Correction to what I wrote: 'þat hrærðist' "it moved" (older spelling
'hroerðisk'), preterite indicative 3rd pers. sg. of 'hrærast' (older
'hroerask'); rather than 'þat hræðist' (older spelling 'hræðisk') "it
is afraid", present indicative 3rd pers. sg. of 'hræðast' (older
'hræðask').

hræða - to frighten
hræðask e-t, við e-t - to be frightened of s-thing
þat hræddisk - it was frightened

hroera - to move (transitive)
hroerask - to move (intransitive), to be moved, be in motion
þat hroerðisk - it moved