How were these rocks formed?

bonzodog2

Can anyone enlighten me on how these rocks might have been formed? They have round grooves about 8cm wide. They are in Merlin woods in Galway. My first thought is that they might be sedimentary, formed around something like a tree, but I'm not sure if a tree would be around long enough for the sedimentary process.

the keen edge

Rock ID from a photograph is always a little sketchy; however I think that they are Limestone, (probably Visean Limestone formed in a tropical shallow marine basin ~330 million years ago when the land mass now know as Ireland was to be found at the latitude of todays North Africa and Spain) sedimentary as you've suggested.

The rocks would have formed in thickly bedded strata(^ Metres in thickness) in an area tens of square kilometres wide from sedimentary accumilations upon the sea bad. These sediments were compressed into horizontal strata and contain fossil corals, sea urchins, sea-lilies (crinnoids) and ammonites.

The cylindrical structure is almost certainly the result of preferential weathering, chemical dissolution of the calcium carbonate composition of the rock by some type of hydraulic flow over the previously found sub-aerial in-situ location of the rock.

bonzodog2

Thanks. By preferential weathering do you mean the grooves are where some softer rock was, that was worn away quicker than what remains? How might this softer portion have been formed in a straight tube?