"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.

Local History

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".

Geological Note

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.