The Drawing Of Hummocky Topography

Bands and tracts of hummocks commonly mark the former margins of the great ice sheets that covered Europe and North America during the Late Pleistocene. These landforms are best explained as forming by the collapse of abundant supraglacial debris during the melting of stagnant glacier ice, and hence are said to make up the supraglacial landsystem. It is also possible that subglacial squeezing of soft till contributes to landform development, and, in that case, this landsystem may be better described as the 'stagnant-ice landsystem.'

By 'lowland terrain' we refer to regions of moderately low relief at the margin of ice sheets. This refers primarily to the non-mountainous margins of the Laurentide and Scandinavian Ice Sheets, large areas of which are dominated by the supraglacial landsystem. The term also implies that the glaciers in these regions received little or no sediment directly from supraglacial sources: the vast bulk of sediment in these landforms was derived from subglacial sources and initially transported to the margin subglacially or englacially.

The most common landform in the supraglacial landsystem is the hummock, but this landsystem also includes ring forms, ice-walled-lake plains, dump moraines, outwash fans and disintegration ridges. In places, these landforms are superimposed on active-ice landforms. Regions of hummocky topography have been referred to as dead-ice moraine, hummocky moraine, stagnation moraine, ice-disintegration features and moraine-mound complexes, to name a few. They have also been referred to as end moraine where the hummocks occur in a distinct band, and ground moraine where hummocks are widespread and of low relief.

A supraglacial interpretation of this suite of landforms above was first developed in the 1940s and 1950s in Europe and North America (Milthers, 1948; Gravenor and Kupsch, 1959). Further studies in the 1960s in North America, Europe, and on modern glaciers (e.g. Clayton, 1964, 1967; Parizek, 1969; Boulton, 1967, 1968) strengthened a supraglacial interpretation. These studies were accompanied by investigations that pointed out the sedimentological complexity of the supraglacial environment (Hartshorn, 1958; Boulton, 1968, 1972a;

Marcussen, 1973; Lawson 1979; Eyles, 1979, 1983b and c), along with the corollary that several till layers of varying genesis could be deposited during one glacial cycle.

Early workers who supported a supraglacial explanation for these features also considered subglacial squeezing to be possible and important (Gravenor and Kupsch, 1959; Parizek, 1969). In recent years, the supraglacial theory has been challenged by proponents who think that subglacial pressing dominates the hummock-forming process or that hummocks are formed by subglacial meltwater erosion. These theories and others for hummock formation are listed below. Only one of the following theories (number 1) occurs in a supraglacial setting: 2 through 7 take place subglacially, 8 and 9 bear the stamp of englacial processes, and 10 and 11 occur proglacially. The theories include:

1. collapse of thick supraglacial debris lying on stagnant ice (Gravenor and Kupsch, 1959; Clayton, 1967; Boulton, 1967, 1972; Parizek, 1969; Clayton and Moran, 1974; Eyles, 1979, 1983b and c; Krüger, 1983; Paul, 1983; Sollid and Sorbel, 1988; Johnson et al., 1995; Ham and Attig, 1996; Mollard, 2000)

2. subglacial pressing of stagnant ice blocks into a soft bed (Hoppe, 1952; Stalker, 1960; Aartolahti, 1974, 1975; Eyles et al., 1999a; Boone and Eyles, 2001)

3. chaotic erosion by subglacial meltwater (Shaw, 1996; Munro and Shaw, 1997)

4. subglacial moulding by active ice, forming hummocks along with drumlins and Rogen moraine (Aario, 1977; Lundqvist, 1981)

5. subglacial glacitectonic thrusting forming 'cupola hills' (Aber et al., 1989; Evans, 2000b)

6. subglacial accumulation on stoss side of patches of frozen bed (Kleman et al., 1999)

7. patchy formation of ground ice beneath stagnant, cold-based glaciers (Aario, 1992)

8. melting of debris-rich stagnant ice containing extensive karst tunnels (Kemmis et al., 1994)

9. deformation of supraglacial and englacial debris by rising diapirs of clean ice (Minell, 1979)

10. groundwater blowouts (Bluemle, 1993; Boulton and Caban, 1995)

11. partly or wholly by periglacial means (Bik, 1968; Mollard, 2000).

Of these, we consider 1 the theory that best explains the observed morphology and sedimentology of these features, although we consider 2 possible as well. Of the remaining, we regard 3 and 9 as being unlikely, for a variety of reasons. Theories 4 through 8 and 10 and 11 likely occur, but are of only local importance or they produce hummocks not associated with the suite of landforms described in this chapter.

10.2 LANDFORMS

10.2.1 Hummocks

A landform with the shape of a hummock can be formed in a variety of environments, some non-glacial. Even in glaciated landscapes other than the supraglacial landsystem, there are likely to be found examples of incompletely formed Rogen moraine or drumlins, the forms of which could best be described as hummocky. But geologists working in glaciated terrain have rarely used 'hummock' in a purely descriptive sense. Rather, the term has been reserved for the knobs, hillocks and mounds occurring in widespread tracts and interspersed with the other landforms mentioned below. These are the hummocks we describe here.

