Wonderful animal. Excellent photo. LIKE. Best Greetings.
Barnacles. They are a small sea creature related to shrimp but they stick themselve to rocks. When the tide is out they close up like this but when the tide comes in fronds pull food out of the water. They are small creatures this picture is only of about four or five centimetres of them. A god article for more information is http://en.wikipedia.org/wiki/Barnacle
All the best
this is a fantastic place to see. It's a must if you are visiting Orkney
This is either No 1 or No 2 barrier, not No 4 barrier as indicated. As the photographer has misplaced several other photos which are at or near No 1 barrier, placing them at No 4, then I suspect this to be No 1, between St. Marys and Lambs Holm.