies several times over the trail erosion excuse to limit access. At Malibu Creek State Park I was told that horses didn't cause as much erosion as my mountain bike because they "Terraced" the trail as they walked (I've studied a lot of trails since and never have I seen any evidence of that) and my bike, I was told, would cut a rut in those terraced steps which would increase erosion.

While backpacking in a wilderness area last Sunday I was on a trail that's used by both a lot of hikers and horsemen and the trail is over 20" deep in a lot of places and just strewn with rocks (and there's no evidence of being terraced by horses here either). There's just no way one of those rolling packs could really impact a trail much, or off trail for that matter. At least not around here.

The truth is, trails cause erosion. I have found evidence of that almost everywhere I've went. I don't say that because I'm anti-trail, I say it because it's a fact.