page_title: Ottawa-Bonnechere Graben
section_title: Geology
hierarchical_section_title: Ottawa-Bonnechere Graben / Geology
caption_attribution_description: English: Astronaut photo of the Monteregian Hills, Quebec, Canada. Credit: International Space Station Crew Earth Observations experiment and the Image Science &amp; Analysis Laboratory, Johnson Space Center The description from that page follows below. This astronaut photograph of the area to the east of Montreal, Canada, captures two striking patterns. The circular features are the central members of a group of unusual rock formations known as the Monteregian Hills: Mont St. Hilaire, Rougemont, and Mont Yamaska. The rectangular pattern blanketing the landscape in the background reveals the intensive agriculture in the fertile lowlands in southern Quebec. The Monteregian Hills are an isolated series of roughly east-west trending plutons, masses of igneous (volcanic) rock that crystallized below the land surface. The magmas that form the Hills intruded into pre-existing sedimentary rocks roughly 118 to 140 million years ago, during the Cretaceous Era. Over time, the sedimentary rock has eroded away, leaving the more resistant igneous rock of the plutons exposed as the Monteregian Hills, standing over the surrounding, flat St. Lawrence Plains. Snow cover on the three mountains provides contrast with surrounding fallow agricultural fields, which are shades of brown and tan. A bright white circle nestled on top of Mont St. Hillaire is snow-covered Lac Hertel (Hertel Lake). In addition to their interesting geology, the Monteregian Hills also serve as important woodland habitat “islands” in the greater Montreal urban area. Several small town and cities are visible in the image, appearing as purplish-gray patches. The largest in the scene is the city of Beloeil, straddling the Richelieu River west of Mont St. Hilaire. The featured astronaut photograph, ISS014-E-19807, was acquired April 18, 2007, with a Kodak 760C digital camera using an 180 mm lens, and is provided by the ISS Crew Earth Observations experiment and the Image Science &amp; Analysis Laboratory, Johnson Space Center. The image in this article has been cropped and enhanced to improve contrast. The International Space Station Program supports the laboratory to help astronauts take pictures of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the Internet. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Français&#160;: Collines Montérégiennes vues de l'espace. Photo par un astraunaute des Collines Montérégiennes, Québec, Canada. Crédit: expérience de International Space Station Crew Earth Observations, adaptation et traduction de la description faite par Image Science &amp; Analysis Laboratory, Johnson Space Center. Cette photographie astraunautique de la région sise à l'est de Montréal capte deux modèles géomorphologiques contrastants. Les trois formes circulaires sont les composantes centrales d'un groupe de formations rocheuses nommées les Collines Montérégiennes: en l'occurence le Mont Saint-Hilaire, le Mont Rougemont, et le Mont Yamaska. Le fonds de paysage couvert de figures rectangulaires témoigne de l'importante activité d'agriculture dans les basses terres fertiles du sud du Québec. Les Collines Montérégiennes sont une série de plutons grossièrement étalés d'est en ouest, massifs formés de roches ignées intrusives qui se sont cristallisées sous la surface de la croûte terrestre. Ces magmas ont fait intrusion dans les couches de roches sédimentaires pré-existantes il y a 118 à 140 millions d'années, durant la période du Crétacé. Au fil du temps l'érosion de la roche sédimentaire a exposé la roche ignée plus résistante des plutons révélant ainsi ces Montérégiennes dominant l'étendue plate de la plaine du Saint-Laurent. La couverture de neige des trois montagnes contraste avec les champs en jachère environnants dans leurs tons de brun et de beige. Le cercle blanc clair niché au sommet du mont Saint-Hilaire est le lac Hertel couvert de neige. En plus de leur intérêt sur le plan géologique, les Montérégiennes sont aussi d'importants ilôts boisés au sein de la grande région urbaine de Montréal. Plusieurs petits villages et villes se distinguent sur cette image par leur apparence de taches gris-pourpre, dont la plus grande est la ville de Belœil, en bordure de la rivière Richelieu située elle même à l'ouest du Mont Saint-Hilaire. La photographie d'astronaute, ISS014-E-19807, fut prise le 18 avril 2007, avec une caméra Kodak 760C digitale utilisant une lentille 180 mm, et est fournie par le ISS Crew Earth Observations experiment et le Image Science &amp; Analysis Laboratory, Johnson Space Center. Pour cet article l'image a été rehaussée pour améliorer le contraste. Le International Space Station Program supporte le laboratoire en vue d'aider les astronautes à prendre des photos de la Terre qui soient de grande valeur pour les scientifiques et le public, et rendre ces images disponibles gratuitement sur l'Internet. D'autres images prises par les astronautes et cosmonautes peuvent être visualisées sur le site de NASA/JSC Gateway to Astronaut Photography of Earth.
