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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 580 miles (930 km) long 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 by and is 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 west of the town of Okotoks, Alberta, Canada south of Calgary) in the SE. 1/4 of Sec. 21, Township 20, Range 1, West 5th Meridian.〔Stalker, A MacS (1975). “The big rock.” In Structural geology of the foothills between Savanna Creek and Panther River, S.W. Alberta, Canada. May 23, 1975. H. J. Evers and J. E. Thorpe, eds., pp. 9-11. Calgary, Alberta, Canada: Canadian Society of Petroleum Geologists.〕〔Cruden, DM, W Langenberg, and RC Paulen (2003). ''Geology of the Frank Slide and southwestern Alberta. Edmonton Geological Society – Geological Association of Canada annual field trip celebrating the 100th anniversary of the Frank Slide Disaster.'' Edmonton, Alberta: Edmonton Geological Society. 34 pp.〕 Big Rock is a glacial erratic that is part of a long, narrow ( wide), linear scatter of thousands of distinctive quartzite and pebbly quartzite glacial erratics between and 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.〔〔〔Stalker, A MacS (1956). ”The erratics train, Foothills of Alberta.” Geological Survey of Canada, Bulletin no. 37, 28 p.〕 Although sometimes claimed to be the largest glacial erratic in the world,〔Anonymous (nd). (“Okotoks Erratic - "The Big Rock.” ), Alberta History, Alberta Government, Calgary Alberta. Last accessed July 20, 2015.〕 Big Rock is not. For example, one large glacial erratic in Germany measures by in dimensions and is thick.〔Shroder, J. F. (2011). “Landforms of Glacial Transportation.” In Encyclopedia of Snow, Ice and Glaciers, Encyclopedia of Earth Sciences Series. V. P. Singh, P. Singh, and U. K. Haritashya, eds., pp. 693-694. Springer, AA Dordrecht, The Netherlands, 1254 pages ISBN 978-90-481-2641-5〕 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 , has a length of and is about thick. Pollen studies indicate that the Lower Cretaceous sedimentary strata that comprise this glacial erratic were transported a minimum distance of about .〔Stalker, A MacS (1976). “Megablocks, or the enormous erratics of the Albertan prairies.” Paper no. 76-1C, pp. 185-188. Ottawa, Ontario: Geological Survey of Canada.〕 ==Geologic history== Near the end of the Pleistocene Period, between 12,000 and 17,000 years ago, a massive landslide occurred within the upper reaches of the Athabasca River valley. As a result of this landslide, millions of tonnes of beige to pinkish quartzite and quartzitic conglomerate slid from the side of a mountain and onto the top of a valley glacier within the Athabasca River valley. On its top, the narrow valley glacier carried eastward this mass of Gog Group quartzite and quartzitic conglomerate. Because it lay on and within the top of this glacier, the highly fractured boulders were neither broken up into smaller blocks nor rounded by movement of the glaciers that transported it. After leaving the Rocky Mountains, the valley glacier collided with the westward moving ice streams of the Laurentide Ice Sheet and both it, other Rocky Mountain valley glaciers, and Laurentide ice streams coalesced as ice streams and were diverted southward and parallel to the eastern flanks of the Rocky Mountains. Together they flowed as far south as northern Montana as an ice sheet before they stagnated and melted. When the ice sheet melted, erratics of Gog quartzite and quartzitic conglomerate were dropped to form the line of rocks known as the Foothills Erratics Train. Big Rock is one of these glacial erratics of Gog quartzite and quartzitic conglomerate that originated as part of a landslide in the Athabasca River valley and carried on the top of a glacier, later ice stream, to its present site.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Big Rock (glacial erratic)」の詳細全文を読む スポンサード リンク
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