Lower Tills Research Articles

Paleomagnetism of tills and associated paleosols in southwestern Alberta and northern Montana: evidence for Late Pliocene–Early Pleistocene glaciations

Sequences of pre-Wisconsinan till and intercalated paleosols were sampled for paleomagnetic study. The tills were deposited during successive glaciations and the paleosols formed during interglacial intervals. Paleoargillic horizons of the paleosols and the carbonate cement (calcrete) found in some till–paleosol units generally yielded excellent data. Magnetizations of paleosols probably were acquired during the formation of the paleosols rather than during initial deposition of the tills in which they were developed. At Mokowan Butte (Alberta), the lowest paleosol has normal polarity, two of the middle tills have reversed polarity, and the uppermost till–paleosol unit has normal polarity. At Saint Mary Ridge (Montana), three of the lower tills have reversed polarity, and the upper two till–paleosol units have normal polarity. At Two Medicine Ridge (Montana), the lowest three tills are reversed, but the paleoargillic horizon on the uppermost (fourth) till is normal. Magnetostratigraphic correlation indicates that at least six glacial and six interglacial episodes are represented in the Kennedy Drift. The upper normal polarity units are interpreted as having been developed during the Brunhes Normal Chron, the underlying reversed polarity sediments during the Matuyama Reversed Chron, and the lowest normal polarity unit at Mokowan Butte during the Gauss Normal Chron. The oldest glaciations here extend into the Pliocene (2600 ka), making these sediments among the oldest glacial deposits in North America. Alternatively, the lowest normally magnetized paleosol at Mokowan Butte may have formed during either the Jaramillo or the Olduvai subchrons, although this is considered less likely.

Regional significance of pre-Wisconsinan till from Nantucket Island, Massachusetts

A major pre-Wisconsinan glacial event is the only possible source of the lower till on Nantucket Island, Massachusetts. The till occurs near the late Wisconsinan drift border and below fossiliferous marine beds of oxygen-isotope stage 5 (Sangamonian) age. It is considered to be Illinoian in age, but the evidence is tenuous. The till is correlated with the lower till of New England, and its presence supports the view that the New England upper and lower tills represent two glaciations. The pre-Wisconsinan (lower) till in New England may correlate with older tills elsewhere in the northeastern United States and in southeastern Canada that are considered to be early Wisconsinan or older in age.

Sequence of till deposition and erosion in drumlins

Whittecar, G.R. & Mickelson, D. M. 1977 06 01: Sequence of till deposition and erosion in drumlins.Extensive sand and gravel workings have exposed structural and compositional features of 17 gravel‐cored drumlins of late Wisconsin age in eastern Wisconsin. The drumlins are blanketed by 3 m of sandy basal till which truncates lower tills of earlier advanccs, outwash gravels, and an overlying till which is conformable to the gravel bedding and indistinguishable in composition from the surface till. Sands and gravels in the interior of some drumlins are deformed into large overturned folds, and into elastic dikes of fine sand and silt which penetrate to the top of the drumlin and warp overlying gravels. Both the folds and horizontal bedding are truncated by either the drumlin edge or the till blanket.We interpret the conformable, truncated, and in some cases folded, till as a basal till deposited during glacial advance. The capping, truncating till is viewed as a basal till left by retreating ice.The following sequence of events is suggested: (1) advance of ice over outwash, and deposition of till in a zone mar the margin; (2) thickening of the ice and erosion of the drumlin shapes; (3) local folding of the gravels and continued erosion; (4) retreat of ice and deposition of basal till under thin ice; (5) deposition of localized ablation till and stratified deposits.

Re-interpretation of Devensian Till stratigraphy of eastern England

THE current reconstruction of the Devensian Till stratigraphy of Holderness1 invokes three units—the Drab, Purple and Hessle Tills—with the Hessle overlapping both lower tills so as to be the only one present in the westernmost part of the area, on the Yorkshire Wolds. Because of the characteristic reddish brown colours (usually 5YR4/4 of the Munsell Soil Colour Charts, 1954) of the uppermost tills throughout eastern England, the upper till in Lincolnshire and the Hunstanton Till of north Norfolk are equated usually with the Hessle Till1–5, although Straw6 correlated his two Marsh Tills in Lincolnshire with the Drab and Purple Tills of Holderness, and Bisat7 equated the Hessle Till of the Yorkshire Wolds with his Middle Drab division.

Pleistocene deposits of the north-western Allegheny Plateau, U.S.A

In North America Pleistocene ice sheets entered the lowland now occupied in part by Lake Erie and expanded far southward in several lobes in the Interior Plains, but less far in two lobes in the Allegheny Plateau to the east in north-eastern Ohio, north-western Pennsylvania and western New York. This maturely dissected plateau, composed of essentially horizontal Carboniferous strata, has a summit level ranging from about 1200 ft in the west to about 1800 ft in the east. In recent years excavations as deep as 135 ft for strip mines (coal), highways and quarries (limestone) in that part of the plateau in Ohio and Pennsylvania have exposed complete sections as much as a mile in length, which reveal lower drifts, mainly till, that have been previously almost unknown. Each till sheet has a characteristic texture and mineral composition and separate sheets can be traced for more than 100 miles, and treated as rock stratigraphical units. The younger tills are much higher in clay than the older tills. The feldspar content of the sand fraction ranges from about 5 per cent in the oldest till to 30 per cent in the youngest till. Carbonates range from less than one per cent in the oldest till to about 15 per cent in the youngest. Within each till sheet there is a decrease in feldspar and an increase in quartz toward the margin. Statistical studies, in part by trend-surface analysis by computer methods, confirm the regularity of this variation, which is ascribed to dilution by local feldspar-poor sandstone. The dilution is much greater in the older tills; at their margins 90 per cent of the quartz content may be locally derived. The lowest till, of pre-Illinoian age, entirely a sub-surface unit, is everywhere thoroughly weathered. The overlying till of probable Illinoian age, consists of two sheets, the upper one of which has a thick palaeosol. The bulk of the drift is early Wisconsinan in age and consists of three or more till sheets. It is named (in Pennsylvania) the Titusville Till and has a 14 C age of about 40 000 years. It is sandy, very compact and oxidizes to a distinct olive-brown colour. The weathering on it is not as intense as that of the two lower tills. The oldest till of late Wisconsinan age, the Kent Till, is a distinctive coarse yellow-brown, sandy till. It is overlain by the chocolate-brown, silty, more highly calcareous Lavery Till, which in turn is overlain by the brown, silty-clayey Hiram Till. Each of the older tills, except the pre-Illinoian, extends farther south and south-east than the later tills, so that each forms the surface drift in a belt, ranging from 2 to 20 miles in width, in which its surface morphology and soil characteristics can be studied and its general character compared with that in the sub-surface.