Figures 1-2. Two views of a large, lightly polished slab of Tyndall Stone. It takes a fine, smooth surface, with no need of a high polish, such as you may find in a harder rock such as granite. Note the modest porosity (pores or vugs), and the pale, wispy traces in the medium-grey matrix, which is a very fine-grained limestone known as micrite.

"Rock of the Month #211, posted for January 2019" ---

Trace fossils in Ordovician limestone:

the making of Tyndall Stone is an interesting tale in itself. This upper Ordovician rock of the Red River Formation, also known as "tapestry stone", is quarried at Garson, some 37 km northeast of Winnipeg. The elongate outlines in the stone are trace fossils, the generic term for footprints and other indirect evidence of past life. These are dolomitic infillings to the burrows of crustaceans (shrimps and their ilk), in limestone of the Selkirk member. Actual fossils may be found in this rock: nautiloids, brachiopods and others. The Ordovician Arctic fauna of northern North America is found in outcrops in Wyoming (Big Horn Mountains), Saskatchewan and Manitoba (Stinchcombe, 2012, pp.79-84). It is noted for some large molluscan fossils, such as cephalopods. The Tyndall Stone quarry displays fossils such as Kochoceras, Beloitoceras, Lamboceras, Armenoceras and Narthecoceras, as well as other fossil forms such as Receptaculites.

Tyndall stone is a distinctively mottled rock used in the Canadian federal parliament buildings and many other buildings across North America: Vancouver, Saskatoon, Ottawa and elsewhere (see, e.g., Fensome et al., 2014). Used as early as 1832, the quarrying began in earnest in 1895. Named for the nearest railway halt, the site has seen five pits opened, of which two remain in operation (Coniglio, 1999). This rock is a dolomitic limestone of the Selkirk member of the upper Ordovician Red River Formation. It contains abundant fossils of the Ordovician marine carbonate sequence. The rock is marketed in buff, grey and (occasionally) golden buff colour variants. The distinctive mottles are branching to dendritic cylindrical structures, most often extended parallel to the bedding. These are the trace fossils, shrimp burrows in this case, Thalassinoides (formerly Spongeliomorpha) is the name for the burrows, the "ichnogenus". The burrow filling is mostly dolomite, whereas the host limestone is calcitic. The host rock became calcified before the burrow fill, which remained soft longer and was eventually preferentially dolomitized. The result, Tyndall stone, is considered "Canada's most beautiful building stone, and one of the finest building stones in the world" (Kent et al., 1972, pp.10-12). Similar rocks occur in north Greenland, evidence of quiet sedimentation over thousands of kilometres (Jin et al., 2013; McCausland, 2018).

The Thalassinoides story covers much of Phanerozoic time, e.g.:

• Oligocene-Miocene - Manipur and Mizoram, N.E. India - Singh et al. (2010); Rajkonwar et al. (2013).
• Eocene-Miocene - Jammu, N.India - Sudan et al. (2002)
• Eocene - Spain - Crimes (1977).
• Cretaceous - Madhya Pradesh - central India - Saha et al. (2010).
• Jurassic - Gujarat, W. India - Pandey and Fursich (1998); Patel et al. (2014).
• Carboniferous - N.W.England - Horbury and Qing (2004).
• Devonian-Carboniferous - Utah and Nevada - Gutschick and Rodriquez (1977).
• Ordovician - Manitoba - Coniglio (1999).
• Ordovician - Newfoundland - Lane (1990).

References (n=16)

Coniglio,M (1999) Manitoba's Tyndall stone. Wat on Earth 12 no.2, 15-18, June.

Crimes,TP (1977) Trace fossils of an Eocene deep-sea fan, northern Spain. In `Trace Fossils 2' (Crimes,TP and Harper,JC editors), Seel House Press, Liverpool, 351pp., 71-90.

Fensome,R, Williams,G, Achab,A, Clague,J, Corrigan,D, Monger,J and Nowlan,G (editors) (2014) Four Billion Years and Counting: Canada's Geological Heritage. Nimbus Publishing / Canadian Federation of Earth Sciences, 402pp.

Gutschick,RC and Rodriquez,J (1977) Late Devonian-early Mississippian trace fossils and environments along the Cordilleran Miogeocline, western United States. In `Trace Fossils 2' (Crimes,TP and Harper,JC editors), Seel House Press, Liverpool, 351pp., 195-208.

Horbury,AD and Qing,H (2004) `Pseudobreccias' revealed as calcrete mottling and bioturbation in the late Dinantian of the southern Lake District, UK. Sedimentology 51, 19-38.

Jin,J, Harper,DAT, Cocks,LRM, McCausland,PJA, Rasmussen,CMO and Sheehan,PM (2013) Precisely locating the Ordovician equator in Laurentia. Geology 41 no.2, 107-110.

Kent,DM, Bannatyne,B and McCabe,HR (1972) Industrial & non-metallic minerals of Manitoba and Saskatchewan (central plains). IGC 24, Montreal, Excursion Guidebook C23, 39pp.

Lane,TE (1990) Dolomitization, Brecciation and Zinc Mineralization and their Paragenetic, Stratigraphic and Structural Relationships in the Upper St. George Group (Ordovician) at Daniel's Harbour, Western Newfoundland. PhD Thesis, xxiv+565pp., Memorial University.

McCausland,PJA (2018) Laurentia - ancestral North America's plate tectonic journey through a billion years. Presentation 31 to Kawartha Geoscience Network (Kawartha and Region Earth Sciences, Engineering and Metallurgy Network, KREEM), Peterborough, ON, 02 October.

Pandey,DK and Fursich,FT (1998) Distribution and succession of Jurassic rocks in Gora Dongar, Pachchham "Island", Kachchh, India. J.Geol.Soc.India 51, 331-344.

Patel,SJ, Joseph,JK and Bhatt,NY (2014) Ichnology of the Goradongar Formation, Goradongar Hill Range, Patcham Island, Kachchh, western India. J.Geol.Soc.India 84, 129-154.

Rajkonwar,C, Tiwari,RP and Patel,SJ (2013) Arenicolites helixus isp. nov. and associated ichnofossils from the Bhuban Formation, Surma Group (lower-middle Miocene) of Aizawl, Mizoram, India. Himalayan Geology 34 no.1, 18-37.

Saha,O, Shukla,UK and Rani,R (2010) Trace fossils from the late Cretaceous Lameta Formation, Jabalpur area, Madhya Pradesh: paleoenvironmental implications. J.Geol.Soc.India 76, 607-620.

Singh,MC, Kundal,P and Kushwaha,RAS (2010) Ichnology of Bhuban and Boka Bil formations, Oligocene-Miocene deposits of Manipur western hill, northeast India. J.Geol.Soc.India 76, 573-586.

Stinchcombe,BL (2012) More Paleozoic Fossils. Schiffer Publishing Ltd, Atglen, PA, 160pp.

Sudan,CS, Singh,BP and Sharma,UK (2002) Ichnofacies of the Murree Group in Jammu area and their ecological implications during late Palaeogene in the NW Himalaya. J.Geol.Soc.India 60, 547-557.