Érudit's RSS Feed

La fin de l’aventure

André G. Roy

The Saint-Narcisse morainic complex and early Younger Dryas events on the southeastern margin of the Laurentide Ice Sheet

Serge Occhietti
AbstractThe Saint-Narcisse morainic complex extends over 750 km along the southern margin of the Laurentian Highlands in Québec, north of the St. Lawrence Valley, between the Ottawa and Saguenay Rivers. To the east, the Laurentide Ice Sheet margin was located in the present St. Lawrence Estuary. To the west, the morainic complex is extended 235 km west of the Ottawa River to the Algonquin Highlands, in Ontario. The general outline of the morainic complex comprises large lobes and reentrants, related to major topographic features. In the lower Saint-Maurice River area, the moraine is composed of reworked clay and till and proximal glaciomarine deposits (Yamachiche Diamicton) and melt-out till and ice-marginal outwash (Charette Drift). The Saint-Narcisse Event can be subdivided in several phases: local readvance in low areas, main phase at the origin of the Saint-Narcisse Moraine s.s., melting-out of the marginal ice with compressive structures and large proglacial outwash features, and slow retreat with secondary ridges. The accuracy of the chronological data is limited by several factors: and a floating chronology is proposed. Two landmarks constrain the age and range of duration of the main Saint-Narcisse phase. The main ridge deposition occured after the onset, ca. 12.9 cal ka, of Champlain Sea in the St. Lawrence Valley, and a rapid ice retreat on the southern edge of the Laurentians. It ended before the drawdowm, in the Lake Huron basin, of Glacial Lake Algonquin ca. 12.5 cal ka. The Saint-Narcisse Event is related to the early cold phase of Younger Dryas, as evidenced by other YD ice readvances in Maine, Nova Scotia, and ice cover on the Gaspé Peninsula. It corresponds to a positive change of the budget of the Laurentide Ice Sheet as a result of climate forcing. After a slow ice front retreat during about 900-700 yr, the final phase of YD is marked by the Mars-Batiscan Moraine, located 17 to 70 km north of the Saint-Narcisse Moraine.

Façonnement du modelé drumlinoïde par deux écoulements glaciaires successifs au sud-ouest de Montréal

Marc Delage and Pierre Gangloff
AbstractLandforms and deposits of Huntingdon plain, southwest of Montréal, reveal a complex glacial history. Of that glacial heritage, we have considered elongated landforms nearly parallel to the St. Lawrence River axis. The core of these drumklin-like (drumlinoids) forms is a fissile and compact lodgement till, occasionally covered by a melt-out till or by coarse littoral material of probable drift ice (ice-rafted) origin. These subglacial landforms were first moulded by an important ice flow from the northeast moving up St. Lawrence Lowlands south-westwards. This is shown by indicator stones of appalachian origin, but also by till fabrics and microforms of glacial erosion both oriented northeast-southwest. Thus, the initial part of their genesis makes the drumlinoids more similar to drumlins than to subglacial transverse landforms like Rogen or ribbed moraines. Characteristics of the lodgement till indicate than the drumlinoids were constructed by accumulation processes, but that erosion processes responsible for an important fraction of the till were in action near by. The first ice flow from the northeast was followed by a second one from the northwest or the north without deglaciation. Even if this second ice flow let its traces all over the studied area, the initial glacial morphology was only slightly modified. The entire glacial portion of the drumlinoids belongs to the same Late Wisconsinan glacial Stage. The last important phase in the genesis of the drumlinoids was accumulation of drift ice sediments in the end of Champlain Sea episode which altered their crest but mainly their southeastern side.

Incision fluviatile et tectonique dans la Montagne Noire (sud du Massif central français)

Jean-Pierre Larue
AbstractRiver longitudinal profiles and erosion products are used to reconstruct the uplift stages of the Black Mountain, a west-east-trending ridge of the southern French Central Massif. The analysis of 27 rivers and 117 knickzones allows us to distinguish three different sets : (1) the rivers with low gradient and low incision mainly flow on the sedimentary cover, (2) the rivers on the south-west slope have high gradient and are deeply incised in the basement, and (3) the most incised rivers include long rivers, like the Orb, and short rivers with very high gradient, like the Heric. The first knickzone from the source, with abnormally high gradient resulting from headward incision in response to the Black Mountain uplift, has a location controlled by gradient and upstream drainage area. The location of knickzones with abnormally high gradient and without lithological origin indicates a recent uplift of the central and western part of this range. The uplift stages appear different on both sides of the Cesse River. To the east, the Pliocene main uplift induced the formation of stepped erosion glacis on the Languedocian piedmont, while the moderate Pleistocene uplift caused an 80 m-deep river incision. To the west, the main Pleistocene uplift led to still ongoing regressive erosion, with incision often exceeding 500 m.

