It's a shame you did not include the entire quote, since I do not remember the details, and I do not have my books at hand at the moment, but I believe this is a reference to Summer, not Shaggydog. Which is also supported by the wikipedia at Westeros.org
These passages are written from the wolves' perspective, and so are easy to misinterpret.
For the first passage, Shaggydog grows more distant because he is traveling another direction with Rickon and Osha. The second passage merely uses the "sense" in a similar fashion, not as an implicit continuation of the former, and it is most likely referring to Bran and Summer being blocked by the Wall (and possibly the wards in place in their cave, not sure about the timeline here).
The wall is built with magic, as is referenced many times, when Coldhands cannot pass the wall, or Melisandre stating that her magic has never been stronger, and that the wall is one of the "great hinges of the world".