| Summary: | Ice encroachment onto land occurs mainly during the period of complete ice cover and takes two basic forms, both of which are associated with sediment transport to differing degrees. In ice ride-up, unbroken sheets slide over frozen to shallowly thawed beaches or barriers for distances from 1 to over 100 m, supplying to the beach sediments from the very shallow (<1.5m) shoreface but scraping into mounds mainly loose subaerial debris. In ice pile-up, the advancing sheets crumble, building rubble piles up to 20 m high that commonly contain a few percent of intermixed sediment entrained from as much as 50 m seaward and 5 m water depth; we hypothesize a likely entrainment mechanism of down-flexing of the thin (<.5m) floating ice sheet, of breaking at bottom contact and mixing with sediments, finally followed by extrusion of the mixture through the growing pile. The sediment contained by the ice settles onto the substrate within one to several summers of melting, forming melt lag deposits as opposed to the above push mounts. The lag mounds, usually within 20 m of the shoreline, may be 2 m high, and may add one m3 of sandy gravel to each meter of coast line per pile-up. Ice can stack sediments well above the elevation reached by waves, but in most areas the shoreline is receding fast relative to the recurrence rate of ice encroachment (~10yr), and the characteristic ice morphologies are short-lived. The materials added to beaches and barriers by ice encroachment remain, and are seen in barrier island pebble lithology and pebble size, pointing to offshore rather than alongshore sediment sources. A stable barrier island we investigated apparently was elevated by repeated ice stacking to the unusual height of 4 m, as opposed to the 1-1.5 m height of wave-dominated barriers. A possible recent decrease in the rate of ice stacking versus wave reworking, or increased fetch, is seen in several other high, but rapidly disappearing barriers capped by coarse gravel to cobbles. Such a possible change in balance between the two processes may be related to a retreat of the summer ice edge during the last century. Recognition of wave- versus ice-dominated barrier islands should be possible along much of the circum Arctic Ocean shoreline, and may provide information about extent and severity of sea ice in the past or changes occurring today.
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