Individual hummocks are round to elongate, conical to flat topped, and interspersed with depressions, which are sometimes called dead-ice hollows, ice-block depressions, or kettles. Hummocks occur in groups of thousands of individuals and generally display no preferred orientation of slopes or elongate elements (Figs. 10.1, 10.2 and 10.3).

As reported in the literature, hummocks are 15-400 m in diameter and are spaced 150-500 m apart (Table 10.1). The relief of hummocks ranges from 2 to 70 m, but most are less than 20 m. Gravenor and Kupsch (1959) classified hummocks in classes of relief:

As reported in the literature, hummocks are 15-400 m in diameter and are spaced 150-500 m apart (Table 10.1). The relief of hummocks ranges from 2 to 70 m, but most are less than 20 m. Gravenor and Kupsch (1959) classified hummocks in classes of relief:

Figure I0.I Aerial photographs of hummocks. These hummocks lack a preferred orientation of elements and are referred to as uncontrolled. A) A hummock tract 35 km north of Vilnius, Lithuania, from archives of the Geological Survey of Lithuania; ice flow direction was to the southeast. B) A stereopair showing high-relief hummocky topography with ice-walled lake plains (A) and some disintegration ridges (B) approximately 125 km southeast of Edmonton, Alberta, Canada (Photo number 160-5216: I368-I0 and -II, Location, T. 46, R. I2, W. 4th. Mer., from Technical Division, Department of Lands and Forests, Edmonton, Alberta). C) Hummocky topography from northwest Wisconsin (BRO-2AA-95, U.S. Department of Agriculture); ice-flow direction was approximately S 30 E, parallel to the elongate lakes.

Figure 10.2 Topographic maps of hummocky landscapes. A) Mosaic of topographic maps showing the Harrison Hills of northern Wisconsin (Ham and Attig, 1997); ice-flow direction was approximately S 45 E. Contour interval is 10 ft except in the upper-right portion where it is 20 ft. The Harrison Hills are dominated by high-relief hummocks (up to 70 m) that were formed along the Late Wisconsin margin of the Wisconsin Valley Lobe. Numerous ice-walled-lake plains are present, two of which are identified. These ice-walled-lake plains are examples of the stable ice-walled-lake environment (see Fig. 10.12). Several ice-contact ridges and outwash fans are present, some of which are identified. B) Mosaic of topographic maps from western Wisconsin near the Late Wisconsin margin of the Superior lobe showing hummocks and three ice-walledlake plains with rim ridges; contour interval 10 ft (Johnson, 2000). These ice-walled-lake plains are examples of the unstable ice-walled-lake environment (see Fig. 10.12); ice-flow direction was approximately S 50 E. C) Hummocks from southern part of the island of Sealand, Denmark, contour interval 2.5 m (Krüger, 1969).

Figure 10.2 Topographic maps of hummocky landscapes. A) Mosaic of topographic maps showing the Harrison Hills of northern Wisconsin (Ham and Attig, 1997); ice-flow direction was approximately S 45 E. Contour interval is 10 ft except in the upper-right portion where it is 20 ft. The Harrison Hills are dominated by high-relief hummocks (up to 70 m) that were formed along the Late Wisconsin margin of the Wisconsin Valley Lobe. Numerous ice-walled-lake plains are present, two of which are identified. These ice-walled-lake plains are examples of the stable ice-walled-lake environment (see Fig. 10.12). Several ice-contact ridges and outwash fans are present, some of which are identified. B) Mosaic of topographic maps from western Wisconsin near the Late Wisconsin margin of the Superior lobe showing hummocks and three ice-walledlake plains with rim ridges; contour interval 10 ft (Johnson, 2000). These ice-walled-lake plains are examples of the unstable ice-walled-lake environment (see Fig. 10.12); ice-flow direction was approximately S 50 E. C) Hummocks from southern part of the island of Sealand, Denmark, contour interval 2.5 m (Krüger, 1969).

• intermediate-relief hummocks — 10—25 ft (3—8 m), and

• high-relief hummocks — greater than 25 ft (8 m).

This distinction may be important in terms of genesis. For example, Ham and Attig (1996) describe high-relief hummocks in north-central Wisconsin, USA, that occur in distinct tracts amidst broad regions of lower-relief hummocks (Fig. 10.2A), implying that the two types of hummocks have had a different developmental history. Other than these rough measurements of hummock dimensions, we have not attempted a detailed analysis of the variation of hummock size, shape, slope and spacing. Considering the modern abilities of imaging technology this may be a rather easy and fruitful area of future geomorphic research.

