I thought it would be relatively easy to identify this caterpillar which was walking among stones across a path.  There are about 400 illustrations of caterpillars in my field guide and I have scrolled through the internet but have come to no conclusion and hope that someone can identify it.   It is a pale green with a black head and very obvious black markings on its sides and the nearby vegetation included thistles, willow herb and brambles.  

(Edit)  Gen2 identifies it as a Pine sawfly