There is an issue with strips of chunks running north/south generating chunks. It appears the noise function is not working properly and returning unintended results. I have seen this in quite a few worlds and tracked it down to ExtrabiomesXL by regenerating seeds after removing all mods other than Forge and EBXL.

The version used was
ExtrabiomesXL-universal-3.5.2.jar

Here is a seed to test the issue and a screenshot:
-1749136237090068508

http://i.imgur.com/3TVm8.jpg

This is a known issue with marshes which has been reported. It is currently being worked on.

Solution: Disable the Marsh biome.

This issue will be addressed by issue #125

Ah, I didn't see that one, I just skimmed through the open issues to check for duplicates.

yes...it was in the closed issues. :P