I'm not sure if this has been submitted before (it probably has though) so I'm sorry if it has, but I think these are bugs in the terrain generation for Minecraft version 1.6.2.
I was trying to find a good seed and upon inspection I found 2 sections of the terrain that, instead of generating normally, were made out of stone.
This may be fixed in the upcoming version of Minecraft, 1.7, due to the terrain updates, but I thought you guys should know.
P.S. I will include pictures of the terrain. The seed is 1348515798. The coordinates will be included in the pictures.