Most authors emphasize the haphazard orientation of linear elements in hummocky terrains, which is the result of what Gravenor and Kupsch (1959) refer to as uncontrolled deposition (Fig. 10.1 and 10.2). However, in some regions, hummock tops are oval to elongate (they are

1 km

1 mi

1 km

Figure 10.3 Aerial photographs showing controlled collapse features. A) Stereopair of linear disintegration ridges located about 200 km east of Edmonton, Alberta, Canada. Ridges are interpreted to represent the location of crevasses in the ice (ice flow slightly east of south) that were filled by supraglacial debris or squeezed subglacial till (Gravenor and Kupsch, 1959) (Photo number 160-5303:1333-36 and -37, Location: T. 48, R.1, W. 4th Mer., from Technical Division, Department of Lands and Forests, Edmonton, Alberta). B) Stereopair of aligned hummocks forming ridges about 65 km SE of Calgary, Alberta, Canada showing control by structures within the ice, presumably thrust planes near the ice margin (Gravenor and Kupsch, 1959); ice flow was to the southeast.

rarely exactly circular), or the hummocks are grouped in clear patterns. Such an inherited geometry is referred to as controlled deposition (Gravenor and Kupsch, 1959; Fig. 10.3). These hummocks are interpreted to have inherited their orientation from similarly oriented structures in the parent ice, mainly crevasses (Fig. 10.3A) and thrust planes (Fig. 10.3B). For example, controlled deposition has been explained as resulting from melting of debris-rich thrust zones to produce hummocks in Scotland (Hambrey et al., 1997; Bennett et al., 1998), the washboard

Reference

Location

Relief

Diameter

Spacing

Slope

(m)

(m)

(m)

angle

Gravenor, 1955

Canada (Alberta)

5

90

Klassen, 1993

Canada (Saskatchewan)

5-20

Schou, 1949

Denmark

5-25

50-200

150-300

Aartolahti, 1975

Finland

15-25

200-300

Okko and Perttunen, 1971

Finland

3-5

20-100

Möller, 1987

Sweden

2-5

15-50

Clayton, 1967

USA (North Dakota)

7-70

5-15°

Ham and Attig, 1996

USA (Wisconsin

5-70

300-500

Johnson et al., 1995

USA (Wisconsin)

5-20

25-400

250-500

2-14°

TOTAL RANGE

2-70

15-400

150-500

2-15°

Table 10.1 Dimensions of hummocks

Table 10.1 Dimensions of hummocks moraines of the Canadian Prairies (Gravenor and Kupsch, 1959), and aligned hummock tracts of Iowa (Colgan, 1996). Additionally, direct deposition in crevasses produces disintegration ridges (see below), and this is also a type of controlled deposition. Controlled formation of hummocks may not be obvious with just a quick inspection of maps or aerial photographs, but may be revealed after measurement of linear elements in a hummock tract (Johnson et al., 1995; Evans, 2000b).

Hummocks are composed entirely or partly of till, stream sediment and lake sediment. Hummocks composed of collapsed lake sediment that displays faults and folds are described in North Dakota (Clayton and Cherry, 1967) and Wisconsin (Attig, 1993). Collapsed stream sediment with hummocky topography is widespread in many glaciated regions and may even be the predominant feature in some areas. A collapse origin is shown by topography, sedimentology and normal faults that cut sedimentary structures.

Many hummocks contain a complex internal stratigraphy consisting of interbedded till and stratified sediment, often with normal faults, and these are well described from the margins of modern glaciers (e.g. Boulton, 1972a; Lawson, 1979; Krüger, 1994a) as well as in Pleistocene examples from northern Europe (Krüger, 1969; Stephan, 1980; Haldorson, 1982, Möller, 1987; Malmberg Persson, 1991) and North America (Sharpe, 1988; Attig and Clayton, 1993; Johnson et al., 1995; Munro and Shaw, 1997) (Figs. 10.4 and 10.5). These sedimentological relationships are almost always interpreted as developed by the interbedding of supraglacial flow till or melt-out till with stream or lake sediment, which subsequently collapses from melting of underlying ice, forming faults and other disruptions of bedding.

However, the most common type of hummock in hummocky regions are those composed entirely or predominantly of till, and these have been described in Canada (Gravenor and Kupsch, 1959; Stalker, 1960) and the Upper Midwest of the USA (Clayton, 1967; Kemmis et al., 1981; Mickelson, 1986; Hansel and Johnson, 1987; Johnson et al., 1995; Ham and Attig, 1996). The origin of the till in these hummocks has been the subject of debate in the literature.

rV>~-| S!lt ant' day "j Sand ' ■ \| Gravel

Diamicton

With silt beds

Faulted

Colluvium

Colluvium

_-j Rhythmic bedding with clay

Colluvium

Up to 12 m thick

Crudely bedded

Laminated

Pebbly and sandy

Colluvium

Colluvium

Rhythmic bedding Sandy

<1 •

A

A ■

A

A

A

A

A ■

A

A

- A

A

- 1

A

A

A 4 ■"

• A

A A ■

- z

A

A

A

A

Sandy silty Sandy

Flow-til layers ing-upward sequence

Flow till

Melt-out till ing-upward sequence

Flow till

Melt-out till

Row-till layers

i ■

A . •

A

- A. .. ■A •

Z

A . •

A

A

A

- A. .. A •

■ _ 1

A . •

A

- A. .. A •

1

A . •

A

A

- A. .. A •

■ _ 1

A . •

A

- A. . . A •

-

A . •

A

A

- A. .. A •

■ _ 1

Flow-till layers separated by breaks and lenses of sorted sediment

Flow-till layers separated by breaks and lenses of sorted sediment

Melt-out till

Figure 10.4 Vertical profiles of sediment described from Pleistocene hummocks on the Wollaston Peninsula, northern Canada, and from modern glaciers. These sequences show interbedded till and sorted sediment in various proportions. A and B = sequences from hummocks on the Wollaston Peninsula (Sharpe, 1988), C = Idealized section of hummock sediment from the Matanuska Glacier, Alaska (Lawson, 1981), D, E, and F = Idealized sections from several modern glaciers showing trough (depression) fillings on the glacier surface where troughs receive mixed flow till and sorted sediment (D), predominantly sorted sediment (E), or predominantly flow till (F). (Paul, 1983).

