Moraine

By: Dr. Pranab Kr. Das, Assistant Professor, Department of Geography, Sree Chaitanya College, Habra

Moraines are the depositional landforms of the glacier, mostly found in valley or mountain glaciers and even in continental glaciers. The glacial deposits are marked by heterogeneity of materials and lack of stratification, unlike fluvial deposits. Moraine is a type of debris, composed of loose stones, soil/regolith, and mud that a moving glacier leaves behind.  Moraines may form through a number of processes, depending on the characteristics of sediment, the dynamics of glacier movement, and the location of the glacier in which the moraine is formed. Moraine forming processes may be divided into active and passive processes. Active processes is linked with the direct accumulation and storage of moving ice or glacier, known as glaciotectonism. Passive processes involve the placing of chaotic supraglacial sediments over a certain landscape.

    Moraines are only found in places that have, or used to have glaciers. Glaciers are extremely large, moving rivers of ice. Glaciers shape the landscape in a process called glaciation. Glaciation can affect the land, rocks, and water in an area for thousands of years. That is why moraines are often very old.

    Moraines are divided into four main categories: lateral moraines, medial moraines, supraglacial moraines, and terminal moraines.

Lateral Moraine 

 A lateral moraine forms along the sides of a glacier. As the glacier scrapes along, it tears off rock and soil from both sides of its path. This material is deposited as lateral moraine at the top of the glacier’s edges. Lateral moraines are usually found in matching ridges on either side of the glacier. The glacier pushes material up the sides of the valley at about the same time, so lateral moraines usually have similar heights. If a glacier melts, the lateral moraine will often remain as the high rims of a valley.

Medial Moraine

 A medial moraine is found on top of and inside an existing glacier. Medial moraines are formed when two glaciers meet. Two lateral moraines from the different glaciers are pushed together. This material forms one line of rocks and dirt in the middle of the new, bigger glacier. If a glacier melts, the medial moraine it leaves behind will be a long ridge of the earth in the middle of a valley.

Supraglacial Moraine

 The supraglacial moraine is a material on the surface of a glacier. Lateral and medial moraines can also be supraglacial moraines. Supraglacial moraines are made up of rocks and regolith that have fallen on the glacier from the surrounding landscape. Dust and dirt left by wind and rain become part of supraglacial moraines. Sometimes the supraglacial moraine is so heavy, it may cover up the entire glacier. The supraglacial moraine is evenly distributed across a valley if a glacier melts.

Ground Moraine

 Ground moraines often show up as rolling, strangely shaped land covered in grass or other vegetation. They don’t have sharp ridges like other moraines. A ground moraine is made of sediment that slowly builds up directly underneath a glacier by tiny streams, or as the result of a glacier meeting hills and valleys in the natural landscape. When a glacier melts, the ground moraine underneath is exposed. Ground moraines are the most common type of moraine and can be found on every continent.

Terminal Moraine

 A terminal moraine or end moraine is the ridges of unconsolidated debris deposited at the snout or end of the glacier. It reflects the shape of the glacier’s terminus. Glaciers act much like conveyor belts, carrying debris from the top of the glacier to the bottom where it deposits it in end moraines. At a terminal moraine, all the debris that was scooped up and pushed to the front of the glacier is deposited as a large clump of rocks, soil, and sediment. End moraine size and shape are determined by whether the glacier is advancing, receding or at equilibrium. The longer the terminus of the glacier stays in one place, the more debris accumulates in the moraine. There are two types of end moraines: terminal and recessional. Terminal moraines mark the maximum advance of the glacier. Recessional moraines are small ridges left as a glacier pauses during its retreat. After a glacier retreats, the end moraine may be destroyed by postglacial erosion.