Mosses on Seasonally Dry Conglomerates of Inland Denman Island, BC: Triple Rock

Conglomerates are a type of sedimentary rock made up from peices of other rocks. With the obligatory reference to this Beavis and Butthead clip, I can't emphasize enough the satisfaction I get seeing a rock made out of rocks. Sometimes, the rock is made out of rocks that themselves are made out of rocks. I don't know what the record is, but locally both the Jackass Mountain Conglomerate (around Lilloett and southwards) and the various coarser units of the Nanaimo Group (which outcrops all along the Salish Sea) offer lots of ponderous exposures.

If you are interested in bryophytes and lichens, there are few substrates that show as much variation. Because of the shapes of both the cobbles and the concavities left behind when said cobbles weather out, you get tremendous variation in aspect over a matter of inches. Furthermore, the cobbles that make up the conglomerate can be both texturally and compositionally heterogenous. As a cherry on top, the intersticies between the cobbles, known geologically as "the matrix", can also be compositionally and texturally heterogenous.

On Denman Island there is a group of three massive conglomerate boulders called "Triple Rock". You can see them on a satellite photo here. They are presumably remnants or erratics from the submarine channel deposits of the Nanaimo Group, perhaps dropped as the ice sheet melted out in the area roughly 20,000 years ago. This conglomerate is made of cobbles that vary from volcanic basalts to intrusive granites to metamorphic quartzites and even calcareous nodules, the ages of the individual cobbles varying from ~80 million years old to roughly 1,500,000,000 million years old. The matrix in this case is a calcite-cemented sand, giving the rocks a basic quality that leads to unexpected bryological surprises-- things typical of Garry Oak bark or limestone make cameo appearances.

What fallows are all the mosses I could kick up on these conglomerates. If you are ever on Denman, stop by and try and add to the list.

Posted on December 9, 2023 03:50 AM by rambryum