Trimlines

Trimlines are erosional features which mark the maximum vertical extent of a past glaciation [1].

Different types of trimlines exist, dependent on the length of time since the last glacial advance [2]. In recently glaciated valleys, they are marked by a distinct change in vegetation. Above the trimline, dense vegetation exists with species characteristic of the region[2,3]. And below the trimline, valley sides are characteristic of bare, ice-scoured rock, or with early stages of vegetational development [2,3]. These are termed ‘Vegetational Trimlines‘, and are commonly associated with glacial activity since the Little Ice Age, within the historical era [2,3].

For older glaciations, occurring prior to the Little Ice Age, the vegetation change may be less distinctive as vegetation succession has occurred [2,4]. Therefore, these trimlines mark the boundary between the smooth, ice-scoured bedrock below the trimline, and the frost-shattered regolith from periglacial weathering above [3]. These can be termed ‘Peri-glacial Trimlines’ [2,3,5].

Image of the Callequeo Glacier and its terminal lake with trimlines visible from past glacial advance.
Valley side trimlines (labelled with white arrows) marking the former thickness of the Callequeo Glacier, Monte San Lorenzo. Photo credit: J. Martin.

What are Trimlines Used For?

Unlike other glacial landforms which show the lateral extent, or behaviour of a past glaciations, trimlines mark the maximum vertical extent of the ice surface [3,4]. This enables the production of 3-D model reconstructions of ice sheets and valley glaciers[4,6].

In currently occupied glacial regions, trimlines can be compared to the modern ice surface elevation to assess the role of ice surface thinning, and responses to climate change [7].

References


[1] McCarroll D. (2014) Trimline. In: Encyclopedia of Planetary Landforms. Springer, New York, NY. doi.org/10.1007/978-1-4614-9213-9_383-1

[2] Benn, D.I., and Evans, D.J.A., 2010. Glaciers and Glaciation. Hodder-Arnold, London

[3] Rootes, C.M., and Clark, C.D. (2020) Glacial trimlines to identify former ice margins and subglacial thermal boundaries: A review and classification scheme for trimline expression. Earth-Science Reviews. 210. 103355.

[4] McCarroll, D. (2016) Trimline Trauma: The wider implications of a paradigm shift in recognising and interpreting glacial limits. Scottish Geographical Journal. 132(2).

[5] Ballantyne, C. K. (1997). Periglacial Trimline in the Scottish Highlands. Quaternary
International, 38, 119–136.

[6] Ballantyne, C. K. (2010). Extent and Deglacial Chronology of the Last British‐Irish Ice Sheet: Implications of Exposure Dating Using Cosmogenic Isotopes. Journal of Quaternary Science, 25(4), 515–534.

[7] Kohler, J., James, T., Murray, T., Nuth, C., Brandt, O., Barrand, N., Aas, H. & Luckman, A.
(2007). Acceleration in Thinning Rate on Western Svalbard Glaciers. Geophysical Research
Letters, 34(18).

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