The familiar red colour of these rocks arises from the presence of iron oxide but not all the Old Red Sandstone is red or sandstone -- the sequence also includes conglomerates , mudstones , siltstones and thin limestones and colours can range from grey and green through red to purple .
The dark red colour of these rocks comes from iron oxide . However , not all the Old Red Sandstone is red or sandstone -- the sequence also includes conglomerates , mudstones , siltstones and thin limestones and colours can range from grey and green through red to purple .