If you go down to the woods today!

While walking in the woods on the northside of the valley between Lieurac and Le Sautel we came upon this incredible sight. Actually 3 of us walked over it and would have missed it altogether if our sharp eyed son had not spotted it.
A moving chain of caterpillars about 6 - 7 meters long in all. Slowly crawling up the hillside nose to tail in a continuous chain.
Some of them we had unwittingly stepped upon and the chain was broken where the caterpillars were injured. We carefully removed the injured ones and put them out of their misery and joined up the chain again by putting the next caterpillars in contact with the preceeding ones and off the living chain went again.
The video above seems a little faster than they were actually moving but it gives a general impression of what was happening.
None of us had ever seen or heard of anything like this before.. comments will be most welcome.