FEarth Science Program University of Calgary Calgary Alberta Canada

Hummocky terrain is comprised of tracts of hummocks and depressions of various sizes and shapes that occur in formerly glaciated areas. Traditionally, this terrain is known as 'hummocky moraine', and is believed to represent deposition via letdown at, or near, the ice margins during ablation. Hummocks therefore have been used to delineate recessional stages of glaciation in many regions. For example, the four prominent north-south trending hummocky complexes (Fig. 5.1) in Alberta, Canada, are commonly identified as terminal or recessional moraines (e.g. Klassen, 1989) deposited by letdown during deglaciation. Observations in these hummocky zones, however, do not support the letdown theory. Other researchers have proposed alternative geneses for hummocky terrain in the region but we propose that these hummocks were formed by erosion and, more specifically, subglacial meltwater erosion. We also suggest that as such variation in hummock theory exists, the descriptive term 'hum-mocky terrain' should replace the genetic term 'hummocky moraine'.

The four major north-south trending hummocky belts in Alberta are known traditionally as (from west to east): the Duffield Moraine, the Buffalo Lake Moraine, the Viking Moraine and the Coteau Moraine (Fig. 5.1). Hummock form in these 'moraines' is typical of forms in most hummocky regions worldwide. We divide these forms into six types based on shape and pattern (Fig. 5.2; Table 5.1). Sediment in the Albertan hummocks, however, is atypical of letdown at the ice-margins. For instance, the 'Buffalo Lake Moraine' contains material including lodgment and melt-out till, in situ and disturbed lake sediments, and local in situ and thrust bedrock (e.g. Kulig, 1985; Tsui et al., 1989; Munro & Shaw, 1997; Munro-Stasiuk, 2003). The 'Viking Moraine' contains pre-glacial lacustrine and glaciofluvial sediment, diamicton and in situ Cretaceous bedrock (Sjogren, 1999). The presence of in situ bedrock in some of these forms demonstrates that they are secondary; the product of erosion rather than deposition. In addition, exposures clearly show that intact regional lithostratigraphies and local sedimentary beds are truncated by hummock surfaces (Fig. 5.3). Thus, the hummocks are erosional, formed by excavation of the intervening depressions. Hummock surfaces are therefore representative of a landscape unconformity. This regional unconformity extends over a broad area at least as wide as the hummocky tracks (upwards of 50 km in places). The agent of erosion was one that involved movement, as simple observations note streamlining and transverse trends (type IV and V hummocks) towards the east-southeast and south-east(e.g. Munro & Shaw, 1997) (Fig. 5.2). These trends are similar to the other erosional fluted terrain observed in the region (Munro-Stasiuk & Shaw, 2002).

Sedimentary observations in the Buffalo Lake Complex point to a subglacial origin for the hummocks: subglacial eskers overlie the hummocks (Munro & Shaw, 1997), and the youngest recorded unit in the hummocks is a well-documented subglacial melt-out till (Munro-Stasiuk, 2000). Additionally, regionally consistent, strongly orientated clast fabrics in the till indicate ice movement towards the south-southwest, which is up to a 70° deviance from the surface trends noted in hummocks. Thus erosion of the hummock surfaces was therefore not contemporaneous with deposition of the underlying till; it occurred after till deposition.

We propose that the erosion was by subglacial meltwater and not by basal ice. Several lines of evidence support this: abrupt ero-sional surfaces are readily explained by fluvial erosion which removed sediment grain by grain, thus cutting into the underlying sediment but leaving beds undisturbed; surface boulders at many locations are best explained as fluvial lags resulting from lower flow competence in some areas; sorting of the lags suggests fluvial transport; many boulders are heavily pitted with percussion marks attesting to clast on clast collisions; type IV hummocks resemble fluvial bedforms and erosional marks produced on the underside of river ice (Ashton & Kennedy, 1972); and horseshoe-shaped troughs are wrapped around the upstream sides of some

Figure 5.1 Distribution of hummocky terrain in central and southern Alberta. Names traditionally assigned to the 'moraine' belts are shown.
Figure 5.2 Hummock types observed in central and southern Alberta. Types refer to those described in Table 5.1.
Figure 5.3 Representative exposures along Travers and McGregor Lake Reservoirs that illustrate the erosional nature of hummock surfaces.

Table 5.1 Hummock types and their description

Hummock type Proposed name

Type I

Type II Type III

Type IV

Type V

Type VI

Mounds with no discernible orientation or shape patterns Mounds with central depressions Linked mounds with central depressions

Ridged mounds

Elongate mounds

Moraine plateaux

Also known as

Stagnation moraine, disintegration moraine and uncontrolled moraine

Prairie doughnuts, rim/ring ridges and uncontrolled moraine Hummock chains, rim ridge chains, donut chains and both controlled and uncontrolled moraine Cross-valley ridges, transverse ridges, transversal morainic hummocks and controlled moraine Drumlinized hummocky moraine, corrugated moraine, humdrums and controlled moraine

Ice-walled lake plains and veiki plateaux


Chaotically distributed mounds with varying size and height

Mostly chaotically distributed mounds with minimal relief (<5m) containing a shallow central depression Mostly chaotically distributed linked mounds with minimal relief (<5m) containing a shallow central depression

Multiple semiparallel ridges resembling rogen moraine usually inferred to have formed at right-angles to ice flow direction Mounds contain a distinct elongation in the shape and are usually inferred to have formed parallel to ice-flow direction; frequently have asymmetric shape and horseshoe shaped troughs around their steepest edges Larger and higher than surrounding mounds; can range from a few metres to several kilometres across; surface is generally flat to undulating, and is commonly surrounded by a discontinuous rim elongate mounds (type V) and irregular shaped mounds (type I) suggesting scouring by horseshoe vortices generated at obstacles in the flow (e.g. Shaw, 1994).

While we propose that hummock formation was by subglacial meltwater erosion, in the absence of observations on hummock formation in the modern environment the agent and mechanics of hummock erosion are obviously open to debate. However, there is one major conclusion to be drawn from the Albertan observations: the Viking, Duffield, Buffalo Lake, and Couteau 'moraines' are not moraines. Consequently, reconstructions of Laurentide deglaciation in the prairies based on this assumption are misguided. We suggest that when relationships between underlying sediment and hummock surfaces are unknown, to avoid confusion and misinterpretation, the terms hummocky moraine, ice-disintegration moraine, stagnation moraine, and ice-stagnation topography should be abandoned in favour of 'hummocky terrain'.

Importantly, the interpretations presented here are based on specific field areas. As hummocks are known to form in subaqueous outwash at the margins of modern glaciers due to letdown and due to thrusting of debris bands, it is imperative that the morphology, sedimentology and structural relationships of hummocky terrain in each region of interest be studied thoroughly before determining landform genesis. Detailed descriptions and interpretations will lead to more accurate palaeoenvironmental reconstructions.

Was this article helpful?

0 0

Post a comment