Les conditions météorologiques propices au déclenchement des avalanches de neige dans les corridors routiers du nord de la Gaspésie, Québec, Canada

Bernard Hétu
AbstractNorthern Gaspésie is one the principal areas of snow avalanche risk in Québec. Snow avalanche hazard represents a treat to back-country recreational activities, but also to the vehicles circulating on national roads 132 and 198 between Sainte-Anne-des-Monts and Grande-Vallée. Each winter, masses of snow cover partially or completely the roadway, stopping circulation and causing accidents sometimes. This paper aims to localize the main avalanche risk zones into Northern Gaspésie transportation corridors and the analysis of the weather situations leading to avalanche activity along transportation roads under the specific climate of Northern Gaspésie. Three sets of data were gathered together, an inventory of avalanche activity realized by the Ministère des Transports du Québec between 1987 and 1993, a series of specific avalanche events having caused road accidents, including three mortals (1935, 1956 and 1971), for a total of 21 avalanche days, and the dates of avalanches were confronted with weather data provided by the stations of Environment Canada. Preliminary results indicate that the coastal slopes of Northern Gaspésie have two major avalanche regimes : a spring regime associated with snowmelt, rain and ice-block fall from rock walls, and a winter regime associated with snowstorms.

Réchauffement climatique et dynamique forestière au 20e siècle : la pinède de reboisement sur éboulis de la Courbe (Massif des Grandes Rousses, Alpes du Nord, France)

Christophe Corona and Georges Rovéra
AbstractForest dynamics were studied on a scree slope located at southern end of the Grandes Rousses Mountains (Northern Alps, France), which had been reforested with Austrian black pine (Pinus nigra ssp. nigricans) at the beginning of the 19th century. Dendrological study of the stages of forest dynamics and tree growth revealed climate fluctuations over the last century. Large scale (1 :5 000) bidecennial diachronic cartography combined with dendrologic samples (157 trees sampled on 12 plots) showed rapid expansion of Pinus nigra between 1950 and 1970, followed by a slow-down since the beginning of the 1980s. This tendency matches low-frequency variations in both dendrochronological and meteorological series. A rise in tree numbers between the 1950s and 1970s thus parallels an increase of 40 % in radial growth and a sequence of cool, wet summers recorded at the nearby meteorological station of Besse en Oisans. In the same way, radial growth has decreased by 20 % since the middle of the 1970s, in relation with a succession of hot, dry summers, resulting in a slow-down in colonization. The high sensitivity of pines to early summer drought (in May, June and July) was corroborated by monthly dendroclimatologic analysis. These results raise questions about the future survival of such pine populations in the context of increasingly-severe intra-alpine drought, a phenomenon as yet insufficiently predicted by macroclimatic scenarios. Further research is also needed into the suitability of protection forests on scree slopes to detect modifications in the pluviometric patterns of mountain regions, currently poorly predicted by climatic models.

La batture de l’anse au Sable à Rimouski : un estran typique de la rive sud de l’estuaire maritime du Saint-Laurent, Québec

Jean-Claude Dionne
AbstractAmong the various cold region shorelines, those of the Lower St. Lawrence estuary have particular characteristics that need to be better known. The shore at Anse au Sable (Sandy Cove), near Rimouski, is of special interest. Located at the foot of an Appalachian rock ridge oriented SW-NE, the shoreline includes a narrow sandy gravel beach at the high tide level, a relatively wide rock platform in the upper half of the strand, and, in the lower half, a clayey substrate which is covered by boulders, 65 % being Precambrian erratics from the Laurentidian Shield located 30 km north of the St. Lawrence valley. Precambrian and Appalachian far-travelled erratics were first moved toward the estuary by Wisconsinian glaciers. Most erratics were then transported and dropped on the sea floor of the Goldthwait Sea by icebergs during deglaciation between 13 and 10 ka. An unknown percentage of Precambrian erratics, first displaced by the Laurentide Ice Sheet and dropped in the Appalachian region, were removed and displaced northward and northeastward at the Late Glacial maximum. The sedimentary errratics at Anse au Sable have two origins. Most are from local bedrock outcrops near the present shoreline, whereas a low percentage are from the Ordovician and Silurian formations of the Appalachians south of the coastal area submerged by the postglacial sea. Erosion by waves and currents during the Holocene of shore and marine clay deposits containing ice rafted coarse debris resulted in a boulder lag from which boulders were removed by shore ice and concentrated on the lower clayey tidal flat.

