HWW~ I'll be dammed!

A puzzle at first sight here. It is actually the back wall and buttresses of the Lochan na Lairige Dam here near Loch Tay in the Scottish central Highlands.. As I walked towards these massive archways I could not tell until stood underneath whether the top inside was lit up by a light through a gap to the top of the dam or was it simply an optical illusion of difference in the rock colouration.

HWW, enjoy the week.

ps A more complete photo of this to come!