Nye channels (flutings) on the Humboldt Massif, northern Venezuelan Andes

摘要

Flutings produced by glacial ice/meltwater action have been described either as deep or shallow gutters or channels, cut into the stoss side of bedrock bars or hills, obstructing glacial advances. As defined by CHAMBERLIN (1888), they are larger than glacial grooves, and are not found on the lee sides of obstacles. However, a variant includes deeper furrows, lineations or a series of small-scale, streamlined grooves, formed in coarse grained granite or gneiss, and oriented parallel to the direction of ice movement on the lee sides of obstacles such as bedrock bars. On the lower flanks of the Humboldt Massif, in northern Venezuela, large-scale glacial flutings or Nye channels constitute imposing longitudinal erosional landforms cut through the stoss and lee sides of major bedrock obstacles. Situated below the three lobes of the present Humboldt Glacier - Este, Central, and Bonpland - at elevations of 4,600 in a. s. I., and below the bedrock bar damming Lago Verde at 3,960 in a. s. I., they are formed in bedrock of felsic gneiss, which is relatively homogeneous throughout the valley. The channels on the lower slopes have variable geometries, ranging from 1-5 in in width and 1-15 in depth. Their depth required a high discharge of meltwater, laden with sufficient clasts, to carve the channels leaving smooth abraded surfaces. Occasionally the channels exhibit polished/striated surfaces. As wave-like forms these channels or flutings are the result of prodigious melting below the Last Glacial Maximum ELA, and most likely of pre-LGM ELAs, in approximately the same topographic position.