_-j Rhythmic bedding with clay

Up to 12 m thick

Crudely bedded

Laminated

With silt beds

Faulted

Colluvium

Back Yard Down Hill Terrace

¡53 Boulders and stones BUI Gravel y=|=J] Silty sand | | Sand 1-7 grain-size analyses | | Diamicton

S1=0.53

¡53 Boulders and stones BUI Gravel y=|=J] Silty sand | | Sand 1-7 grain-size analyses | | Diamicton

S1=0.53

Sand and gravel |A a| Diamicton

Sylvan Lake site

Because the topography of till hummocks is identical to hummocks composed of collapsed lake sediment and outwash, it is likely that most till hummocks formed in the same way, that is, by supraglacial collapse.

Another argument indicating that till hummocks are supraglacial collapse features is their close association with ice-walled-lake plains (described in more detail below). Fossils in ice-walled-lake plain sediment in North Dakota indicate that ice-walled lakes existed long after the period of initial ice stagnation into a time of warmer climate. Such a climate would cause rapid ablation unless the surrounding stagnant ice was covered with thick supraglacial debris (Tuthill, 1967; Clayton and Cherry, 1967). These ice-walled-lake plains are today surrounded by the hummocks produced when the insolating supraglacial debris was eventually let down by the melting of the underlying ice.

The few sedimentological studies that exist also indicate a supraglacial-collapse origin for till hummocks. For example, Ham and Attig (1996) described high-relief hummocks (Fig. 10.2A) with thick uniform till that they interpret to be flow till. They observed flow-till bedding dipping into the centres of hummocks, presumably from an adjacent supraglacial source, as well as pebble fabrics that are randomly oriented, suggesting flow till. Johnson et al. (1995) describe hummocks with thick uniform till that they interpret to represent supraglacial melt-out till. They cite till fabric measurements that parallel regional ice-flow direction (Fig. 10.5C), a relationship that would not be expected with a flow-till or a squeezed-till hypothesis. Supraglacial sandy melt-out till, like outwash, would be well drained and would collapse by faulting leaving the till between faults undisturbed. Recently, the till of hummocks on the Canadian Great Plains has been reinterpreted as subglacial squeezed till (Eyles et al., 1999a; Boone and Eyles, 2001), but this interpretation is not based on sedimentological measurements.

The matrix grain-size composition of till has an effect on the geomorphic characteristic of hummocks. Boulton (1972a) noted that till with higher water content would flow more easily, producing hummocks with lower slopes. Thus, hummocks composed of poorly drained, clayey till would have lower slopes and generally lower relief than hummocks composed of well-drained, sandy till. The more poorly drained clayey sediment would tend to collapse and flow,

Figure 10.5 Sketches of exposures through hummocks containing mixed till and sorted sediment. A) Outcrop sketch from a hummock in southern Sweden showing interbedded till and sorted sediment. Weak fabrics are found in the till layers and interpreted to indicate flow till (Malmberg Persson, 1991) (reprinted with kind permission from the author and the Geological Society of Sweden). B) Outcrop sketch of till with some sorted sediment from a hummock in southern Sweden. Weak fabrics are interpreted to indicate flow till (Andersson, 1998) (SI = primary eigenvalue) (reproduced from 'Genesis of hummocky moraine in the Bolmen area, southwestern Sweden,' by G. Andersson, from Boreas, www.tandf.no/boreas, 1998, volume 27, pages 55-67, by permission of Taylor and Francis AS). C) Outcrop sketch of a portion of a hummock in northwestern Wisconsin showing flow till (fabric B) and sorted sediment overlying thick, uniform till with strong, ice-flow-parallel fabric (fabric A) (SI = primary eigenvalue) that is interpreted as melt-out till (reproduced from 'Composition and genesis of glacial hummocks, western Wisconsin,' by M.D. Johnson, D.M. Mickelson, L. Clayton, and J.W. Attig, from Boreas, www.tandf.no/boreas, 1995, volume 24, pages 97-116, by permission of Taylor and Francis AS).

where the better-drained, stiff sandy sediment would flow less often and fault, and produce a stable debris cover that would insulate the glacier, cause ice to melt slowly, and allow for the preservation of melt-out till. This difference in grain size explains why high-relief hummocks in Wisconsin are associated with sandy till (Johnson et al., 1995; Ham and Attig, 1996), and

Lithuania Last Glacial MarginLithuania Last Glacial Margin

Figure 10.6 Photos showing ring forms. A) Stereopair of ring forms composed of thin lake sediment over clay till southwest of Long Valley, Saskatchewan, Canada, highlighted by a prairie-grass fire (A6729-I2 and 13 (R4I-I2 and 13); Lat. 50° 42', Long. 107° 03'). B) Stereopair of ring forms composed of clay till east of Kenaston, Saskatchewan, Canada (95675-08 68 and 69; Lat. 5I° 33', Long. I06° 08'). C) Stereopair of low-relief ring forms in clay till north of Steelman, Saskatchewan, Canada (A2I749-I4 and I5; Lat. 49° 22', Long. I02° 37').

