Wire fence (woven OR multiple strand high tension) - put the posts in the same height above ground down into the hollow and back up the other side, put the wire on at the same place wrt ground level on each post. The top of the wire will follow shape of the ground down and back up.

Two situations where this approach may not be the best:
1) If the hollow is very narrow, with steep slopes. But I have seen the above method work well on slopes as steep as 3:1.
2) If water will flow through the hollow - you may want to allow some open space at the bottom so brush doesn't catch on the fence. If the hollow is more of a ditch that will have frequent flow, often the fence is run straight across and a (folding) extension is hung below (which bends downstream to pass heavy water flow and brush). I have seen this used on board fence, not on wire as yet.

We normally place the bottom of the fence about 6" above ground (keeping horses and cows in, not sheep or pigs or smaller stuff), and don't have to worry about water flows underneath it.


"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
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