>If the circle builders wanted to corral livestock, why didn't they use 4 ft tall thin stones...<

How many 4ft tall thin stones are going to be lying around on your average site? If you're in an area with suitable stones (of whatever size or shape) you're likely to use them just as they are rather than spend hours chiselling them into regular sizes and shapes.

>Why go to all that effort when there are easier ways?<

Actually, I don't think there <i>are</i> easier ways to construct a secure corral other than to have posts (of stone or wood) to support the fences.

The link to the Stones List discussion came under the <b>Big Avebury Pigpen<b> (I think :-)