Glaciolacustrine Landforms

There is a wide variety of different types of glacial lake, although two broad types can be identified: those that form along an ice margin; and those that occur in a supraglacial setting. Ice-marginal lakes may form in front of glaciers or when ice dams water in a valley or against a hill side (see Figure 2.7). Supraglacial lakes can develop either where an ice-dammed lake expands over an ice margin (Figure 11.2) or in areas of complex ice-cored topography. The geomorphological products of these two categories of lake are different and will be considered in turn.

Figure 11.2 Supraglacial lake submerging part of an ice margin in Greenland. Note the debris bands on the glacier surface formed by ice-marginal thrusting. [Photograph:

Figure 11.2 Supraglacial lake submerging part of an ice margin in Greenland. Note the debris bands on the glacier surface formed by ice-marginal thrusting. [Photograph:

There are two types of glacier-lake interface at ice-marginal lakes; those at which large fans or deltas form, separating the ice margin from the lake, and those at which the ice front is actively calving (Figure 11.3). At calving margins small fans and moraine banks tend to form depending on whether the ice margin is stationary and upon the amount of meltwater discharge. The development of large deltas is, for example, favoured by a stable ice margin with a high meltwater-sediment discharge. The landforms associated with these different lake margins and with lakes in general are reviewed below.

A: Calving margin

Grounding line

B: Ice-contact delta

B: Ice-contact delta

Figure 11.3 Type of glacier-lake interface. (A) Calving margin. (B) Ice-contact delta-type margin.

Figure 11.3 Type of glacier-lake interface. (A) Calving margin. (B) Ice-contact delta-type margin.

1. Glaciolacustrine landforms at non-calving ice margins. At a stationary ice margin with high meltwater discharge an ice-contact delta may develop. Where sediment is delivered from a single or narrowly confined group of channels, delta fronts are arcuate in planform. In contrast, where meltwater streams switch from one side of a valley to another the delta front may be more linear in planform following the outline of the ice margin. The ice-contact face of the delta may be deformed and raised by post-depositional ice pushing and this ice-contact face is frequently subject to collapse and slumping when the ice support is removed. A delta moraine may develop where an ice front remains stationary for a considerable period of time. A delta moraine consists of a series of deltas or fans that coalesce along an ice margin to form a continuous ridge. The moraine is not formed from one but from many fans and deltas. Probably the longest delta moraine exposed on land is the Salpausselka Moraine in Finland, which is over 600 km long and formed in the Baltic ice lake (Box 11.1).

2. Glaciolacustrine landforms at calving ice margins. Moraine ridges may develop at the grounding line. This is the line at which the ice begins to float and loses contact with its bed. These moraines are frequently referred to as De Geer moraines, washboard, or cross-valley moraines. They are low (<5m), often asymmetric, ridges that are either straight or slightly concave in planform. This planform reflects the linear or concave morphology typical of a calving glacier margin in plan. They appear to form at the grounding line, which may be some

Iceberg

Iceberg

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