Moraine Types and Formation
Where a glacier has been, it leaves evidence. The most visible and widespread of that evidence is the moraine — an accumulation of rock debris deposited or shaped by glacier ice. Moraines take many forms depending on where the debris originates within the glacial system, how it is transported, and where and how it is deposited. They are read by geologists as a physical record of glacier extent and behaviour: the outermost moraine marks the maximum advance, recessional moraines record pauses during retreat, and the internal moraines of an active glacier reveal the complexity of tributaries and flow patterns in the ice. No study of glaciated landscapes is complete without understanding these features.
The debris that forms moraines comes primarily from two sources. Supraglacial debris falls onto the glacier surface from valley walls as rockfall and avalanche, riding passively on the ice surface and concentrating at margins and along flow boundaries. Subglacial debris is quarried from the bedrock by ice plucking and grinding, transported in or close to the basal ice layer, and deposited as the ice melts or as glacial pressure decreases. The contrast between these two debris sources produces important differences in the character of moraines.
Lateral Moraines
Lateral moraines form along the sides of valley glaciers, where debris from the valley walls accumulates on the glacier surface and is transported downstream. As the glacier moves, debris supplied by rockfall and freeze-thaw from the valley walls above the ice accumulates at the glacier margins. The ice surface at the margin is typically lower than the central ice, creating a trough that concentrates debris. Over time, as the debris load grows, lateral moraines can build into prominent ridges tens to hundreds of metres high.
When a glacier retreats, the lateral moraine is left as a pair of ridges high on the valley walls, marking the former surface level of the glacier. The contrast between the vegetated upper slopes of a valley and the bare, unvegetated inner face of a recently exposed lateral moraine ridge is one of the clearest visual indicators of recent glacier retreat. In the Alps, the lateral moraines of the Little Ice Age maximum — the period of maximum glacier extent around the seventeenth to nineteenth centuries — are prominent ridges of bouldery till often visible immediately above the current ice surface.
Medial Moraines
When two glaciers join — either two valley glaciers merging at a confluence or a tributary entering a main glacier — their inner lateral moraines combine into a single debris-covered band running down the centre of the combined glacier. This structure, called a medial moraine, is visible on the glacier surface as a dark stripe of rock debris against the white or blue ice. In large glacier systems with multiple tributaries, several medial moraines may be present, their positions reflecting the number and location of tributary junctions.
Medial moraines are most prominent on high-debris-supply glaciers where rockfall input is large. Some of the most visually striking examples are on the heavily rock-mantled glaciers of the Karakoram, where multiple tributary junctions create complex patterns of medial moraines that converge and spread as they are carried downglacier, visible from satellite as dark stripes crossing tens of kilometres of ice surface. Baltoro Glacier in Pakistan is a classic example of this multi-moraine geometry.
Terminal Moraines
The terminal moraine, also called an end moraine, marks the furthest downvalley extent of a glacier at a particular time. It is built by the continuous delivery of debris to the glacier front during a phase when the terminus is stationary or only slowly retreating — the rate of ice advance and the rate of ablation at the snout are roughly balanced. Subglacial debris is carried forward by ice flow and deposited at the snout; supraglacial debris slides off the stagnant terminus; and bulldozing of proglacial sediments by the ice pushes material into a ridge.
Terminal moraines are the key stratigraphic markers of glacial chronology. Their positions record the maximum extent of past glacier advances, and their relative positions tell the sequence of advances from most distal (oldest) to most proximal (most recent). Terminal moraines from the Last Glacial Maximum, roughly 18,000 to 20,000 years ago, are found across the northern European plain, the northern United States, and at the margins of the major mid-latitude valley glaciers of the Alps and Andes. The moraine ridge enclosing Puget Sound in Washington State, the terminal moraine forming Long Island in New York, and the moraine ridges across the North German Plain all mark the maximum advances of different lobes of the last major ice sheets.
Recessional Moraines
As a glacier retreats following a maximum advance, it does not typically retreat steadily. Periods of relatively stationary terminus position — when ablation and accumulation are again temporarily balanced — can result in the deposition of recessional moraines: smaller ridges built upvalley of the terminal moraine, marking successive stages of retreat. A series of recessional moraines, increasingly fresh-looking and closer to the current terminus, provides a direct visual record of the deglaciation chronology.
In the Alps, the series of recessional moraines left by glaciers retreating from their Little Ice Age maxima through the nineteenth and twentieth centuries are particularly well-documented. At some glaciers, historical photographs and survey records provide dates for specific moraine ridges, calibrating the deglaciation record and connecting it to instrumental climate records of the same period.
Push and Squeeze Moraines
Push moraines form when a glacier re-advances over its proglacial area and bulldozes previously deposited sediment into a ridge. The resulting moraine has a characteristic asymmetric cross-section: a steep, sometimes overridden and deformed proximal face showing the direction of ice push, and a gentler distal face of bulldozed material. Surging glaciers — those that undergo periodic rapid flow events — commonly produce multiple stacked push moraine ridges at their maximum advance positions.
Squeeze moraines form where soft, water-saturated sediment is squeezed upward through marginal crevasses or along the ice margin as the weight of the ice pressurises the sediment below. These are typically smaller features, sometimes just tens of centimetres high, but are diagnostic of specific subglacial conditions.
Hummocky Moraine and Dead-Ice Terrain
When a large area of stagnant, debris-rich ice melts in place rather than retreating as an active glacier, the result is a chaotic, hummocky landscape of irregular mounds, basins, and ridges called hummocky moraine or dead-ice topography. This terrain forms where the glacier downwastes vertically rather than retreating horizontally, often because a thick supraglacial debris cover has insulated the ice and slowed melt. The ridges and mounds represent subglacial and englacial debris bands that collapse and settle as the ice supporting them melts. Dead-ice terrain is particularly common in formerly glaciated valleys in Alaska, the Yukon, and Scandinavia.
Explore the map to identify the glaciated valleys and formerly glaciated landscapes worldwide where the full range of moraine types is accessible and visible.