Dedicated to the memory of Janice Mary Wybourn (1944-2022), artist and fellow traveller,
on whose watch the first 250 "rocks of the month" evolved,
shown here with Laddie the Dog (circa 2000-2009).
Fig. 1: Now, this is a Big Rock! Its formal name today is the Bleasdell Boulder, though it has also been known widely as the Glen Miller Rock. A glacial erratic, composed largely of marble, the massive rock is conspicuously out of place in the limestone plain of the Lower Trent. In 1997, the land on which the Boulder sits was purchased by local conservationists Paul and Maria Heissler. They set up the Bleasdell Boulder Preservation Corporation to ensure conservation of the property and its boulder, clearly a site of special scientific interest. This photograph is a view of the Boulder in October 2005. By that time, a wealth of information was available on signs around the rock, since regional and provincial authorities had shown a strong interest in this local attraction. The Lower Trent Regional Conservation Authority now administers the big rock, the short trail leading to it, and the land upon which it sits. The Ontario Geological Survey was involved in describing it and deducing its probable source. The official signage, and other sources, are used to briefly explain the boulder in this short article.
Bleasdell (back in 1862) estimated the weight of the erratic to be close to 1,000 tonnes, with dimensions about 13.5 metres long, 7.3 metres across, and average height 6.3 metres (44 x 24 x 20 feet).
"Rock of the Month # 249, posted for March 2022" ---
This month's Rock is a large one, located in southeastern Ontario, Canada, near the right (west) bank of the River Trent just north of the north shore of Lake Ontario, between Toronto and Ottawa.
The Bleasdell Boulder is a spectacular glacial erratic, thought to have been brought tens of km south from the Madoc region, which is noted for its modern (20th century) marble quarries. The erratic made its way to the Trenton region much earlier, borne by the Laurentian ice sheet. Composed largely of granular calcitic marble, a close inspection reveals zones of deformation, where the marble has been brecciated (broken up) and recemented. Some zones contain silicate minerals formed in metamorphism, in addition to the host calcite and trace quartz. These include two related amphiboles, green actinolite and pale, whitish tremolite.
The boulder is located just west of the lower reaches of the Trent River, near Lock 1 on the navigation, upstream from Trenton and the Bay of Quinte. It lies a short walk from the hamlet of Glen Miller, south of Batawa.
The Reverend William Bleasdell was born in Preston, Lancashire in 1817. He served as headmaster of a grammar school for several years, before electing to study theology at Trinity College, Dublin. From Ireland he moved to Canada in 1848, and was rector of St. George's Anglican church in Trenton until his death in 1889. Rev. Bleasdell was evidently the classic Victorian clergyman- naturalist, his interests including geology, botany and astronomy. He drew the attention of botanist Dr. George Lawson of Queen's College, who eventually gave lectures referring to the "Bleasdell boulder".
The marble is of Precambrian age, formed by metamorphism of limestone and preserved in the Grenville province, the southern margin of the Canadian Shield. Its age is probably in the range of 1300-1000 Ma (million years): perhaps deposited from a warm sea, as a limy mud, near 1300 Ma, and metamorphosed and recrystallized to a granular marble nearer 1000 Ma. Moving on a billion years or so, the growth of the Laurentide ice sheet, peaking some 20,000 years ago, generated vast erosive power, and a vast southward conveyor belt, and moved the erratic to its present area. Around 12,000 years ago, retreat of the ice sheet scattered erratics, as well as larger depositional features such as eskers and drumlins. Description of regional Quaternary geology can be found in sundry good publications, e.g., Barnett (1989).
Some Additional Context
The boulder has long been a local feature of interest (e.g., Theberge and Theberge, 1978). The Big Rock Café at Glen Miller was long a starting point, on the west side of highway 33, for the short walk to the boulder. The Lower Trent Conservation Authority produced a nice brochure for the site (my copy is dated 2012), after the 2010 construction of a 300-metre trail linking the rock (and its existing 1,000-metre loop to Glen Miller) to the Lower Trent Trail.
Local marbles are variously either calcite- or dolomite-dominant. Relevant studies include Bourque (1982), Meyn (1988) and the detailed review of Grant et al. (1989). Besides the importance of attractive marbles (of diverse colours, textures and grain size) for building stones, these rocks may serve as hosts for mineral deposits, such as talc in dolomitic marble (Hewitt, 1957) and wollastonite in zones of contact metamorphism around plutons (Mackinnon and Kingston, 1987). Easton (1989) describes styles of mineralization around the Deloro granite, including fluorite veins, and gold in quartz veins with arsenopyrite. Hewitt (1968, 1969) provides good reviews of the local bedrock geology and mineral deposits.
