The Bleasdell Boulder, aka the "Big Rock"

Glen Miller, lower Trent River valley, southeast Ontario, Canada

Bleasdell boulder [500 kb]

Fig. 1: Now, this is a Big Rock! Its formal name is the Bleasdell Boulder. 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. Thery set up the Bleadale 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 1000 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" ---

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. 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 million years: perhaps deposited from a warm sea nearer 1300 Ma, and metamorphosed to marble nearer 1000 Ma. Moving on a billion years or so, the growth of The Laurentian 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 deposited erratics, as well as depositional features such as eskers and drumlins. To be continued.

Bleasdell boulder [408 kb]

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.


References

Bleasdale 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. Excellent source of insights into both local history and geology.

To be added: more on local history, the Grenville province, and marble from Madoc.

Graham Wilson, draft version posted 22 November 2021

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