By: Mario Coniglio
Introduction
Newscasts of federal politicians within the proud halls of our Parliament Buildings offer much more than just insight on the latest political maneuverings or scandals. In fact, the best way to enjoy these newscasts is to mute the volume on the television set, quickly cast your gaze past the silenced rhetoric and feast your eyes on the walls! Here, in merciful silence, you will be mesmerized by unusual patterns of dendritic, branching dark-coloured streaks beautifully highlighted against a lighter background. This is the remarkable 'Tyndall Stone' which ranks as one of the most beautiful building stones in the world. Canadians from coast to coast regularly see this rock on television, but fail to truly observe it. What a shame! The interior of the Parliament Buildings, as well as the exteriors, interiors, steps, walkways, columns, fireplaces and floors of countless other buildings throughout Canada and the United States are adorned by these distinctively mottled rocks. As you can see from Figures 1 and 2, Tyndall Stone is also sometimes referred to as 'tapestry stone'.
Tyndall Stone is quarried from Garson, Manitoba, which is located 37 km northeast of Winnipeg. The Garson deposit was opened in 1895, although the first record of construction with Tyndall Stone extends even further back to 1832, when it was used to build the fort warehouse and walls of Lower Fort Garry in Manitoba. Tyndall Stone acquired its name from Tyndall, the closet railway point to Garson. A total of five pits were developed in the area by various firms over the years, but only two remain in operation. Gillis Quarries Limited started working the stone in 1910, acquired the property soon after and continues to this day to supply Tyndall Stone. Gillis Quarries Limited remains a family-operated business, and the company is involved in all aspects of the extraction and fabrication of Tyndall Stone. At the beginning, Winnipeg and nearby areas were the principal market for Tyndall Stone, but soon after, the stone saw much more widespread use.
Geology
Tyndall Stone is a widespread dolomitic limestone that formally belongs to the Selkirk Member of the Red River Formation. The formation is exposed in the Manitoba outcrop belt, and similar, correlative rocks are also known from the subsurface of Manitoba and Saskatchewan. Tyndall Stone occurs in the lower half of the Selkirk Member, which is 43 m thick. The Selkirk Member is overlain by evaporitic laminated, finely crystalline dolomite of the Fort Garry Member of the Red River Formation and it overlies calcareous dolomite and dolomitic limestone of the Cat Head Member of the Red River Formation.
Figures 1 and 2: These two photographs of fossiliferous Tyndall Stone demonstrate the characteristic dark mottles offset against the lighter-coloured background. Two large Maclurites snails (M) and the problematical fossil Receptaculites (R), sometimes referred to as the 'sunflower coral', are clearly shown in Figure 1. There is a penny in the upper left of the photograph for scale. Figure 2 shows a longitudinal view through an ellesmeroceratid nautiloid (N) and a high-spired snail, Hormotoma (H). There is a penny in lower left of the photograph for scale. Photographs were provided courtesy of Dr. Brian Pratt, University of Saskatchewan.
Tyndall Stone dramatically illustrates the impressive diversity of organisms that inhabited Ordovician seas approximately 450 million years ago. Intact and fragmented nautiloids, corals, stromatoporoids, bryozoans, crinoids, trilobites, brachiopods, gastropods, bivalves and calcareous algae all abound (Figs. 1, 2). Some fossils, especially nautiloids, corals and stromatoporoids, can reach impressive sizes up to several tens of centimetres, but sizes of a few centimetres or less are more typical. Although geologists and collectors might be most inclined to view the most fossiliferous rocks as the most intriguing part of Tyndall Stone, it is interesting to note that product specifications for cut dimension stone (see below) state that 'fossils and other natural markings are permitted only to the extent that they do not disfigure finished appearance.' The mottled rock enclosing the fossils consists of broken up bits of fossils, lime mud, and fecal pellets from the wide range of skeletal and soft-bodied invertebrates.
The wide range of fossil organisms, combined with paleomagnetic information, indicate that this area of Manitoba was a relatively warm, inland sea that just south of the Ordovician paleoequator. The shallow, warm waters of the Bahamas Banks are a reasonable modern-day analog for the paleoenvironment of the Tyndall Stone, although the ancient seas that covered much of the continent during the Ordovician were significantly larger than the Bahama Banks.
Mottles part one the shrimp factor
The mottles mentioned above are the defining characteristic of Tyndall Stone, which is available in two distinct shades - buff ('a light creamy beige with pastel brown mottling') and grey ('a pale bluish grey with grey-brown mottling'). A third colour, 'golden buff' is sometimes available in limited amounts. In three dimensions, the mottles in Tyndall Stone are seen to be irregularly branching to dendritic cylindrical structures up to 3 cm in diameter, usually sharply offset from the surrounding lighter-coloured rock. The mottles penetrate the rock in all directions but they mostly extend and branch parallel to bedding. Tyndall Stone is usually dressed (finished) parallel to bedding to maximize the radiating and dendritic branching patterns of mottles.