First recognition of a Laurentide Ice Stream : Robert Bell on Hudson Strait

Ian A. Brookes
AbstractIn papers published in 1895 and 1901, and in undated notes for a 1907 paper he did not deliver or publish, Robert Bell of the Geological Survey of Canada interpreted the pattern of glacial striae, stossing of rock knobs, and surficial sediment composition along the margins of Hudson Strait, between Labrador, Ungava Bay and Baffin Island, as evidence of what he called an ice-stream, a long river-like glacier, fed from Hudson Bay and Foxe Basin, that had moved eastward along the Strait during the Late Glacial period. This was the earliest mention of such a glaciological feature within the Laurentide Ice Sheet (LIS). It was not until ice-streams were recognized in the West Antarctic Ice Sheet in the 1970’s that Bell’s concept was revived in the next decade and subsequently, in recognition of several ice-streams within the Late Wisconsinan LIS.

Geological evidence of a palynologically defined cooling in southeastern Canada at 10 000-8000 14C yr BP

Ian A. Brookes and Douglas R. Grant
AbstractPost-glacial pollen spectra over a wide area of southeastern Canada have been interpreted as showing that the general warmth-adapted trend of regional vegetation change was interrupted between 10 000 and 8000 14C yr BP, reverting to conditions associated with a markedly cooler climate. This biotic reversal has been attributed to a climatic cooling caused by discharge of frigid water from glacial Lake Agassiz, through the Great Lakes to the Goldthwait Sea in the Gulf of St. Lawrence basin. Here, we assemble geological evidence from widely scattered localities in and around the Gulf of St. Lawrence, all previously reported, a majority unexplained, and ascribe it collectively to the same climatic cooling. We interpret the marine diamicts and other faunal reversals as the products of cooling, which intensified sea-ice conditions during the interval 10 000-8000 14C yr BP, specifically at 9300 14C yr (9170 yr, reservoir corrected), conditions resulting from that meltwater influx. This age was several centuries too young to be correlated with the Preboreal Oscillation, but several easterly overflows of Lake Agassiz occurred before and after this date, and may have increased sea-ice in the Goldthwait Sea, singly or by hysteresis. Other truly glacial features in southeastern Canada, such as moraine systems and diamictons, are also referred to this cooling.

Vertebra of a Bowhead whale (Balaena mysticetus) in Late Wisconsinan deglacial marine sediments, Trout River, Newfoundland, and local late-glacial sea-level change

Ian A. Brookes
AbstractThis note reports the geological contexts and radiocarbon and calibrated ages of whalebone and bivalve recovered from emerged deglacial marine sediments at Trout River, Newfoundland (49°28.6’ N, 58°07’ W). The rate of sea-level fall is calculated for the first deglacial millennium, and discrepancies are noted between calibrated ages of bone and shell.

Jaccard, Mark, 2006. Sustainable Fossil Fuels. The Unusual Suspect in the Quest for Clean and Enduring Energy. Cambridge University Press, New York, 381 p., 23,0 x 15,5 cm, 24,99 $ US, ISBN 0-521-67979-6 (couverture souple)

Pierre André

André, Marie-Françoise, Étienne, Samuel, Lageat, Yannick, Le Coeur, Charles et Mercier, Denis, sous la direction, 2007. Du continent au bassin versant. Théories et pratiques en géographie physique. Hommage au professeur Alain Godard. Presses Universitaires Blaise Pascal, Clermont-Ferrand, 592 p., 201 fig., 30 tabl., 25,0 x 19,0 cm, 40 €, ISBN 978-2845160-335-5

Jean-Claude Dionne

Ancey, Christophe, édit., 2006. Dynamique des avalanches. Presses Polytechniques et Universitaires Romandes, Lausanne, 352 p., 110 illustrations, 19,0 x 24,0 cm, 55 €, ISBN 2-88074-648-5 (couverture souple)

Bernard Hétu

Soutter, Marc, Mermoud, André et Musy, André, 2007. Ingénierie des eaux et du sol. Processus et aménagements. Presses Polytechniques et Universitaires Romandes, Lausanne, 312 p., illustré, 19,0 x 24,0 cm, 37,45 €, ISBN 978-2-88074-724-4 (couverture souple)

Hélène Lamarre

Ingebritsen, Steve, Sanford, Ward et Neuzil, Chris, 2006. Groundwater in Geologic Processes. Cambridge University Press, New York, 562 p., 154 fig., 13 tabl., 24,7 x 17,4 cm, 75 $ US, ISBN 0-521-60321-8 (couverture souple)

René Therrien

Index du volume 61 — 2007

None