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What is a Moraine?

By Brendan McGuigan
Updated Jun 04, 2024
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A moraine is debris that is moved by a glacier as it traverses a continent. This may be either rocks and soil that is still on a living glacier, or rock that has been transported and left behind by a now extinct or receded glacier. There are eight distinct types of moraine, six of which describe rock that has become a steady landform, and two of which describe rock interacting with a living glacier. The eight types are: ground, lateral, medial, push, recessional, terminal, supraglacial, and englacial. The last two exist only when a glacier is still active, while the other six may exist after the glacier has receded.

A glacier is essentially a river of ice, formed on the land and moving slowly because of gravity’s force on the large mass. Glaciers are roughly permanent features, although various atmospheric conditions may cause them to recede and eventually vanish. They exist on every continent on earth, so moraine can be found on every continent as well. Glaciers move as a result of a property of ice when it reaches a certain thickness, at which point it becomes somewhat plastic and can flow in a way similar to, though much slower than, water. Because of the amazing mass of a glacier, it is able to sheer through rock, creating geologic formations.

Ground moraine is simply the mixture of rock and debris, known as till, that has dropped to the floor of the glacial valley once a glacier has melted away. Its distribution is somewhat regular along the area the glacier once inhabited, and it has no particularly interesting characteristics.

The lateral type is formed on the sides of a glacier. As the glacier moves through rocky areas, the intense cold and pressure break off large chunks of rock, which fall onto the edges of the glacial body. This rock is carried along as the glacier moves, and when the glacier melts away, the lateral moraine drops down and forms large ridges marking the edge points of the now-gone glacier. Often, it bounds the glacier that formed it when it recedes in size, traveling through the ridges it created when it was a larger size.

The media form is created when two glaciers run into each other and merge. The piles of lateral moraine on the merging edges come together and form a new ridge, now in the center of the new, larger glacier. If the new glacier melts away, it leaves a large pile of medial moraine in the center of where it once existed. This also serves as good evidence that a glacier at some point in the past was formed by the merging of two smaller glaciers.

Push moraine is so named because it forms when a glacier recedes and leaves a pile of debris, then expands again and pushes that debris more forward. Often, this process repeats many times, surging forward and dropping back, pushing newer debris on top each time. This can serve as evidence of changes in climate, since it represents a cooling and warming cycle.

Recessional moraine occurs where a glacier stops its movement long enough for debris to build up at the end, and then slowly melts over time, leaving strewn debris across the edge of the glacial valley. Similarly, terminal moraine also forms at the edge of a glacier’s furthest point, but does not have the characteristic formation of recessional debris that indicates that the glacier paused for a period of time before receding.

Supraglacial moraine is simply the collective name for these different types of moraine when they exist on a living glacier — before they have dropped down to the valley below. Englacial moraine similarly encompasses these types, but comprises debris that is trapped within the ice, either through mild thawings and refreezings, rocks falling into glacial chasms, or ground rock being swept along.

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