I have come across those farm fences online. Also used on the plains where lumber was not readily available.
They are bolted. 1/4 bolt in a 3/8 hole for expansion/contraction of the pourous stone (I know sounds bizarre, but I guess it really does change like wood.)
I was going to ask here. The pickets, some of them were height adjusted by putting them in the ground a couple inches. Do you think the freezing thawing could cause a problem?