The location of rare is almost exactly in the centre of the Grand River basin. The basin is home for 800,000 people and is forecasted to have over 1,000,000 residents in less than 20 years. The river drains approximately 7,000 square kilometres as it flows almost 300 km from the Dundalk highlands to Lake Erie. The rare property is of geological significance since it is one of the areas of the central Grand basin that has an extensive bedrock outcrop along the river.

Opportunistically sited, rare has a moderately large bedrock outcrop. However, the outcrop thickness is not too well developed and is only easily seen along the south bank of the Grand River and in parts of the wooded area.

Just outside rare is an old stone quarry which provides access to some of the Silurian dolostone and the interesting, but poorly preserved, fossils contained within it.

Oil and gas drilling near Roseville, some 15 km west of rare, provides us with a “deep” view of the geology under the region. This is summarized in Figure 3. At rare the glacial deposits directly overlie the upper part of the Middle Silurian Guelph Formation. The Upper Silurian Salina Formation that lies above has been eroded away. The Salina contains gypsum and rock salt and does outcrop 20 km to the south at Paris, where the rocks were exploited for the plaster of Paris industry. However, just west of rare the outcrop is cut out by the regional dip that rises to the surface approximately 7 km west of Blair.

The geology beneath rare

The deeply hidden strata under rare is moderately well understood from deep boreholes elsewhere in the Grand basin and also from outcrops farther to the east; for example east of Guelph and along the Niagara Escarpment. A large number of deep boreholes drilled for oil and gas companies in southwestern Ontario reveal the presence of a continuation of the Canadian Shield at a depth of close to one kilometre under much of the region. The distribution of these rock units or “domains” as they are known is shown in Figure 6.

Situated very close to the boundary of the Waterloo and Cambridge domains, rare is represented by quite different suites of rocks, but both probably date to about the same general Middle Proterozoic time frame (Fig. 7). The Waterloo domain has a high magnetic susceptibility trending north-south with more pronounced and elliptical magnetic highs. These represent deeply buried “plutons” (igneous bodies) that have higher concentrations of magnetite. Most of the rocks are granodiorites to quartz-mica-diorites. The Cambridge domain, located a few kilometres to the east, contrasts by having a low aeromagnetic signature and by being composed of dolomitic marble and formerly clastic sediments that have been metamorphosed. The Cambridge domain also has a north-south trend. Rocks that exhibit the same ages and geology are present north of the exposed edge of the Shield at the southern edge, and east of, Georgian Bay.

All of these rocks were formed in the last one-third of Precambrian time and were originally deposited as sediments. The sediments were later crumpled into mountains built on the super-continent of Rondinia. Today, remnants of these mountains — that in Grenvillian time (about one billion years ago) were about the height of the Himalayas — can be seen in the Gatineau hills running from Ottawa into Québec.

Because of mountain building, erosion and rifting, a large time block exists when materials were being continuously eroded. Geologists call these time gaps “unconformities”. A huge unconformity representing perhaps ~ 450 - 500 million years is present between the Precambrian formations under rare and the next rocks (seen in Figure 3) that represent the Paleozoic sequences of the Cambrian, Ordovician and Silurian. To give you some impression of the length of this time gap, it is approximately equivalent to the same amount of time that has existed from the upper Cambrian to the present! In Figure 5 the unconformity between the Precambrian and the younger, overlying, rocks is represented by a wavy line.

The interval represented by this time break has been seen in many boreholes and varies from a heavily weathered surface where circulating water has greatly altered many of the minerals to a relatively clean break with a minimal amount of weathered Precambrian rock. The variability of this contact probably has depended on water movement as the Cambrian seas re-invaded the area over 500 million years ago.

The Paleozoic sequence

A huge block of time — the Paleozoic, or “the time of ancient life”— followed the Precambrian. It covered from 542 Ma (million years ago) to 251 million years ago. If the reader notices some slight time discrepancies it is because geological time is based on radiometric decay series of minerals. These are often presented as 542+1.0 Ma, so there is slight variance in the dates. Furthermore, they are constantly being refined so additional minor modifications can be expected. Also without knowing precisely where one is in a given rock sequence it is possible that there might be minor time breaks. For example at rare we know, from the fossil content, and from regional mapping, that we are dealing with exposed rocks that belong to the Silurian Period. We know that the Silurian Period commenced 443.7 + 1.5 Ma, and we know that it ended 416.0 + 2.8 Ma. We can narrow this down farther to the Guelph Formation which is believed to be equivalent to the Middle Silurian between about 422.9 + 2.5 Ma to 418.7 + 2.7 Ma. None of these dates come from this region but they are from radiometrically dated rock sequences of the same age elsewhere in the world. The linkage is made from widespread macro- and micro-fossils that allow time-specific correlation to take place.

