To add to what Doyle said on a more tactical note, when a regiment was in combat one of the most dreaded events was a unit on your flank to retreat or not keep up, thereby exposing your flank to enemy fire. You wanted your flanks anchored or "tied to" a natural obstacle (mountain, stream, swamp etc) or another rock solid regiment. Therefore, the statement by the Texans that the "3rd Ark will do to tie to" meant that with the 3rd Ark on your flank, the TExans would not have to worry about their flanks. Quite a high compliment of the times.