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The Col-Gully and Glacial Deposits at Court Hill, Clevedon, Near Bristol, England

  • D. D. Gilbertson (a1) and A. B. Hawkins (a2)

Abstract

An outline is given of the Quaternary geology and geomorphology of Court Hill Col in Failand Ridge near Clevedon, Avon County, from observations made during the construction of the M5 Motorway.

A glacial col-gully about 100 m wide and approximately 25 m deep is described. The col-gully, eroded through the Carboniferous Limestone, opens and deepens northward. Associated with the Col and the col-gully is a complex sequence of Quaternary deposits. Uppermost in the sequence is a layer of red sandy silt (cover sand) approximately 0.5 m thick, of periglacial origin, probably of Devensian (Weichselian) age. Largely confined to the col-gully are unstratified tills, stratified ice-contact deposits and glacio-lacustrine deltaic deposits. The glaciogenic deposits are up to 25 m thick. Boulders of about 8 Mg in weight have been observed.

The geomorphology of the col-gully, and the stratification and composition of the glaciogenic deposits, demonstrate that an ice sheet at least 85 m thick had impinged against the south flank of Failand Ridge and was discharging immense quantities of water and sediment down an ice-contact slope through the Col into a small ice-marginal lake north of the col-gully. The ice sheet is regarded as being Wolstonian, or Anglian, in age.

The precise origins of the col-gully and the interpretation of the glacial sequence are not yet completely clear. However, it is believed that the balance of evidence indicates that both the col-gully itself and the glaciogenic deposits represent a complex sub-, en- and pro-glacial sequence associated with the downwasting and division of an ice mass into two parts by the "emergence" of Failand Ridge. The possible extent and geomorphological implications of ice-sheet penetration into the Bristol area are briefly discussed.

Résumé

Les dépôts glaciaires et torrentiels de Court Hill, Clevedon, près de Bristol, Angleterre. On donne un aperçu de la géologie et de la géomorphologie du quaternaire du Court Hill Col dans la Failand Ridge près de Clevedon, Avon County, à partir d'observations réalisées durant la construction de l'autoroute M5.

Un défilé glaciaire d'environ 100 m de large et approximativement 25 m de profondeur est décrit. Le défilé, creusé dans les calcaires du carbonifère, s'ouvre et s'approfondit vers le nord. Associée avec le Col et le défilé, on trouve une séquence complexe de dépôts quaternaires. Au sommet de la séquence il y a un niveau de sables rouges limoneux (sables de couverture) épais d'environ 0,5 m, d'origine périglaciaire d'âge probablement Devensian (Weichselian). Largement limitées au défilé sont des argiles morainiques non stratifiées, des dépôts stratifiés de contact à la glace et des dépôts déltaïques glacio-lacustres. Les dépôts d'origine glaciaire ont jusqu'à 25 m d'épaisseur. Des blocs d'environ 8 Mg en poids ont été observés.

La géomorphologie du défilé, la stratification et la composition des dépôts d'origine glaciaire démontrent qu'une calotte glaciaire d'au moins 85 m d'épaisseur s'est bloquée contre le flanc sud de la Failand Ridge et débitait d'énormes quantités d'eau et de sédiments au bas d'une pente au contact avec la glace à travers le Col dans un petit lac pro-glaciaire au nord du défilé. On pense que la calotte est d'âge Wolstonian ou Anglien.

Les origines précises du défilé et l'interprétation de la séquence glaciaire ne sont pas encore complètement claires. Cependant, on pense que le bilan des preuves indique que tant le défilé lui-même que les dépôts d'origine glaciaire représentent une séquence complexe sub-, en- et pro-glaciaire associée à la destruction et à la division d'une masse de glace en deux tronçons par l'"émergence" de la Failand Ridge. On discute brièvement de l'extension possible et des implications géomorphologiques d'une pénétration d'une calotte glaciaire dans le secteur de Bristol.

Zusammenfassung

Die rinnengebundenen und glazialen Ablagerungen am Court Hill, Clevedon nahe Bristol in England. Die Quartär-Geologie und -Geomorphologie des Court Hill Col im Failand Ridge bei Clevedon, Avon County, wird auf der Grundlage von Beobachtungen beim Bau der Autobahn M5 dargelegt.

Eine glaziale Joch-Abflussrinne von etwa 100 m Breite und annähernd 25 m Tiefe wird beschrieben. Die Rinne führt durch Karbonischen Kalkstein und verläuft unter Eintiefung nordwärts. Verbunden mit dem Joch und der Rinne ist eine komplizierte Folge quartärer Ablagerungen. Zuoberst in der Folge findet sich eine etwa 0,5 m dicke Schicht aus rotem, verschlammtem Sand (Decksand) periglazialen Ursprungs, vermutlich aus der Devensian- (Weichsel-) Eiszeit. Im wesentlichen auf die Rinne beschränkt, folgen ungeschichtete Schotter, geschichtete Eisrand-Ablagerungen und deltaartige Aufschüttungen in einen Eisrandsee. Die glaziogenen Ablagerungen sind bis zu 25 m mächtig; sie enthalten Felsbrocken bis zu 8 Tonnen Gewicht.

Die Geomorphologie der Abflussrinne und die Schichtung und Zusammensetzung der glaziogenen Ablagerungen lassen erkennen, dass eine Eisdecke von mindestens 85 m Dicke sich an die Südflanke des Failand Ridge herangeschoben hatte und riesige Mengen von Wasser und Schutt über einen Hang am Eisrand durch das Joch in einen kleinen Eisrandsee nördlich der Jochrinne ergoss. Die Eisdecke dürfte der Wolstonian- oder Anglian-Eiszeit angehört haben.

Der Ursprung der Jochrinne und die Deutung der glazialen Ablagerungsfolge bedürfen noch weiterer Untersuchungen. Doch lässt die Gesamtheit der Erscheinungen glaubhaft erkennen, dass sowohl die Jochrinne selbst wie die glaziogenen Ablagerungen ein kompliziertes sub-, in- und proglaziales System darstellen, das mit dem Verfall und der Spaltung einer Eismasse in zwei Teile durch den "Aufstieg" des Failand Ridge verbunden war. Das mögliche Ausmass und die geomorphologischen Folgen des Vordringens der Eisdecke in das Gebiet von Bristol werden kurz diskutiert.

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