Gassy lava near Dockey Tarn?

 On the way up towards Nab Scar from Alcock Tarn, I found this boulder inn the path. There are rows of little indentations.

I was wondering if it was artificial but it continued into rocks nearby. This means that it is probably due to gas bubbles in lava. But why would the bubbles be so uniform in rows? It doesn't look as though there were repetitive layers forming on layers and that in each the bubbles rose to the top. So a mystery.

We went on to visit lonely Docket Tarn which sits on one of those wide ledges that is characteristic of the lava flows in the Borrowdale Volcanics around St John's in the Vale.