context_page_description: The Ottawa-Bonnechere Graben is a geological structure that coincides with a 55 km wide topographic depression extending from near Montréal through Ottawa. It is part of the St. Lawrence rift system that also includes the seismically active Saguenay graben. This rift valley was formed when the Earth's crust moved downward about a kilometre between two major fault zones known as the Mattawa and Petawawa faults. The length of the graben is about 700 km.
  The Ottawa-Bonnechere Graben runs from the Montreal area on the east to near Sudbury and Lake Nipissing on the west. On the east, it joins the Saint Lawrence rift system, a half-graben which extends more than 1000 km along the Saint Lawrence River valley and links the Ottawa and Saguenay Graben.
  The 200 km segment of the Ottawa-Bonnechere Graben west of Ottawa was the first to be recognized as a graben. Since then, it has been traced west to Lake Nipissing, and northwestwards from the confluence of the Mattawa and Ottawa Rivers up the valley of the latter stream to Lake Timiskaming and the Montreal River valley. This latter branch is the Timiskaming Graben.
context_section_description: Since the Late Proterozoic to Early Paleozoic, erosion has removed the volcanic peaks, exposing a number of relic volcanic pipes, such as Callander Bay and the Manitou Islands in Lake Nipissing.
  These features are subterranean geological structures formed by the violent, supersonic eruption of deep-origin volcanoes. Batholiths and dikes were also exposed by erosion, such as the Timber Lake, Mulock, West Arm, Powassan and Bonfield batholiths. The expressions of a thick pile of dominantly mafic, bimodal volcanics and the Tibbit Hill volcanics in the Humber Zone of the Quebec Appalachians are believed to be related to the formation of the Ottawa-Bonnechere Graben. The precise age of these volcanics is unknown but they are either early Cambrian and late Precambrian. This volcanism was probably coeval with the emplacement of the Grenville dike swarm.
  Minor but significant igneous activity occurred during the Mesozoic era, including kimberlite emplacement during the Jurassic period, and the development of alkalic intrusions along the Ottawa-Bonnechere Graben and elsewhere in Ontario. This second episode of alkalic volcanism occurred along the eastern part of the graben in the early Cretaceous.
  The products of this event are the Monteregian Hills in Montérégie, Quebec. These are thought to have formed as a result of the North American Plate sliding westward over a long-lived center of upwelling magma called the New England hotspot, and is the eroded remnants of intrusive stocks.
  These intrusive stocks have been variously interpreted as the feeder intrusions of long extinct volcanoes, which would have been active about 125 million years ago,  or as intrusives that never breached the surface in volcanic activity. Of all these features, Mont Saint-Hilaire is the best known as a source of rare specimens.
  Along the northern side of the Ottawa-Bonnechere Graben lies a dramatic escarpment that forms the southern edge of the Gatineau Hills. This escarpment, called the Eardley Escarpment, makes this part of the graben an attractive location for rock climbers and hikers, offering a beautiful view of the relatively flat fields below, which extend to the Ottawa River.
  On or near a branch of the Ottawa-Bonnechere Graben lies the Brent impact crater.  It is 3.8 km (2.4 mi) in diameter and the age is estimated about 400 million years (Early Devonian). The impact crater, which was first recognized in 1951 from aerial photographs, formed in Precambrian gneisses.
    Geophysical and diamond drilling investigations show that the crater has a present depth of about 425 m (1,390 ft) but is partly filled with sedimentary rocks with a thickness of about 274 m (899 ft). The rocks beneath the crater floor are thoroughly fragmented over a depth of about 610 m (2,000 ft). Like the similar Pingualuit crater, the Brent crater is attributed to the high speed impact of a giant meteorite. It is calculated that the impact released energy equaling 250 megatons of TNT and occurred when this area was probably covered by a shallow sea.
caption: Three of the central Monteregian Hills viewed from space (from left: Mont Saint-Hilaire, Mont Rougemont and Mont Yamaska).
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