http://imgur.com/a/h95TX

http://imgur.com/a/Hm8qs

RTG-1.7.10-0.4.0 BiomesOPlenty-1.7.10-2.1.0.1640-universal

Those kinds of biome-surface transitions are an intended feature of version 0.4.0.

However, they will be removed in version 0.5.0 in favour of 'decorative' biome transitions.

Thanks