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Okotoks Erratic

Big Rock (also known as either Okotoks Erratic or, by the Blackfoot Indians, as Okotok) is a 16,500-tonne (18,200-ton) boulder that is about the size of a two-story house and lies on the otherwise flat, relatively featureless, surface of the Canadian Prairies in Alberta. It is part of the 930-kilometre-long (580 mi) Foothills Erratics Train of typically angular boulders of distinctive quartzite and pebbly quartzite.

This massive angular boulder, which is broken into two main pieces, measures about 41 by 18 metres (135 by 60 feet) and is 9 m (30 ft) high. It consists of thick-bedded, micaceous, feldspathic quartzite that is light grey, pink, to purplish. Besides having been extensively fractured by frost action, it is unweathered. Big Rock lies about 8 km (5 mi) west of the town of Okotoks, Alberta, Canada, 18 km (11 mi) south of Calgary in the SE. 1/4 of Sec. 21, Township 20, Range 1, West 5th Meridian.

Big Rock is a glacial erratic that is part of a 930 km (580 mi) long, narrow (1.00 to 22.05 km (0.62 to 13.7 mi) wide), linear scatter of thousands of distinctive quartzite and pebbly quartzite glacial erratics between 30 cm (1 ft) and 41 m (135 ft) in length. This linear scatter of distinctive quartzite glacial erratics is known as the Foothills Erratics Train. The Foothills Erratics Train extends along the eastern flanks of the Rocky Mountains of Alberta and northern Montana to the International Border. The boulders and smaller gravel, which comprises the Foothills Erratics Train, consist of Lower Cambrian shallow marine quartzite and conglomeratic quarzite, which occurs only within the Gog Group and is found in the Athabasca River Valley of central western Alberta. Big Rock is the largest erratic within the Foothills Erratics Train. Lying on prairie to the east of the Rocky Mountains and like all the larger erratics, it is visible for a considerable distance across the prairie and likely served as a prominent landmark for Indigenous people.

Although sometimes claimed to be the largest glacial erratic in the world, Big Rock is not. For example, one large glacial erratic in Germany measures 3 by 6 km (2 by 4 mi) and is 9 m (30 ft) thick. Near Cooking Lake, Alberta, one of several large glacial erratics, which is called the Cooking Lake (Number 6) megablock, covers an area of at least 10 km2 (4 sq mi), has a length of 4.0 km (2.5 mi) and is about 10 m (33 ft) thick. Pollen studies indicate that the Lower Cretaceous sedimentary strata that comprise this glacial erratic were transported a minimum distance of about 260 km (160 mi).

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