The process of glaciation, where vast sheets of ice sculpt the Earth’s surface, is one of the most powerful forces in geomorphology. These moving masses of ice erode, transport, and deposit enormous amounts of material, fundamentally reshaping landscapes. Moraines are among the most common and visible results of this activity. They are structured accumulations of debris that serve as geological records, marking the former extent and movement of glaciers that have long since retreated.
Composition and Defining Characteristics of Moraines
A moraine is formally defined as a ridge, mound, or sheet of unconsolidated glacial debris that has been transported and deposited directly by a glacier or ice sheet. This material is known as till.
Till is characterized by being unsorted and unstratified, meaning the mixture contains a chaotic jumble of sediment sizes without any layers or sorting by water. The physical composition of till ranges widely, containing everything from fine, silt-sized rock particles, often called glacial flour, to large, multi-ton boulders.
These larger rocks, moved far from their original source, are known as glacial erratics. The mixture of clay, sand, gravel, and boulders gives till a wide-ranging appearance, but its lack of organization identifies it as material deposited directly by ice. The entire deposited mass of till, along with other water-reworked glacial sediments, is collectively referred to as glacial drift.
The Glacial Process of Moraine Formation
Glaciers acquire material through two primary erosional processes: plucking and abrasion. Plucking occurs when meltwater seeps into bedrock fractures, freezes, and expands, ripping out chunks of rock as the glacier flows over them.
Abrasion is the grinding action where rock fragments already embedded in the base of the ice act like sandpaper, scouring and polishing the underlying bedrock. The eroded material is then transported within the ice mass, categorized by its position: sub-glacial (at the base), englacial (within the ice), and supra-glacial (on the surface). Supra-glacial debris often originates from rockfalls onto the glacier from adjacent valley walls.
Moraine formation, or deposition, happens when the glacier’s capacity to transport its load decreases, typically at the snout or margins of the ice. When the glacier’s advance and melting are balanced, or when it stagnates and melts, the transported sediment is released. This debris is either dumped in place as melt-out till or pushed and squeezed into ridges, especially at the leading edge.
Classification of Moraines by Location and Structure
Moraines are classified based on their distinct structure and their position relative to the glacier that formed them.
Terminal Moraine
A terminal moraine, also known as an end moraine, is a curved, ridgelike accumulation of till that marks the absolute farthest extent of a glacier’s advance. These features can be vast, sometimes reaching over 100 meters in height and extending for many kilometers, such as those left by continental ice sheets.
Recessional Moraine
Recessional moraines form behind the terminal moraine and are created during temporary halts in a glacier’s overall pattern of retreat. These are smaller, parallel ridges that represent secondary end moraines. Each ridge marks a brief period where the ice front stabilized, revealing the step-by-step history of the glacier’s melting.
Lateral Moraine
Along the sides of a valley glacier, lateral moraines develop as elongate ridges of debris that run parallel to the valley walls. This material primarily consists of rock fragments that have fallen onto the glacier from the surrounding slopes. When a glacier melts, these features are left stranded high on the valley sides, often forming impressive, linear embankments.
Medial Moraine
When two separate valley glaciers converge and merge into a single, larger ice stream, their adjacent lateral moraines are compressed together to form a medial moraine. This appears as a single, dark stripe of debris running down the center of the combined glacier. When the ice eventually melts, the medial moraine is deposited as a ridge running down the middle of the valley floor.
Ground Moraine
The most widespread type is the ground moraine, which is not a ridge but a generally flat to gently undulating sheet of till deposited beneath the ice as the glacier retreats. Sometimes called a till plain, it can cover large areas, creating irregular, rolling topography. This material is often thin, seldom exceeding 20 meters in thickness, and is responsible for creating the fertile, yet poorly drained, soils found in many formerly glaciated regions.