160 Unusual Things to See in Ontario ~ Glen Miller Erratic

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.

Reverend William 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). 

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. 

The boulder has long been a local feature of interest and 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 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.

Other erratics are known in the region.  A a nice example of metagabbro boulder can be found opposite the Tim Horton’s café in Campbellford. Striking boulders may be used as trail markers, or even as memorials for glacial geologists such as A.P. Coleman (1852-1932).