PDF LAlaska en bref (French Edition)

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Search was then extended to neighbouring regions. In this process, it became rapidly obvious that many features were preferentially associated with specific substrates, for example old Lower Pleistocene and Tertiary alluvial sand and gravel deposits for polygons, chalk with a thin loess cover for soil stripes… Blind tests in other regions were also made but were most of the time unsuccessful.

These data were rigorously selected and all entries not accompanied by drawings, photographs or good descriptions were rejected from the database. We were forced to exclude a significant amount of features because the sources did not document them enough lack of description or figure or did not give satisfying geographic information e.

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Cailleux and J. Many were also mentioned in the explanatory notes accompanying the sheets of the geological map of France. As a consequence, with few exceptions e. In aerial photographs, a site corresponds to a single land parcel or a few adjacent parcels. The CSV format was chosen because of its simple use and the possibility of easy transformation into an attribute table in a GIS. Each CSV file is accompanied with a folder that contains photographs and sketches of the features when available.

The ID number allows linking the listed features and corresponding pictures, drawings, or aerial photographs. They include:. The chronological data have not been included in the database, they are accessible through the cited references.

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Thermal contractions cracking forms large polygonal networks of fissures, evolving into V shaped wedges as a consequence of repeated filling by ice or sediment. Cross-sections discovered in the vicinity of a few sites suggest that the walls correspond to sand wedges or composite wedges. In the Paris basin and the north of France, some polygons have more rounded outlines and walls of irregular width fig.

They are thought to correspond to former ice-wedge polygons that have undergone thermokarstic degradation. Although not very conclusive by themselves, some photographs have been kept in the database because of the discovery of sand wedges in nearby cross-sections Lenoble et al.

Although a periglacial origin seems probable, their classification as thermal contraction features rather than very large patterned grounds remains doubtful see aerial photographs and cross-sections in Agache, , and needs further confirmation by field survey. As a consequence, this kind of features was not added to the database.

Severe selection was made among the reported features, 90 ice wedge pseudomorphs are listed in the database. According to Black , Washburn , Vandenberghe , Gozdzik and Murton , the main criteria for their identification and consideration in the database are: 1 a V shape showing collapse structures due to the replacement of ice; these consist in block faulting common in the coversands of northern Europe, Murton, ; no example has been found in France so far, fig. The filling may be massive or show steeply dipping or U-shaped layers.

Blocks of sediment may be present within the filling fig. Thermokarstic degradation of permafrost may lead to the erosion and deformation of the wedge, the formation of tunnels, ponds and gullies so that the relict ice-wedges are no longer identifiable or visible. The distribution of ice wedge pseudomorphs is therefore probably significantly underestimated in our dataset.

They are narrow and lack any clear indication of host material collapsed in the wedge after the melt of an ice body fig.


These features may represent either incipient ice wedges ice veins or soil wedges in seasonally frozen ground Friedman et al. Sand wedges do not show evidence of host material slumped in the wedge. They are usually V-shaped but may be irregular with multiple elementary sand veins extending out of the toe of the wedge into the host material this typically occurs in sands, Romanovskij, ; Murton et al.

The origin of the bulbous shape, illustrated by Murton et al. Deformation of the host material induced by wedge growth is usually poorly developed and upward bending of strata occurs only occasionally. Field exposures generally show multiple sand wedges a few metres apart, which testifies to a polygonal pattern. The sand wedges identified in France have generally a massive filling fig. Only the wedges with a minimum width of 0. Antoine ; the shovel is 50 cm long. The host material is colluviated aeolian silt. Both features located in the southwest of France form small-scale polygons in plan view 0.

Although the digitations suggest repeated cracking, no evidence for ice melt and subsequent filling is visible. The fissures were secondarily affected by redox processes and bleaching due to waterlogging. In photo B, bleached horizontal fissures suggest that ice lenses formed in connection with the vertical fissures.

The scale is 1 m long. The composite wedges share therefore common features with ice-wedge pseudomorphs and primary sand wedges. The most diagnostic feature is the evidence of a secondary filling occupying a significant part of the wedge. This filling appears typically either as cross-stratified to U-shaped beds of sand or gravel material fig. In many cases, however, the wedges show different types of filling cross-cutting each other, suggesting a succession of dominantly icy or sandy phases fig.

One example fig. The gully is thought to be of thermokarstic origin and, therefore, suggests that the wedge filling contained a significant amount of ice. Overall, 10 composite-wedge pseudomorphs are listed in the database. However, this number may be underestimated because of difficulties in identifying secondary sandy fillings. Potential composite wedge pseudomorphs may, therefore, be classified here as sand-wedges.

They reflect alternating bands of light coloured usually chalky debris and darker loess or clay loam material or uneven plant growth due to contrasts in water content fig. A small number of aerial photographs show spotted patterned ground on flat surfaces stretching into stripes on slopes fig. A total of sites of relict soil stripes have been identified in France, mostly in the Paris basin.

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The spacing between the stripes range from 2 to 13 m, but the distribution can be adjusted to two Gaussian functions peaking at 4. In modern periglacial environments, soil stripes form in the ground layer subjected to freezing and thawing cycles. Vernet ; the scale is 1 m long. Coutard ; the pickaxe is 1 m long.

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The data presented here should not be viewed as fully exhaustive; because of limited time and funds all the potential areas where periglacial features could be found in cross-sections have not been prospected yet. This is particularly the case for large areas of northeast France.

Conflicts and discrepancies may remain as shown by Andrieux et al. To solve these issues further research on the origin and palaeoclimatic significance of some features and more thorough dating is needed. The file of photographs and drawings is available on request from the first two authors E.

Agache R. Andrieux E. Permafrost and Periglacial Processes , 27 1 , Antoine P. Quaternary Science Reviews , 28 , Arnal H. Ballantyne C. Cambridge, Cambridge University Press, p. Bastin A. Bertran P. Boreas , 43 3 , Black R. Quaternary Research , 6 1 , Bockheim J. Permafrost and Periglacial Processes, 20 3 , Eiszeitalters und Gegenwart , 1 , Cailleux A. Fortier D. Friedman J. Geografiska Annaler , 53A , Genty D. Quaternary Science Review , 29 , Gozdzik J.

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