Giant Grooves Made by Concentrated Basal Ice Streams

Abstract

Abstract Glacial grooves are known on all the flat-lying limestone islands in western Lake Erie. The largest megagroove complexes are on Kelleys Island, each 2–6 m deep, 5–20 m wide, and 100–400 m long before quarrying. Each megagroove floor is rolling with second-order rounded ridges and “sine-shaped” grooves 10–90 cm deep, and 5–40 m long. Furthermore, these are gouged laterally by sharply curving and nesting gouges up to 10 cm deep. All grooves exhibit third-order striation up to 1–2 mm deep and 5–200 cm long made by individual tools of debris traveling mostly 240–260°. Ice of the Erie lobe was under compressive flow as it rose from 9 m below present lake level to 12 m above. Some sort of ice vortices (fixed eddies in the basal ice stream) are postulated to make the scoop marks and ridge ends. Fast-moving ice rather than water or a slurry is favored because there are no percussion marks. Very likely, a till mat of interfering engraving points is required. A strong convergence (2× to 10×) of striae into the deepest grooves indicates squeezing together of the debris tools and increase in local ice velocity. Interglacial subareal streams may well have shaped the initial trough up which these ice streams concentrated, because dendritic tributary grooves intersect, and main groove sets curve as much as 10° or 20°. Furthermore, sharp 100–180° meander curves are preserved at the bottom of the deepest grooves. Hiram-age clay-till with rare erratics half fills the original grooves; it shows that this groove cutting was completed before 15 500 14C years ago.