Abstract

Glaciers on Mount Rainier, Washington, may be divided into two types: valley glaciers that bear large superglacial-debris loads, and "ridge" glaciers that occupy the divides between the valleys and essentially lack superglacial debris. Nearly all valley-glacier tills have median particle sizes of about 64 mm and resemble superglacial debris, but some ridge-glacier tills have median particle sizes of about 1 mm and appear to be lodgment till. The latter tills display strong fabrics with a mean orientation parallel to ice flow, but almost all tills of valley glaciers show weak fabrics Thus, ground moraines and end moraines of ridge glaciers can be distinguished on the basis of texture and fabric, whereas those of valley glaciers cannot. Pebbles of ridge-glacier tills tend to be less angular than those of valley-glacier tills, but running water apparently is necessary to produce rounded and well rounded clasts. Striated clasts are rare in all deposits, probably because of unfavorable lithologies. Clasts that can be identified on the basis of their lithology as subglacial in origin are rare in valley-glacier tills, suggesting that the latter are composed mainly of superglacial debris. Such clasts are common in outwash from these glaciers, however, indicating that subglacial erosion is important.

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