Blackhead fossil gas-seep concretions

A wide shore platform, cut into Late Miocene Whangaehu Mudstone, contains many conspicuous hard, pipe- and bulb-like concretions with tubular holes in them. The concretions can be several metres long and over 1 m across, and are calcareous (lime rich, 50−85% carbonate). They are thought to have formed by the precipitation of calcareous minerals (dolomite and some calcite) derived from methane ("natural gas") within the host mudstone at shallow burial depths, probably less than 100 m below the sea floor. (Since then of course uplift and erosion of the overlying sediment has exposed the rocks we see here today). These concretions are interpreted to reflect methane migrating upwards through layers of sediment, and their presence highlights parts of an ancient (about 10 million years ago) subsurface plumbing network of cold gas seeps. These same types of features, many actively and naturally venting methane gas, occur today on and below the sea floor in many places off the modern eastern North Island coastline.

At low tide, the extensive shore platform is well exposed. See how many different shapes of concretions you can find. Do they occur randomly, or do they occur in some sort of line or pattern?