Other erratics are known in the region: a nice example of metagabbro boulder can be found opposite the Tim Horton’s café in Campbellford (see ROM 13, Gabbro Boulder). Striking boulders may be used as trail markers, or even as memorials for glacial geologists such as A.P. Coleman (1852-1932), buried in Toronto (Hannibal, 1999).
Erratics of the World
It is logical to expect erratics to be created by ice flow in times of glaciation, by mountain glaciers as well as continental ice sheets, and in earlier Ice Ages, extending back into Precambrian time. Globally, erratics may be traceable for hundreds of km from source, an example being the striking siltstone blocks known as omars, across northern Ontario. Large erratics have been reported widely around the northern hemisphere. Often, the largest are of local origins, such as the anorthosite blocks scattered around the feet of the Adirondack mountains in upstate New York. Other examples can be cited around the rugged north end of the isle of Arran, on the west coast of Scotland. Tyrell (1928, pp.261-262) described some examples, amidst the generally southward dispersion of granite blocks across the lower southern part of the island. He noted that “the heaviest is probably the Clach Mhor, on the highest raised beach south of the Corrie Burn and about three-quarters of a mile south of the hotel. It is 30 feet square at the base, about 15 feet high, and must weigh about 400 tons” ** (for comparison with the Bleasdell boulder, that’s about 393 tonnes, 9.1x9.1x4.6 metres). The Beddington boulder, a substantial Okotoks block (see below), is some 24x23x15 feet (7.3x7.0x4.6 metres).
Along with such valuable field indicators as glacial striations and the orientation of landforms such as drumlins, roches moutonnées and eskers, the down-ice dispersion of “trains” of glacial erratics helps constrain the direction of movement of now-vanished ice sheets and glaciers. Reversing the observations, going up-ice, can lead back to the source of the erratic, whether an outcrop of a distinctive rock type or, perhaps, a mineral deposit. Such studies have been conducted for, e.g., a line of quartzite blocks in Alberta, including the famed Okotoks boulders (Stalker, 1956), granites in the Canadian Arctic (Atkinson, 2007), and highly-shocked rocks (such as breccias) in Europe (Forster et al., 2010), as well as examples in the Himalaya (Prashra, 1997). Possibly the biggest transported block anywhere, albeit somewhat battered in appearance today, is the Okotoks "Big Rock", an unparalleled 41x18x9 metres in size, estimated weight 16,500 tonnes (see also Mussieux and Nelson, 1998, pp.176-177)! Not surprisingly, the Okotoks Big Rock is one of a number of geological features on the Plains that has been venerated by the Blackfoot native people.
Mineralized boulders, like much smaller grains of “indicator minerals” may also be valuable leads to potential orebodies, whether they have travelled many km, or just a little way from source (see, e.g., Plouffe et al., 2011). Clearly, any rock that has even a moderate strength can travel 1, 10, perhaps even 100 km. In southeast Ontario, as we have seen, the list includes marble and gabbro, for starters.
Fig. 2: A brecciated zone on the "rear" side of the erratic (far side of the view seen in Figure 1), as noted in July 2014.
** To put this in its local context, the site referred to here is: near sea level; on the east coast of the island of Arran, north of Brodick; south of the hamlet of Corrie (with the hotel); and some 3.5 km east of the granitic summit of Goat Fell, highest point of the rugged north half of the island, elevation 874 metres (2,866 feet). The provenance of erratics, that is, the tracing of boulders back to their bedrock source, was recognized as a valuable means of reconstructing the directions of ice advance in regions such as the British Isles. A recent talk by teacher and mountaineer Noel Williams of Lochaber Geopark (Williams, 2022) cast a valuable spotlight on the work of the Boulder Committee of the Royal Society of Edinburgh, which collected data in 1871-1884 and published 10 reports on the distribution of large boulders across Scotland. The Committee solicited reports on boulders, first just the largest, of estimated weight >50 tons, then again, >20 tons.
This pioneering work was taken up and continued, south of the Scottish borders. A sequence of letters from regional correspondents was collated by another Committee, under the auspices of the British Association for the Advancement of Science, and published as a series of progress reports. As an example, Hull et al. (1895) reported finds across England, Wales and Ireland of rocks from northern England, Scotland and as far away as Scandinavia. Distinctive lithologies with identifiable sources included Lake District andesites, Eskdale granite, Buttermere granophyre, Silurian grit, and intrusive rocks from Scotland, such as Galloway granites and Criffel granites, as well as Jurassic sediments with fossils such as ammonites and bivalves. Not all these transported blocks were large, and many were only on the order of one foot (30 cm) in diameter.
