| Summary: | Cryostratigraphic mapping in the main shaft of the CRREL tunnel indicates secondary modification of original Pleistocene‐age syngenetic permafrost. Layered, lenticular‐layered and micro‐lenticular cryostructures within undisturbed silt characterise the syngenetic permafrost. Thawed and refrozen deposits are characterised by massive and reticulate‐chaotic cryostructures. The micro‐morphology of typical cryostructures, as observed through an environmental scanning electron microscope (ESEM), is described. It is inferred that preferential cryogenic modification has occurred due to fluvio‐thermal erosion operating along ice wedges. Soil and ice pseudomorphs are the manifestation of this process. Gravel, silt, ice and mixed (silt/ice) pseudomorphs occur within silt. Ice pseudomorphs are formed by thermokarst‐cave ice (‘pool ice’) which filled thermokarst pits, channels and gullies. Copyright © 2006 John Wiley & Sons, Ltd.
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