What are moraines? And how do they form?
Moraines are piles of debris deposited by glaciers. Such landforms can form along the margin of a glacier – at the front or its sides – or can build up on the glacier surface. One common form of the latter, termed medial moraine, typically forms at confluences of two glaciers in mountain areas (Figure 1 and 2). These are deposits that only last for as long as the glacier is around – once the ice melts, any material on its surface is redistributed by meltwater and gravity, so that medial moraines do not normally survive after a glacier has retreated. They are said to have a low preservation potential. Far more important for understanding the climate of the past are those moraines that form along the margins of the glacier, because they usually survive after the glacier has retreated from an area. This means that we can easily establish the former glacier dimensions.
Such ice-marginal moraines may take circular to elongated shapes (Figure 1) and can reach heights ranging from < 1 m to several tens of metres. In mountain environments, walkers often follow the highest points of moraine ridges – termed the ridge crest or crestline – because these are relatively straight and have a uniform gradient, ideal for walking and observing (Figure 3). Moraines are often very prominent in the landscape. In the Alps, they were some of the key indicators used as evidence for formerly-larger glacier extent, where, during the Ice Age, glaciers extended far beyond the mountains. Much like the famous far-travelled erratic boulders, moraines were used by the pioneering 18th and 19th Century scientists such as Charles Lyell and Louis Agassiz in formulating the glacial theory. No doubt, their observations of moraines being pushed up by glaciers that were advancing at that time (during the so-called ‘Little Ice Age’ which terminated around the year 1850 in the Alps) helped them to establish the clear link between a moraine and the glacier margin that formed it.
So, the relationship between glaciers and moraines should be simple then – where there is (or was) a glacier, there should be a moraine?
Unfortunately, this is not always the case: Not every glacier forms a moraine, and not all glaciers form moraines all the time. Moraines can only be built when the glacier is stationary (in balance with climate) or advances (has a surplus of mass). The glacier also needs to transport enough sediment to the front in order to build a moraine. This means that, if a glacier retreats over longer time periods without interruption, moraines are not normally formed. Also, many glaciers in the Arctic do not form moraines, because they flow very slowly and do not transport enough sediment to build a moraine. Generally, where a lot of snow falls on a glacier and where the temperature is not extremely low year-round, glaciers flow faster and pick up, transport, and deposit more sediment (such as in Iceland, Norway and the Alps). Those types of glaciers (termed ‘temperate’ glaciers) form more prominent moraines than those existing in areas where little snow falls and the glacier is frozen to much of its bed due to lower air temperatures year-round (such as in the Arctic).
We can use our knowledge of how and when moraines form to link this to the interior of moraines, their sediments. This allows us to investigate key questions such as:
- How have moraines formed and deformed?
- Which forces were generated by the glacier?
- Was the glacier advancing or stationary?
Each style of advance produces a distinct ‘signature’ within the sediments. Identifying different styles of advances is important, because moraines are often very large and complex features that build up during different depositional phases, but in the same spot, over many centuries and millennia. This is possible, because any material arriving at the snout is dumped and pushed over existing piles of debris which often hem in the glacier at its sides, thereby leading to the formation of a higher moraine rather than multiple ones (Figure 4). Therefore, any clear association of these moraines with former climate is not straightforward, and the sequence of climatic fluctuations in mountain environments is often more patchy than in other areas. This is also complicated by several active processes such as fast debris flows, snow avalanches and rigorous river activity taking out previously-formed moraines.
Ultimately, we need a thorough understanding of when a moraine was formed and how fast the glacier was advancing/retreating to certain points in order to link the patterns of glacier response in one valley to another, an approach we refer to as correlation. It is partly this correlation, locally (between valleys), regionally (across individual mountain belts) or globally that allows climate scientists to extract meaningful information about how former climate change has affected our planet and what factors are causing, or controlling, this change. So, a little pile of dirt can go a long way.