Ellandvahe erratic in Jõelähtme, Estonia. Source: Zosma, http://upload.wikimedia.org/wikipedia/commons/1/1a/Ellandvahe_r%C3%A4ndrahn.jpg. Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported licence.
If you live in a temperate or subarctic climate and you come across a giant boulder looking incredibly out of place, don’t be perplexed. It most likely came to be there naturally, having been deposited by a continental ice sheet.
Glaciers move so slowly that we think of them as static at times. Yet, glaciers are so massively powerful that as they slowly advance or retreat, they scrape the landscape beneath and alongside it barren, leaving huge striations and scars on the bedrock. Glaciers also pick up a fair amount of debris as they travel. This debris gets incorporated into the ice and is slowly carried away, ending up removed some distance from its original location when the ice finally melts; sometimes dozens or even hundreds of kilometres away depending on the size of the ice sheet. Typically, the debris is in the form of dirt, gravel or small rock, which usually wind up deposited as part of an esker, moraine, or drumlin. Sometimes, though, a large boulder is carried away by the glacial ice. When the ice retreats, the boulder is left stranded in a very alien landscape. This is what is known as a glacial erratic.
Angular erratic, Lembert Dome, California, USA. Source: D. Meyer, http://upload.wikimedia.org/wikipedia/commons/d/dd/Angular_glacial_erratic_on_Lambert_Dome-750px.jpg. Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported licence.
Technically, erratics can include any piece of self-standing rock larger than a pebble to those the size of houses. It is the large rocks, of course, which are the most interesting and prominent. As erratics are easily identifiable, typically with a composition completely different from that of the surrounding ground, they can be used to determine the direction of the glacial flow. These huge rocks end up on (or even in) the ice for many reasons: they fell onto the ice from above; the glacier intercepted a talus slope or landslide debris; the glacier scoured the rock from the bedrock below or plucked it off of the side of a mountain as it passed by. Those rocks which are trapped directly inside the ice may be deposited as the ice melts into a proglacial lake; where the rock is deposited can serve as an indication as to what the altitude of the lake was. Ice-borne erratics can also get carried far away from glaciers when ice dams break, as what happened during the cataclysmic Missoula Floods between 15 000 and 13 000 years ago. It is important not to confuse standard erratics with megablocks or rafts, which are incredibly large chunks of bedrock taking up multiple square kilometres that are carried away in glaciers (one incredibly enormous block near Esterhazy in southeastern Saskatchewan was discovered to measure 1 000 km2).
Yeager Rock in Douglas County, Washington sits on top of glacial till.
Erratics are most plentiful in northern North America and northern Europe, the two locations of the largest continental ice sheets during the previous Ice Age. The largest erratic (that we know of) is found just to the west of Okotoks, Alberta, Canada. Known officially as the Okotoks Erratic, but universally referred as simply ‘Big Rock’, it is part of a band of erratics stretching from Jasper National Park to northern Montana along the eastern front of the Rocky Mountains; the result a of mountain glacier carrying landslide debris from the Athabasca River valley colliding with the Laurentide ice sheet, which carried the erratic hundreds of kilometres south before depositing it. On Google Street View, it is plainly visible next to the highway. Below is a home video found on YouTube that gives you a good feel for the size of Big Rock, along with some facts at the beginning of the film (and copious amounts of kids illegally enjoying themselves in the nooks and crannies of the giant boulder).
Aber, J.S. and A. Ber (2007). Chapter 7: Megablocks and Rafts. In Glaciotectonism, 101-110. Amsterdam: Elsevier.
Alden, A. (2004). Glacial Erratic. About.com: Geology. Available at http://geology.about.com/library/bl/images/blerratic.htm. Accessed 23 October 2010.
Collins, G. (2005). The Very Cold Case of the Glacier. New York Times, 14 September 2010. Available athttp://www.nytimes.com/2005/09/14/nyregion/14glacier.html. Accessed 23 October 2010.
Davis, N.K. et al. (2005). Glacial Lake Musselshell: Late Wisconsin slackwater on the Laurentide ice margin in central Montana, USA. Geomorphology 75 (3-4): 330-345.
Government of Alberta (2010). Okotoks Erratic. Government of Alberta: Culture and Community Spirit. Available athttp://www.culture.alberta.ca/museums/historicsiteslisting/okotokserratic/default.aspx. Accessed 23 October 2010.
Hall, A. (2010). Glacial erratics. Cairngorm Landscapes, 10 October 2010. Available athttp://www.landforms.eu/cairngorms/about%20this%20site.htm. Accessed 23 October 2010.
Yanko, D. (2010). Erratics Rock. Virtual Saskatachewan. Available at http://www.virtualsk.com/current_issue/erratics.html. Accessed 23 October 2010.