Figure 10.6 Photos showing ring forms. A) Stereopair of ring forms composed of thin lake sediment over clay till southwest of Long Valley, Saskatchewan, Canada, highlighted by a prairie-grass fire (A6729-I2 and 13 (R4I-I2 and 13); Lat. 50° 42', Long. 107° 03'). B) Stereopair of ring forms composed of clay till east of Kenaston, Saskatchewan, Canada (95675-08 68 and 69; Lat. 5I° 33', Long. I06° 08'). C) Stereopair of low-relief ring forms in clay till north of Steelman, Saskatchewan, Canada (A2I749-I4 and I5; Lat. 49° 22', Long. I02° 37').

low-relief hummocks of Alberta, Saskatchewan and North Dakota are associated with clayey till (Clayton, 1967).

10.2.2 Ring Forms

A type of landform common on the Great Plains of central North America are fields of low-relief rings; circular ridges with depressions in their centres (Fig. 10.6). These ring forms have also been referred to as doughnuts, circular disintegration ridges, closed ridges, rim ridges, rimmed kettles and humpies (Gravenor and Kupsch, 1959; Parizek, 1969; Mollard, 2000). They are essentially a type of hummock, but one with a central depression. Most of these ring forms are clearly glacial in origin, but some associated with lake sediment in cold regions likely have a periglacial origin.

Ring forms in North Dakota, Alberta and Saskatchewan (Fig. 10.6) are up to 200 m across, are generally of low relief, uniform size, and uniform spacing, and composed predominantly of clayey till. Nearly identical ring forms can form tracts consisting of hundreds or thousands of rings. Ring forms are much less common in sandy-till regions, though some composed of sandy till have been reported in northern Sweden (Melander, 1976). Ring forms can be formed in several ways (Fig. 10.7). According to several authors (Gravenor, 1955; Clayton, 1967; Mollard, 2000), these features form supraglacially by flowing of till into a glacier sinkhole (Fig. 10.7A, B). A subglacial-squeezing origin (Gravenor and Kupsch, 1959; Aartolahti, 1974; Eyles et al., 1999a; Mollard, 2000) suggests that ring forms may be produced by till being squeezed into the sinkhole from below (Fig. 10.7C) or by squeezing up around individual, foundering ice blocks.

10.2.3 Ice-Walled-Lake Plains

Ice-walled-lake plains (Figs. 10.2, 10.8, 10.9, 10.10) form where sediment accumulates in broad, water-filled sinkholes in stagnant ice. After the ice completely melts, the sediment

Figure 10.7 Mechanisms for the formation of ring forms. A) A supraglacial collapse mechanism where a block of ice is insulated by flow till in an ice depression (Clayton, 1967).

B) A supraglacial collapse mechanism where flow till is deposited around the margins of sinkhole.

C) A subglacial squeezing mechanism for the formation of hummocks and ring forms. See text for description of a fourth mechanism.

Figure 10.7 Mechanisms for the formation of ring forms. A) A supraglacial collapse mechanism where a block of ice is insulated by flow till in an ice depression (Clayton, 1967).

B) A supraglacial collapse mechanism where flow till is deposited around the margins of sinkhole.

C) A subglacial squeezing mechanism for the formation of hummocks and ring forms. See text for description of a fourth mechanism.

| | Fine offshore sediment

| | Fine offshore sediment

Figure 10.8 An ice-walled-lake plain in North Dakota, USA. A) Portion (shown in B) of the Ross 7.5' series topographic map showing a rim ridge with ice-walled-lake plain to the north and hummocks to the south; contour interval 10 f B) Geologic interpretation of the features in A.

remains as a roughly circular, flat-topped landform. Ice-walled-lake plains are rounded in map view, similar to the shape of lakes where headlands tend to be eroded and the lake shape smoothed out by erosion. They stand out amidst the surrounding hummocky terrain by being flat to dish-shaped, and they are often, but not always, marked on their edge by distinct rim