What are these mottles? The Gillis Quarries Limited web site claims that 'No satisfactory explanation can be given for the formation of the mottling which intersects through the mass like a sponge network giving structural reinforcement to the stone.' Although this statement may have been accurate several decades ago, it certainly no longer remains true. Early studies of Tyndall Stone and similar rocks suggested that such mottles were probably algal in origin. Geologists now widely agree, however, that the mottles represent burrows made while the sediment was still soft on the ancient sea floor.
Burrows, along with tracks and trails, are types of traces left in a sediment by a variety of organisms, such as worms or crustaceans, as they forage for food, escape from predators, develop living shelters, travel, or simply rest on the sea floor. When lithified, these markings are recognized as 'trace fossils'. Trace fossils contrast with the better known 'body fossils', such as corals, gastropods and trilobites, to name a few, because the latter are actually the skeletal remains of an organism. Because trace fossils reflect the behavior of an organism rather than an actual biological part, in many cases the identity of the originator of a trace marking is uncertain.
'Ichnologists' (paleontologists who specialize in the study of trace fossils or 'ichnofossils') have refined their craft to an impressive degree over the last couple of decades, however, and they can usually make a reliable guess as to the identity of a particular trace maker based on careful comparisons with traces left by modern organisms.
In modern environments, traces similar to those observed in the Tyndall Stone are made by a number of marine organisms, including anemones and fish, to name a few, but most importantly by decapod crustaceans (e.g., lobsters, crabs), particularly thalassinid shrimp, such as Callianassa (also known as the 'ghost shrimp' - Fig. 3). Visitors to the warm waters of the Caribbean might recall small, conical mounds that dot the shallow sea bottom. These mounds are the surface expression of a complex gallery of branching burrows below the sediment surface that are maintained by the ghost shrimp.
Similar branching cylindrical burrow systems are seen in the rock record (Fig. 4) and are classified as Thalassinoides (formerly referred to as Spongeliomorpha). This trace fossil is generally considered to be a dwelling structure of a creature that had legs. It is common in Mesozoic and younger strata, which coincides with the earliest paleontological record of shrimp in the Jurassic, although a few decapod fossils have been described in rocks as old as Devonian. Thalassinoides burrows, however, are now recognized as a common constituent in Ordovician shallow water limestones as well, suggesting that the early decapod record is quite incomplete, or alternatively, some other unidentified organism, perhaps not even crustacean, was responsible for these burrows.
Mottles part two - the change from sediment into rock
Mottling in Tyndall Stone results from the distinctive colour difference between the sediment representing the burrow system and the sediment that was burrowed. Let us set the problem of the identity of the burrower aside for the moment and examine another perplexing question. How does the simple act of burrowing produce the mottled effect we see in Tyndall Stone? This is far from a trivial question, and to answer it, we need to first more closely examine the nature of the mottles and the surrounding rock. The colour difference between mottle and surrounding rock reflects a fundamental compositional difference. Mottles are mostly composed of finely crystalline dolomite (CaMg(CO3)2), whereas the surrounding rock is mostly limestone composed of the mineral calcite (CaCO3). The mineralogical difference is nicely expressed in naturally-weathered outcrops where the darker, dolomitic mottles are slightly more resistant than the surrounding, calcitic, lighter-coloured rock. Clearly, during the post-depositional alteration ('diagenesis') of the sediment, the burrow-fill sediment and the burrowed sediment behaved quite differently. One became dolomite, the other limestone. Why? Burrowing must have occurred while the sediment was still relatively soft. The excavation made by the burrower would have been subsequently filled with sediment, either backfilled by the burrower, or piped-in after the burrow was abandoned. Thus, there developed a very slight porosity and permeability difference between the burrow fill and surrounding sediment, perhaps as a result of differences in particle size or sorting. In addition, there may have been slight differences in the amount of organic material incorporated in the burrow fill, maybe even from the burrower's fecal pellets, and the sediment hosting the burrow.
Accepting that the burrow-fill and surrounding sediment were compositionally different, even if only to a very minor extent, this difference was evidently adequate to allow the sediment hosting the burrows to become partially lithified (calcite-cemented) while it was still just below shallow sea floor. In contrast, the burrow-fill sediment remained relatively unlithified - in fact, some of the burrows are themselves burrowed by a younger and smaller generation of burrowers, testifying to the hardness contrast between the softer burrow-fill and surrounding, stiffer, partially lithified sediment. This younger generation of burrowers may have been seeking organic matter preferentially concentrated in the older burrows.
