Page 282 - Fundamentals of Geomorphology

GLACIAL AND GLACIOFLUVIAL LANDSCAPES 265

Hummocky moraines, also called dead-ice moraines Second, they may be the result of textural differences in
or disintegration moraines, are seemingly random subglacial debris.Third, they may result from active basal
assemblages of hummocks, knobs, and ridges of till and meltwater carving cavities beneath an ice mass and after-
other poorly sorted clastic sediments, dotted with kettles, wards filling in space with a range of stratified sediments.
depressions, and basins frequently containing lacustrine Some large drumlin fields, the form of which is redolent
sediment. Most researchers regard the majority of hum- of bedforms created by turbulent airflow and turbu-
mocky moraines as the product of supraglacial deposi- lent water flow, may have been formed by catastrophic
tion, although some landforms suggest subglacial origins. meltwater floods underneath Pleistocene ice sheets (Shaw
Far-travelled erratics are useful in tracing ice move- et al. 1989; Shaw 1994).
ments. De Geer and Rogen moraines lie transversely to the
direction of ice flow. De Geer moraines or washboard
Subglacial landforms moraines are series of small and roughly parallel ridges
of till that are ordinarily associated with lakes or former
A wealth of landforms form beneath a glacier. It is lakes. Rogen moraines, also called ribbed moraines and
convenient to class them according to their orientation cross-valley moraines, are crescent-shaped landforms
with respect to the direction of ice movement (parallel, composed largely of till that are formed by subglacial
transverse, and non-orientated). thrusting. They grade into drumlins.
Forms lying parallel to ice flow are drumlins, drum- Various types of ground moraine display no particu-
linized ridges, flutes, and crag-and-tail ridges. Drumlins lar orientation with respect to ice flow. A ground moraine
are elongated hills, some 2–50 m high and 10–20,000 m is a blanket of mixed glacial sediments – mainly tills and
long, with an oval, an egg-shaped, or a cigar-shaped out- other diamictons – formed beneath a glacier. Typically,
line. They are composed of sediment, sometimes with a ground moraines have low relief. Four kinds of ground
rock core (Plate 10.11), and usually occur as drumlin moraine are recognized: till plain, gentle hill, hummocky
fields, giving rise to the so-called ‘basket of eggs’ topog- ground moraine, and cover moraine. Till plains are the
raphy on account of their likeness to birds’ eggs. They thickest type and cover moraine the thinnest. A review of
are perhaps the most characteristic features of landscapes subglacial tills argues that they form through a range of
created by glacial deposition. The origin of drumlins is processes – deformation, flow, sliding, lodgement, and
debatable, and at least three hypotheses exist (Menzies ploughing – that act to mobilize and carry sediment and
1989). First, they may be material previously deposited lay it down in a great variety of forms, ranging from
beneath a glacier that is moulded by subglacial meltwater. glaciotectonically folded and faulted stratified material
to texturally uniform diamicton (Evans et al. 2006).
Moreover, owing to the fact that glacier beds are mosaics
of deformation and sliding and warm- and cold-based
conditions, most subglacial tills are likely to be hybrids
created by a range of processes active in the subglacial
traction zone. Nonetheless, glacial geologists can identify
three distinct till types (Evans et al. 2006):

1 Glaciotectonite – rock or sediment deformed by
subglacial shearing or deformation (or both) and
retaining some structural characteristics of the parent
material.
2 Subglacial traction till – sediment released directly
Plate 10.11 Rock-cored drumlin, New Zealand. from the ice by pressure melting or liberated from
(Photograph by Neil Glasser) the substrate (or both) and then disaggregated and