Figures 1-2. A close-up of a weathered limestone surface reveals a mass of shell (brachiopod) fragments and mm-scale ring-like crinoid ossicles. Limestone flagstones comprise more than 95% of the elegant yet sturdy drystone walls. The view of the August 2018 walling workshop reveals a few elements of the technique, which uses simple templates of wood and/or steel rebar, plus string, to set out a three-dimensional space to be filled, within a centimetre or two, by the finished wall. In the foreground is a glimpse of the first stage: to separate raw materials into flagstones large and small, cobbles, and small stones suitable for interior "fill". A few key points:

• The local limestones are of middle Ordovician age (470-458 million years ago), from the Verulam Formation of the Simcoe Group.
• Some of them are almost devoid of fossils, whereas others represent coquina, a wave-swept accumulation of bioclasts (elements of the rock composed largely of fossil detritus, such as shell fragments). Coquinas can be quite attractive, and so find use in lapidary and jewellery applications (Thompson, 2012).
• Besides brachiopods and the odd stick-like bryozoan, an assemblage of colonial organisms, we can see circular crinoid ossicles (and one pentagonal [5-sided] variety from a different species).
• The circular crinoid sections are up to 7 mm in diameter. The walls show at least 2 genera (each) of bryozoans, crinoids and brachiopods. There are also burrows, and some oddly uneven, lumpy bedding planes.

"Rock of the Month #208, posted for October 2018" ---

Sediments of the Michigan Basin

overlie a wide area extending westwards from Montreal and Ottawa past Toronto, London, and Windsor, down through southern Michigan into the midwestern USA. The carbonate platform sediments, limy muds, that formed the limestone by compaction and cementation, were laid down along a seaway thousands of miles long, in a warm tropical sea. Evidence of this paleogeography, which seems so exotic in light of the current mid-latitude, strongly seasonal climate, is available in light of palaeomagnetic data (estimates of latitude at the time of deposition of the rocks), paleoecology and sedimentology (Jin et al., 2013). Further relevant discussions of paleoecology can be found in, e.g., Pickerill and Brenchley (1991) and Johnson (2006).

The carbonate sediments of the district are described by, e.g., Liberty (1971) and Carson (1980), and their nature and industrial uses are catalogued by Dolar-Mantuani (1975) and DMBW/OGS (1989). Some additional notes on the strata of the Simcoe Group can be found elsewhere on this web site.

THANATOCOENOSIS

As I understand it, imperfectly, this long word means "death assemblage" in Greek, and is an ecological term used by paleobiologists. When a pile of fossils or bones is unearthed, there are 2 possibilities for the geologist or forensic expert -

a) the luckless subjects were trapped by some fate in their living space, such as a coral reef buried in sand (a "biocoenosis"), or the victims of the volcanic eruption at Pompeii, or

b) they were going about their business, when a sudden event such as a flood swept them to their doom, and piled them up in some random place, NOT where they used to live. I think that is what we have here: the fossils were swished back and forth in the circulation near the beach, dumped in random orientations, then buried in sand / mud.

It occurred to me that we see crinoid ossicles individually, not in the typical stacks of them that corresponded to the stem of the sea lily, let alone the rare, perfectly preserved complete creature, with calyx (see a superb example from Guizhou in southeast China). That is consistent with a higher-energy environment, where the parts of the delicate organism are easily disarticulated.