Experimental formation of sorted patterns in gravel overlying a melting ice surface

Abstract

Experiments were performed to investigate the processes involved in the formation of sorted patterns which occur in the unconsolidated sandy gravel deposits covering the edge of the ice cap southeast of Thule, northwest Greenland. Four different glacier ice surfaces were covered with various thicknesses of sandy gravel without fines in order to observe the effect of differential melting on the formation of sorted patterns. A thin gravel cover of 5 cm allowed more rapid melting than did a cover of 15 cm, with the result that mounds and depressions were formed. Coarse particles were segregated in the depressions by a natural sorting of the various particle sizes when set in motion by differential melting and resulting uneven collapse of the gravel cover. The sorting produced well-developed stone rings in three cases where the gravel cover varied in thickness. In a fourth area a uniform gravel cover over a smooth ice surface produced no sorted nets, although a poorly developed stone stripe was formed in a melt-stream channel. A large stone ring formed around the test area as the surrounding poorly insulated ice melted down at a more rapid rate. It is concluded that the sorted nets and stripes occurring naturally in the moraine deposits on the edge of the ice cap were initiated by mechanical sorting induced by differentia! melting of the ice under a non-uniform layer of sandy gravel. This proces very likely plays an important part in the formation of collapse patterns observed in newly thawed permafrost from which the active layer had been removed.