I have seen plenty of collapsed dry stone walls.

Often the reason for the collapse isn't obvious - I suspect rather than a catastrophic event, it's more that the stones right on the top of the wall aren't held in by interlocking and friction as well as the other stones, so storms and animals can dislodge them. Plants growing in the top of the wall can knock stones off too.

If plant matter gets inside the wall, pools of water can form between stones, which then freezes and pushes stones apart and cracks them.

Do that for 2000 years, and the walls turn into a scattered line of stones in the ground 30 feet wide.

> rather than a catastrophic event

I'll give you catastrophic. Quite a few years ago me and some mates were camping in a field at night and a lady misjudged a turn and took her car through the dry stone wall. The car threw hefty blocks for an astonishing distance. What was odder was that after the initial smash I could hear heavy thuds as blocks dropped from the sky for what seemed to be up to 10 seconds. Far, far too many seconds to be credible, but that's what I noticed at the time.

Being dark we heard it but didn't see it happen. The full glory was revealed next morning.

If that wall hadn't been in the way the car would have gone all over our tents - and us.

We owe some anonymous derbyshire dry waller a lot of beer for putting it in the way.

(The lady was uninjured if extremely shocked)

That's interesting. I wonder if another factor is a wide variation in the level of skill used to build the walls. It's easy for a complete beginner to start building dry stone walls based on a limited amount of information - and using only materials found on site. Those walls might last the lifetime of the builder, but are less likely to hold up to stress over the ages.

I don't know about thousands of years off hand, but if you spend some time in New England you will see the truth to the poet's words:

  Something there is that doesn't love a wall,
  That sends the frozen-ground-swell under it,
  And spills the upper boulders in the sun;
  And makes gaps even two can pass abreast.

Many, most, all of the dry stone walls once in pasture and now in forest used to be straight, neatly stacked walls, and are now linear heaps that only outline and no longer divide old paddocks.

While Frost is beautifully right here, of course, there is a mild irony in that many of the rocks in New England walls were brought there by repeated glaciation, so cold both made the walls possible, and, as frost-heave, both surfaces buried rocks, and breaks down dry walls made with the generally rounded glacial rocks.

A secondary cause for spilling the upper boulders is the behavior of herds of dairy cattle, with each cow reaching over the wall to reach that one extra blade of tasty grass, with eventual wall collapse.

I have a friend there and they say it's pretty common to hire someone to restack them

The title of the poem is, in fact, "Mending Wall", and the overall subject is the yearly work to maintain the stone walls and musing on the necessity of the work.