Too fragile to be erratics?

These are three of the standing stones at Callanish.  The top two are at Callanish One, and the lower photo was taken at Callanish Two.  They are all taken side-on, showing the thinnest face.  They are in reality elongated flattish slabs.  And they are very delicate indeed -- I cannot see these as having been transported over a great distance by glacier ice, either in the glacier or on its surface.  So I would not call them erratics, and prefer to think that they have been collected by the builders of the stone monuments from the broken rock outcrops in the vicinity.  Other more chunky rocks (of which there are many at the three Callanish monuments which I visited) could well have been partly moved by ice, and may have been picked up by the Neolithic builders very close to the sites where they were used.