References to the Bleasdell Boulder (the "Big Rock") and other erratics
Atkinson,N (2007) A statistical technique for determining the source area of glacially transported granite erratics in the Queen Elizabeth Islands, Nunavut. CJES 44, 43-59.
Barnett,PJ (1989) Quaternary Geology of the Bancroft Area, Southern Ontario. OGS Report 262, 93pp. plus OGS map 2500, 1:50,000 scale.
Bleasdell Boulder Preservation Corporation (undated) The excellent, undated signs at the site, beautifully written and designed, installed by 2005, with funding from the Ontario Trillium Foundation. These are a great source of insights into both local history and geology. The only likely error may be the age of the rock: quoted at some 2.3 billion years, the actual maximum age is more like 1.3 billion years.
Bourque,MS (1982) Stratigraphy and sedimentation of carbonate metasediments within the Grenville Supergroup in the Havelock- Madoc- Bancroft area. OGS Misc.Pap. 106, 89-91.
Easton,RM (1989) Regional alteration patterns and mineralization associated with the Deloro granite, Grenville province, Madoc area. OGS Misc.Pap. 146, 275pp., 158-168.
Forster,L, Schmieder,M, Bartoschewitz,R and Buchner,E (2010) Fennoscandian impactite erratics in northern Germany tracers of Pleistocene long-distance glacial reworking. Meteoritics & Planetary Science 45, A56.
Grant,WT, Papertzian,VC and Kingston,PW (1989) Geochemistry of Grenville marble in southeastern Ontario. OGS MDC 28, 266pp. plus 1:250,000 scale map.
Hannibal,J (1999) Glacial erratics mark the graves of glacial geologists. Wat on Earth 12 no.2, 11-12.
Hewitt,DF (1957) Talc deposit of Canada Talc Industries Limited, Madoc, Ontario. In `The Geology of Canadian Industrial Mineral Deposits' (Goudge,MF, Haw,VA and Hewitt,DF editors), CIMM, 247pp., 240-243.
Hewitt,DF (1968) Geology of Madoc Township and the North Part of Huntingdon Township. ODM Geol.Rep. 73, 45pp.
Hewitt,DF (1969) Geology and Scenery, Peterborough, Bancroft and Madoc Area. ODM Geological Guide Book 3, 114pp.
Hull,E, Prestwich,J, Boyd Dawkins,W, Hughes,TMcK, Bonney,TG, De Rance,CE, Kendall,PF, Tiddeman,RH, Woodall,JW and Miall,LC (1895) Erratic blocks of England, Wales, and Ireland - twenty-third report of the Committee. Report of the British Association for the Advancement of Science, 430-436.
MacKinnon,A and Kingston,PW (1987) Wollastonite occurrences associated with the Deloro pluton, Marmora and Madoc townships, southeastern Ontario. OGS Misc.Pap. 137, 429pp., 307-315.
Meyn,HD (1988) Stratigraphy and sedimentation of marbles and associated metasedimentary rocks of the Grenville supergroup in southeastern Ontario. OGS Misc.Pap. 141, 507pp., 339-341.
Mussieux,R and Nelson,M (1998) A Traveller's Guide to Geological Wonders in Alberta. Federation of Alberta Naturalists and Can.Soc.Petrol.Geol., Calgary, 254pp.
Plouffe,A, Anderson,RG, Gruenwald,W, Davis,WJ, Bednarski,JM and Paulen,RC (2011) Integrating iceflow history, geochronology, geology, and geophysics to trace mineralized glacial erratics to their bedrock source: an example from southcentral British Columbia. CJES 48, 1113-1130.
Prashra,KC (1997) Glacial erratics in the Higher Himalayan region of Lahaul, Himachal Pradesh: their geomorphological significance. J.Geol.Soc.India 49, 47-54.
Stalker,AM (1956) The Erratics Train, Foothills of Alberta. GSC Bull. 37, 28pp., 3 plates plus map.
Theberge,CB and Theberge,E (1978) The Trent-Severn waterway, a Traveller's Companion. Samuel-Stevens, Publishers, Toronto, 171pp.
Tyrell,GW (1928) The Geology of Arran. British Geological Survey Memoir, HMSO, Edinburgh, viii+296pp., reprinted 1987.
Williams,N (2022) Big boulders of Scotland: the work of the Boulder Committee, 1871-1884. Geological Society of Glasgow, hybrid presentation, live and via Zoom, 10 March.
Graham Wilson, draft version posted 22 November 2021, extended and polished,
24-28 December 2021, wee updates on 01,07,24,28,29 March, 03 April, 01 May 2022.
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or, visit the Turnstone "Rock of the Month" Chronological Archives!