Hummocks isrands-kames

Ji'-.^. uregelm&ssige ■■■i-"'^ mor&nebakker

End moraines

^^^ israndsbakker, mindre

Ice-walled-lake plains

(^¡fi Plateaubakker

Hat-formed hills esker hatformade bakker

Hummocks isrands-kames

Ji'-.^. uregelm&ssige ■■■i-"'^ mor&nebakker

End moraines

^^^ israndsbakker, mindre

Ice-walled-lake plains

(^¡fi Plateaubakker hatformade bakker

Figure 10.9 A portion of Smed's (1962) geomorphologic map of the island of Funen, Denmark. Only selected geomorphic features from Smed's map are shown, and we have identified these as hummocks, end moraines, ice-walled-lake plains, hat-formed hills, and eskers. Names in italics are Danish terms for these features. Among the features present in the area but not shown on the map include outwash plains, tunnel channels and drumlins, none of which are prominent in this area dominated by supraglacial landsystem landforms. These stagnant-ice features developed in ice left from the East Jylland advance. Prominent Danish ice-margins are shown in the inset: H = Main Stationary Line, E = East Jylland ice limit, B = B^lthav advance ice limit, R = R.0snes advance ice limit (from Houmark-Nielsen, 1983). End moraines shown on the map are associated with the B^lthav advance.

Figure 10.9 A portion of Smed's (1962) geomorphologic map of the island of Funen, Denmark. Only selected geomorphic features from Smed's map are shown, and we have identified these as hummocks, end moraines, ice-walled-lake plains, hat-formed hills, and eskers. Names in italics are Danish terms for these features. Among the features present in the area but not shown on the map include outwash plains, tunnel channels and drumlins, none of which are prominent in this area dominated by supraglacial landsystem landforms. These stagnant-ice features developed in ice left from the East Jylland advance. Prominent Danish ice-margins are shown in the inset: H = Main Stationary Line, E = East Jylland ice limit, B = B^lthav advance ice limit, R = R.0snes advance ice limit (from Houmark-Nielsen, 1983). End moraines shown on the map are associated with the B^lthav advance.

Figure 10.10 Stereopair of an ice-walled-lake plain in Mountrail County, North Dakota USA (BAL-4V-9 and 10, USDA). The ice-walled-lake plain is surrounded by hummocks and ring-forms.

Reference

Location

Relief (m)

Diameter (m)

Area (km2)

Thickness of lake sediment (m)

Stalker, I960

Canada (Alberta)

2-5

6-180 (90)

Up to 30

'Thin'

Parizek, 1969

Canada (Saskatchewan)

7-50

Up to I3

1-10

Klassen, 1993

Canada (Saskatchewan)

5-25

200-800

Brehmer, 1990

Denmark

I0

200-750

Hansen, 1940

Denmark

10-15

I50-4,000

Schou, 1949

Denmark

5

300-700

Smed, 1962

Denmark

25-35

500-2,000

2-5

Strehl, 1998

Germany

9-13

300-1,200

0.1-1.0

9-18

Bitinas, I992

Lithuania

I0-50

1-30

Westergard, 1906

Sweden

I5-30

500-4,500

1-3

Clayton and Cherry, 1967

USA (North Dakota)

400-2,500

0.5-13

Ham and Attig, 1996

USA (Wisconsin)

100-1,500

15

Johnson et al., 1995

USA (Wisconsin)

5-10

500-4,000

I-I3

20

Syverson, 2000

USA (Wisconsin)

40-60

500-1,000

50 (stable environment)

Syverson, 2000

USA (Wisconsin)

I0-35

1,000-1,500

23 (unstable environment)

TOTAL RANGE

2-60

I00-4,500

0.1-30

Table 10.2 Dimensions of ice-walled-lake plains

Table 10.2 Dimensions of ice-walled-lake plains ridges that rise up to 10 m above the central parts of the ice-walled-lake plains. The rim ridges slope gently towards the plain centres, but have slopes close to angle of repose on their outer slopes. Ice-walled-lake plains are generally 1—15 km2 in area but can be up to 30 km2 (Table 10.2). They may occur as isolated forms but generally appear in clusters amidst their hummocky surroundings (Figs. 10.2B, 10.9).

Ice-walled-lake plains were referred to as plateau clay-hills by Milthers (1948) and Schou (1949) in Denmark, and as moraine plateaux by Stalker (1960) in Canada. The high kames of southern New England are likely ice-walled-lake plains (Stone and Peper, 1982) as are rimmed Veiki moraine plateaux of northern Sweden (Hoppe, 1952; Lagerbäck, 1988). Ice-walled-lake plains have also been described in Poland (Niewiarowski, 1963), Lithuania (Bitinas, 1992), Germany (Strehl, 1998), and Russia (Ekman et al., 1981).

Ice-block depressions may occur in ice-walled-lake plains, and there are tracts of hummocky topography underlain entirely by collapsed lake sediment (Figs. 10.11A and B). However, as noted above, the majority of ice-walled-lake plains show little sign of collapse in their centres, and this is due to the ability of lakes, once formed on the ice surface, to melt completely through the stagnant ice due to the high heat content of water. Multiple rims positioned concentrically within an ice-walled-lake plain indicate progressive widening of the ice-walled lake, with each rim forming from ice-contact sedimentation (Fig. 10.11D).

