Mega-terrain edges - problem

I apologize if this would be the wrong place to post this particular issue. I'm trying to get a mega-terrain to work and the trouble I'm having is that along it's edges there is something of a wall (vertical terrain) that shoots up in various places. I'm using a 512 x 512 jpg reference when creating it and have attempted several different heightmaps; all have the same effect!

Could someone point me in the right direction here?

Thanks