Rocks of Paleozoic age in the Blenheim borehole near Roseville start at depth of 809 m below ground surface and are present to 72 m below ground level. The remaining sequence to the surface is sediment of Quaternary age (Fig. 5). Because the regional dip is gently to the west, there are at least 30 m more of still younger rocks at Blenheim than are present at rare. These rocks are Upper Silurian Salina Formation beds which have been eroded completely at rare.

Rocks exposed at rare

Bedrock is at variable depth beneath rare, ranging from some 50m at the height of land and is exposed in a substantial area along the Grand River in the bluffs east of the confluence of the Grand and the Speed (Figs. 1, 4, 9). These exposures continue east of the rare property particularly along the southwest bank of the Grand.

The rocks of the Guelph Formation are largely light-grey to light buff coloured fine to medium crystalline dolomite. When broken they exhibit a slight petroliferous odour. Rock descriptions and stratigraphic settings are provided by Armstrong and Goodman (1991), Frizell (1999) and personal observations.

The early settlers exploited the thicker units of the Guelph Formation for building stone and several building in the older parts of Cambridge have utilised this unit. At rare the slit barn along the Cambridge – Blair road is built predominantly of this rock type. Closer examination shows that most of the blocks exhibit numerous specimens of a pelecypod — a clam — called Megalomus (now Megalomoides) canadensis Hall. (Figs. 10, 11, 12).

Unfortunately the Guelph Formation is heavily dolomitised in the vicinity of rare. What were moderately abundant and likely well-preserved fossils have been destroyed by a process called diagenesis. Circulating ground water in the sediments following burial of the fossils carried magnesium ions that replaced the former calcium ions. Since the magnesium takes up more space than the calcium, the fossil shapes were disrupted. Further water movement through time has enlarged the voids further destroying fossil preservation. However, there have been enough fossils found in the area to paint a partial picture of the marine environment when these organisms were alive some 420 million years ago.

Other interesting specimens reveal both the inside and the outside of the snail.

Beside snails and clams there were other molluscs present. Perhaps the most interesting of these are the cephalopods which are distantly related to the modern squid and octopus. These remarkably intelligent animals took a variety of forms and several fragmentary specimens have been found at rare. The illustration below shows a reconstructed marine seafloor with several different cephalopods. The rare form is indicated by the arrow that points to the fossil specimen (centre) and to where its counterpart is in the reconstruction.

Brachiopods, or “lamp shells” as they are commonly known, are a different group of marine organisms that occupy similar habitats to clams. Brachiopods still live in oceans today, but in Paleozoic time they were far more common. Some specimens have been found at rare.

Stromatoporoids are rather enigmatic fossils. At the time that the rocks at rare were being deposited as sediments mounded masses of stromatoporoids formed reef structures. Stromatoporoids consist of laminated layers of calcareous sheets and several fragments have been found at rare.

In Figure 19 the stromatoporoids, that are now believed to be an extinct group of sponges that formed calcareous skeletons, can be seen forming the main reef mound (wall on the left side). The mound would have had other organisms present that would have helped to contribute to the main mass. These would have been compound colonial corals as well as calcareous algal masses. Marine plants are present in the background. The reef face would have also had individual horn corals (solitary forms), as well as other adhering organism such as brachiopods and clams. It is likely that trilobites would have also been present, although these animals have not yet been found at rare.

Colonial corals are known from several specimens at rare. Colonial corals of the Paleozoic are distantly related to modern corals and fall into two major groups. The first are tabulate corals. Tabulate corals were particularly abundant in the Paleozoic and occupy the time range from the Ordovician to the Permian (488 to 251 million years ago). Several specimens have been recovered from rare.

Solitary corals have also been found at rare. These are also members of an extinct group of corals called the Rugosa. Figure 19 shows a number of solitary corals, both living (frilly tops that are the polyp tentacles) and dead (grey specimen in foreground). Figure 22 below shows a pair of solitary corals recovered from an archaeological site at rare. The brownish staining is an oxidation effect on the surface of the specimens.

The ridges that can be seen inside the top of the coral calyx are septa that run down through the organism. A few growth ridges can be seen on the outside (left).

The fossils recovered from rare provide a fleeting glimpse of the Paleozoic Silurian sea and they indicate that rare was, at the time of deposition, located in a shallow, warm, equatorial ocean. We know that latitudinally rare was situated between 20 and 30 degrees south of the equator — approximately in the same latitude as the Great Barrier Reef of today. Hopefully more fossil finds will help to expand this picture of the geological setting of rare. 


Alan V. Morgan; Earth Sciences, University of Waterloo, Waterloo, ON N2L 3G1