Clayton and Cherry (1967) described two end-member types of ice-walled-lake plains. The unstable environment type (Figs. 10.2B, 10.12) is formed in ice with thin surface debris, resulting in ice-walled-lake plains of lower relief but having well-defined rim ridges. Stable environment settings occur where debris cover is thick, and the ice-walled-lake plains that result have higher relief, lack rim ridges, have thick till and are often among the highest points in the landscape (Figs. 10.2A, 10.12). Because of the ability of thicker till to insulate ice, stable environment lakes last a longer period of time and have the potential of preserving long sediment records.

Rim ridges on ice-walled-lake plains are composed of sand and gravel interbedded with layers of flow till (Fig. 10.11C). Deltaic sediment underlies rim ridges (Stalker, 1960; Johnson, 1986, 2000; Syverson, 2000), and indicates stream flow on the adjacent ice surface. The sorted sand and gravel in the ridges is interbedded with beds of flow till dipping towards the centres of the plains, indicating flowage from the adjacent ice surface. In a few places, till makes up the entire rim ridge (Parizek, 1969), some of which may be subglacial-squeeze till (Stalker, 1960).

The centres of ice-walled-lake plains contain finer sediment, which is clayey or silty, well bedded and even varved in places (Hansen, 1940; Smed, 1962; Stalker, 1960; Brehmer, 1990). In Denmark, an asymmetrical distribution of grain sizes in ice-walled-lake plain sediments (e.g. sandier on the southern half, finer on the northern half) has been interpreted to represent, for this example, dominantly northward flow of sediment-laden streams on the stagnant-ice surface (Smed, 1962; Brehmer, 1990).

There is a range of ice-walled-lake plains composed entirely of lake sediment to ones composed entirely of till, even within a given geographic region (Stalker, 1960). Most ice-walled-lake plains seem to contain sorted sediment that accounts for the total relief of the plain, and lake sediment can be 20-50 m thick (Fig. 10.13B) (Stalker, 1960; Clayton and Cherry, 1967;

Glacial Marine StratigraphyHummocky ReliefDiagram Frogs Mouth

Figure 10.11 Features of ice-walled-lake plains. A) and B) Sketches showing the development of an ice-walled-lake plain near Lehr, North Dakota in which a portion of the ice-walled-lake plain collapses. C) Outcrop sketch from the rim of an ice-walled-lake plain in western Wisconsin showing deltaic sediment (Johnson, I986). D) Map of ice-walled-lake plain in western Wisconsin showing multiple rims of different age (Johnson, I986). The dashed lines are local township boundaries.

Figure 10.11 Features of ice-walled-lake plains. A) and B) Sketches showing the development of an ice-walled-lake plain near Lehr, North Dakota in which a portion of the ice-walled-lake plain collapses. C) Outcrop sketch from the rim of an ice-walled-lake plain in western Wisconsin showing deltaic sediment (Johnson, I986). D) Map of ice-walled-lake plain in western Wisconsin showing multiple rims of different age (Johnson, I986). The dashed lines are local township boundaries.

Unstable ice-walled-lake plain development

Unstable ice-walled-lake plain development

Ice Walled Lake Plains
Medium-relied
Stable ice-walled-lake plain development
Ice Age Lake Origen
(D) High-relief Ice-walled-lake plain

Figure 10.12 Figure showing the differences between unstable and stable ice-walled-lake plains. A) and B) illustrate the development of the unstable ice-walled-lake plain, where supraglacial debris is relatively thin and the surrounding thin ice melts fairly rapidly. The icewalled-lake plain features well-developed rims and is surrounded by low-to-medium relief hummocks. C) and D) illustrate the stable ice-walled-lake plain, where supraglacial debris is thick, and the ice-walled lake exists for a comparatively longer period of time. The ice-walled-lake plain lacks well-developed rims and is surrounded by high-relief hummocks. (From Clayton and Cherry, 1967).

Figure 10.12 Figure showing the differences between unstable and stable ice-walled-lake plains. A) and B) illustrate the development of the unstable ice-walled-lake plain, where supraglacial debris is relatively thin and the surrounding thin ice melts fairly rapidly. The icewalled-lake plain features well-developed rims and is surrounded by low-to-medium relief hummocks. C) and D) illustrate the stable ice-walled-lake plain, where supraglacial debris is thick, and the ice-walled lake exists for a comparatively longer period of time. The ice-walled-lake plain lacks well-developed rims and is surrounded by high-relief hummocks. (From Clayton and Cherry, 1967).

Johnson et al., 1995; Ham and Attig, 1996; Syverson, 2000). However, in a few places, the thickness of lake sediment is greater than the relief of the ice-walled-lake plain (Strehl, 1998; Johnson et al., 1995; Fig. 10.13C). This is significant, because such a relationship would not be expected if subglacial pressing is called on to explain the formation of surrounding hummocks.

Some ice-walled-lake plains are composed nearly entirely of till, with only a thin surficial cover of lake sediment (Fig. 10.13A; Table 10.2), and the term 'moraine plateau' has been applied by some to this type of ice-walled-lake plain. Ice-walled-lake plains with thin lake sediment have been described in Canada, the USA, Lithuania, Sweden and Denmark (Westergard, 1906; Hansen, 1940; Lagerbäck, 1988; Brehmer, 1990; Bitinas, 1992; Klassen, 1993). The till in these forms is likely flow till derived from the surrounding ice surface, although some melt-out till and squeezed till may occur towards the base of the form. In southern Minnesota, the flat-topped circular hills of Patterson (1997b) are made entirely of bedded till, which she interprets as indicating supraglacial flow till. It is unlikely that squeezing can account for all the till in these forms. Stalker's (1960) squeezing hypothesis was applied only to till in the rim ridges and not across the centre of the ice-walled-lake plains.

