Erratics are boulders that have been brought from somewhere
else by glaciers, they fall onto the glacier or become
incorporated somehow and are carried along by it, the glacier
then melts and deposits the erratic somewhere a bit "odd". Often
they are a different kind of rock from that where they are deposited,
though this one seemed to be very similar to the surrounding
rocks so maybe it hadn't come that far.
Such rocks are
quite common around this region and seem to frequently be deposited
somewhere very obvious, on top of a rocky outcrop like this
one rather than in a trough for some reason, perhaps the trough
ones just became over grown or buried in eroded rocks.
Picture copyright Paul Ward 2009.
Pentax digital equipment.