Moraine

I

Introduction

Moraine, accumulation of sediment, or rock fragments, that is deposited by the slowly creeping mass of ice of a glacier. There are two major types of moraine: subglacial moraine, which is deposited as a relatively smooth surface beneath a glacier, and ice marginal moraine, which forms as bands of irregular hills around the edges of a glacier. Ice marginal moraines are particularly useful for providing evidence of the previous size and location of glaciers that have shrunk or completely vanished.

As a glacier flows, rock particles in its path are often trapped within the ice and carried away by the glacier, sometimes great distances before it melts and releases them. The material deposited as a glacier melts is called glacial till. Till consists of a mixture of particles of all sizes, ranging from tiny clay flakes up to boulders several metres across. Generally, till does not occur in layers, instead reflecting the random way in which sediment is deposited from a glacier. Till is the most common material found in moraines.

A melting glacier produces water known as meltwater. The vast amount of glacial meltwater that drains over the surface and away from a melting glacier affects the sediments that the glacier deposits. When meltwater flows over or through a deposit of till, it tends to wash away the finer clay and silt particles. The meltwater moves coarser sand and gravel particles a shorter distance and redeposits them. This material, called glacial outwash, is also commonly found within moraines. A moraine may contain only till, only outwash, or—more commonly—a mixture of till and outwash known as glacial drift.

II

Subglacial Moraines

Subglacial moraines are accumulations of drift that have been deposited beneath the bottom of a glacier. They are usually composed primarily of till and generally have little topographic relief, or difference in height between the tops of hills or ridges and the bottoms of depressions. Subglacial moraines come in three main types: ground moraines, streamlined or fluted moraine ridges, and transverse moraine ridges.

A ground moraine is composed of a broad, thin, often broken blanket of drift, with usually less than 10 m (33 ft) of topographic relief. Groupings of distinct, linear streamlined or fluted moraine ridges result when the ice has moulded the drift beneath a glacier parallel to the direction of flow. These ridges are usually no more than 3 m (10 ft) wide and 2 m (7 ft) high but may be hundreds of metres long. Transverse moraine ridges result when subglacial drift is formed into ridges that are perpendicular to the direction of the ice flow. These ridges may be up to 30 m (100 ft) high and 100 to 200 m (330 to 660 ft) wide, with the crests up to a few hundred metres apart.

III

Ice Marginal Moraines

Ice marginal moraines form around the lowest edge of a glacier, along the sides of a glacier, or between two adjacent glaciers. Those that form around the lowest edge of a glacier are called end or terminal moraines, those that form along the sides of a glacier are known as lateral moraines, and those forming between two adjacent glaciers are referred to as medial moraines.

Some end moraines form when rock fragments that have been carried by the flowing ice within a glacier are released as the ice melts. This loose sediment collects as an irregular ridge around the ice margin, or the lowest edge of the ice. Moraines can also form when the force of the flowing ice literally pushes and shoves some of the underlying bedrock up to the surface.

In both types of end moraine, some of the accumulating glacial drift ends up directly in front of the glacier, while the rest is deposited on the surface of the ice near its margin. As the ice gradually melts, the drift on its surface settles onto the underlying landscape at a lower level than the drift that had been deposited in front of the ice. In the highly irregular moraine landscape that results, the lower-level deposits appear as scattered depressions, called kettles, separated by hills where the glacial drift is thicker. This chaotic landscape is called hummocky topography. The topographic relief of this landscape may be up to 100 m (330 ft) or more. In some regions, the depressions partially fill with water, creating numerous kettle lakes. The thousands of kettle lakes of northern Siberia and many other once glaciated regions formed in this manner.

Once a glacier reaches its maximum size, it often remains at the position of its maximum extent for hundreds or even thousands of years. The end moraine that forms at this maximum advance is called a terminal moraine. As the glacier melts and becomes smaller, recessional end moraines may form around the ice margin at one or more of its reduced positions. Because glaciers usually reside at their maximum positions for longer periods than at their recessional positions, terminal moraines are often larger than recessional moraines. The central portion of New York’s Long Island and the islands of Martha’s Vineyard and Nantucket in Massachusetts are part of the Ronkonkoma terminal moraine, named after Lake Ronkonkoma, a large kettle lake in central Long Island. The Ronkonkoma terminal moraine marks the position where the southern edge of a portion of the North American (Laurentide) Ice Sheet sat 20,000 years ago. The extreme limit of the former North European Ice Sheet is marked by a terminal moraine which runs east from Nissum Fiord on the west coast of Denmark’s Jutland peninsula towards Viborg in the centre of the country. In New Zealand the proglacial Tasman Lake, at the foot of the Tasman Glacier in Mount Cook National Park, is contained by a contemporary, retreating terminal moraine.

Lateral moraines form along the sides of valley glaciers. These moraines contain rocky debris that originates from the cliffs above and collects on top of and along the side of a glacier. When two separate valley glaciers merge to form a single larger valley glacier, two lateral moraines, one from each glacier, merge and become trapped between the two adjacent ice masses. Once in this position, these moraines become medial moraines. In modern valley glacier systems, such as those found in south-eastern Alaska, the number of individual glaciers that have merged to form a complex valley glacier system may be determined by counting the number of medial moraines present.