10.2.4 Disintegration Ridges

Crevasses open to the surface can localize deposition of supraglacial materials. As stagnant ice continues to disintegrate, crevasses extending through the ice to the bed may allow soft

Figure 10.13 Sketches of ice-walled-lake plains containing a range of thickness of lake sediment from A) thin to B) as thick as the relief of the plain to C) a thickness greater than local relief. Examples of A) are described by Hansen (1940), Westergard (1906), Bitinas (1992), Stalker (I960), and Klassen (1993); examples of B) are described by Clayton and Cherry (1967), Johnson (1986), and Syverson (2000) and are thought to be the most common type; examples of C) are described by Johnson et al. (1995) and Strehl (1998). A), B), and C) can all form supraglacially, and the till in the lake plain would be flow till derived from the ice surface. Though the type in A) could be formed with subglacial pressing, subglacial pressing would be difficult to form type B) and impossible to form type C).

Figure 10.13 Sketches of ice-walled-lake plains containing a range of thickness of lake sediment from A) thin to B) as thick as the relief of the plain to C) a thickness greater than local relief. Examples of A) are described by Hansen (1940), Westergard (1906), Bitinas (1992), Stalker (I960), and Klassen (1993); examples of B) are described by Clayton and Cherry (1967), Johnson (1986), and Syverson (2000) and are thought to be the most common type; examples of C) are described by Johnson et al. (1995) and Strehl (1998). A), B), and C) can all form supraglacially, and the till in the lake plain would be flow till derived from the ice surface. Though the type in A) could be formed with subglacial pressing, subglacial pressing would be difficult to form type B) and impossible to form type C).

subglacial till to squeeze into crevasses from below. These processes may result in linear to hummocky landforms that have a complex internal structure. Gravenor and Kupsch (1959) call these features linear disintegration ridges, a type of controlled deposition (Fig. 10.1B, 10.3A). The features they describe are 1-10 m high, 8-100 m wide, and extending from a few metres to up to 10 km in length. The ridges may form two sets in a boxwork pattern resembling ice-crevasse patterns (Fig. 10.3B). They are made mostly of till, although crevasse sediments can contain stratified material as well. Gravenor and Kupsch prefer 'disintegration ridge' to 'crevasse fill' because they recognize that sediment may get into the ridge by subglacial squeezing. Subglacial squeezing into crevasses has been called on to explain radial till ridges in Finland (Aartolahti, 1995), minor moraines in Minnesota (Patterson, 1997b), and disintegration ridges in Spitzbergen (Boulton et al., 1996). In addition to ridges associated with crevasses, the term 'disintegration ridge' has also been applied to sinuous, curved and irregular ridges of unknown genesis that are associated with other landforms of the supraglacial landsystem.

10.2.5 Ice-Contact Dump Ridges

Supraglacial debris transported off the terminal margin of an ice sheet as flow till or outwash may form a dump ridge. This ridge may form at the margin of an active glacier, in which case it is an end moraine, or at the peripheral margin of a stagnant ice mass. Melting of ice may collapse parts of the ridge producing hummocks. Dump ridges have been observed to form this way along modern glaciers (Boulton, 1968; Eyles, 1979, 1983b; Krüger, 1994a). Sediment in dump ridges flows from supraglacial positions close to the ice margin where englacial material had been released from the ice by ablation. Figure 10.14 shows a topographic map of such a dump ridge in central Minnesota, USA, together with an interpretation of its origin. Similar ridges have been described in Poland (Kasparek and Kozarski, 1989; Kozarski, 1981). Ham and Attig (1996) show examples of ice-contact ridges formed concentrically around a former stagnant ice mass (Fig. 10.2A).

10.2.6 Outwash Fans

Though the majority of glacial outwash deposits are derived from subglacial streams emerging at the ice margin, small outwash fans can develop derived solely from supraglacial material. These have been well described by Krüger (1997) who refers to these fans as hochsander fans, a term first used by Gripp (1975). Hochsander fans are finer grained than other outwash fans, being primarily composed of horizontally stratified sand with some gravel lenses: coarser material is often preferentially left on the glacier surface. Vertical variations in the sedimentology of the fan deposit reflect the character of surface processes and variations in rainfall and surface drainage. Similar fans of Pleistocene age occur in Denmark, Germany, and Poland (Krüger, 1997), as well as Wisconsin, USA, (Ham and Attig, 1996) where the source of sediment for the fans was a slowly melting debris-covered, stagnant-ice mass (Fig. 10.2A). Like the dump ridges described above, outwash fans need not form at an active ice margin.

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Responses

  • Henri
    What is hummocky topography?
    7 months ago

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