Continued burial slowly transformed the burrowed sediment into rock. At a later stage in the burial history of the Tyndall Stone, perhaps when the evaporitic sediments of the overlying Fort Garry Member of the Red River Formation were being deposited, magnesium-rich brines infiltrated preferentially into the burrows because this part of the rock was still relatively permeable compared to the surrounding more lithified and less permeable rock. Dolomite was preferentially precipitated within the burrows, replacing the original calcareous sediment. The darker colour of the mottles could be the result of the oxidation of some of the iron that forms a trace constituent within the dolomite, or it might have resulted from the oxidation of dispersed pyrite (FeS2) that precipitated along with dolomite within the mottles.
Production of Tyndall Stone
The quarried bedrock at Garson is overlain by a relatively thin mantle (up to 4 m) of Quaternary stony till. Once this overburden is removed, a combination of giant circular 8 foot-diameter diamond-tipped saw blades, wedging and drilling yield large blocks of rock that are then carried by front-end loaders to the processing plant. Here the blocks are further cut into the desired proportions using various diamond-tipped saws and finished using an array of grinding, impact and splitting methods to impart the final surface texture. Specially equipped trucks deliver the final product to the building site.
Tyndall Stone is typically marketed under two broad classifications: 'cut stone', also know as 'dimension stone', which is cut and shaped to specific dimensions from architectural drawings, and 'random ashlar' which are pre-cut stone strips cut into various standard sizes or 'uncoursed' pieces of rock of random size. These are laid in mortar in a linear or irregular pattern.
Tyndall Stone is quite legitimately marketed as an aesthetically pleasing, low-maintenance 'natural product' characterized by structural strength, durability, fire resistance, and sound deadening qualities. The rock is also extremely versatile, being equally at home in an urban setting as in a rural one, and in traditional as well as in contemporary residential and commercial architecture. Gillis Quarries has available a series of brochures that illustrate numerous, well-photographed examples of houses, apartments, condominiums, office buildings, universities, museums, hospitals, banks and churches that prominently demonstrate the natural beauty of Tyndall Stone. Part of the versatility of using Tyndall Stone comes from the wide range of finishes which are possible. The 'rubbed' or machined-smooth face is the most typical, and is the surface which most clearly shows the beautiful mottling effect. 'Bushhammered' and 'pointed face' finishes give the surface more texture and three-dimensionality, but the mottling effect is somewhat reduced as a result. Some surfaces are processed to deliberately preserve the circular saw marks, whereas others are modified by sand blasting. Ashlar facings are often produced with 'split-finish', which is achieved by splitting the stone with powerful hydraulic shears. The resulting surface shows the colour mottling but also exhibits irregular shadow effects and texture from the artificially produced fracture surface. 'Rustic finish' is another type of textured surface produced by splitting the rock along natural, bedding-parallel partings (known as 'stylolites' in the scientific literature). Most stone is produced in a standard grade, but a 'select' grade is available for certain applications where fossils or other natural markings are needed.
In addition to product brochures you can read more about Tyndall Stone and its extraction and processing, as well as view examples of spectacular projects using this rock, in the Gillis Quarries Limited web site. Tyndall stone is as scientifically interesting as it is beautiful! The next time you see your favourite politician facing a scrum of eager reporters in the Parliament Buildings, turn down the volume, and enjoy the view!
Acknowledgments
I would like to thank my colleagues, Dr. Nancy Chow at the University of Manitoba, and Dr. Brian Pratt, at the University of Saskatchewan, for their help in reviewing and illustrating this article.
Selected Bibliography
Ekdale, A.A., Bromley, R.G. and Pemberton, S.G., 1984. Ichnology - trace fossils in sedimentology and stratigraphy. SEPM Short Course No. 15, 317p.
Friedman, G.M., Sanders, J.E. and Kopaska-Merkel, D.C., 1992. Principles of sedimentary deposits. Maxwell Macmillan Canada, 717p.
Gillis Quarries Limited. Tyndall Stone - a natural quarried limestone. 10p.
Goudge, M.F., 1933. Canadian limestones for building purposes. Canada Department of Mines. Mines Branch No. 733.
Kendall, A.C., 1977. Origin of dolomite mottling in Ordovician limestones from Saskatchewan and Manitoba. Bulletin of Canadian Petroleum Geology, v. 25, p. 480-504.
Myrow, P.M., 1995. Thalassinoides and the enigma of early Paleozoic open-framework burrow systems. Palaios, v. 10, p. 58-74.
Sheehan, P.M. and Schiefelbein, D.R.J., 1984. The trace fossil Thalassinoides from the Upper Ordovician of the eastern Great Basin: deep burrowing in the early Paleozoic. Journal of Paleontology, v. 58, p. 440-447.
Also,
visit
the
following
web
site
for
more
information
on
thalassinoides:
http://192.206.48.3/collect/geology/Grafton/Acid-bath.html
Mario
Coniglio
is
Associate
Professor
and
Undergraduate
Officer
in
the
Department
of
Earth
Sciences
at
the
University
of
Waterloo.
His
email
address
is:
coniglio@sciborg.uwaterloo.ca