Abstract

Texture of twenty-six sediment samples from ice-cored deposits at the termini of ten Alaskan valley glaciers was determined. The glaciers are eroding five different types of bedrock. Results of pebble roundness and grain-size frequency distribution analyses reveal two groups of samples: (1) those with mean pebble roundness values in Pettijohn's (1957) subangular class and which do not exhibit distinct grain-size modes (amodal sediment), and (2) those with mean pebble roundness values in the subrounded to rounded class and which exhibit either distinct modal grain-size classes (modal sediment) or pronounced deficiencies in particles finer than coarse or medium sand. The characteristics of samples in the first group suggest these sediments are the product of primary sediment-forming processes associated with valley glaciers (glacial erosion of bedrock and/or frost action on valley walls). These processes produce particles of all sizes irrespective of bedrock type. The occurrence of modal proglacial sediment of valley glacier origin, reported in the literature, is therefore probably a function of the environment of transport and deposition and is independent of the nature of the bedrock type or initially eroded detritus. The characteristics of samples in the second group suggest they are primarily mixtures of former proglacial deposits. Prior to present deposition, proglacial deposits were probably incorporated and mixed in the glacier load during ice advance.

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