When basaltic lava erupts under water it takes on a particular lumpy morphology which gives it it's name: pillow lava. The water supercools the outer layer of basalt to form an ultra fine grained chilled margin which acts as a skin to each pillow.

The stone "pillows" continue for a short period to behave behave as fluid bodies within their chilled carapace, and sag into the pillows below them solidifying completely.

This sagging allows for the way-up of pillow lava flows to be deduced
Pillow lavas of Ordovician age may be found outcropping in North Sannox Burn as part of the North Glen Sannox Beds. These are thought to represent remnants of ocean crust caught up in continental collison.

You can also examine a thin-section of the Arran pillow lava shown above.