Glacial till is a type of sediment known as glacial drift, which is material transported and deposited directly by a glacier without being significantly altered by meltwater streams. Till represents the direct record of a glacier’s erosive power and movement, providing geologists with important clues about past ice flow patterns and the extent of former ice sheets.
Defining Characteristics and Composition
Glacial till is recognizable by its lack of sorting, meaning it contains a complete jumble of materials from the smallest clay particles to massive boulders. Unlike sediments deposited by water or wind, which sort materials by size, the ice drops everything it carries at once, resulting in this highly mixed composition. This unsorted mixture is technically called a diamicton, and is often described as “boulder clay.”
The fine-grained portion of the till, consisting of clay, silt, and sand, forms the matrix that supports the larger fragments. The larger rock fragments, known as clasts, are often angular rather than rounded, since they have not been tumbled and smoothed by flowing water. Many of these clasts exhibit facets, polish, or striations—fine scratches—that were etched onto them from grinding against the bedrock while they were held fast within the moving ice.
Another defining characteristic is that till is unstratified, meaning it lacks the distinct layers or bedding planes found in water-deposited sediments. This absence of layering is a direct result of the material being dropped directly from the ice, rather than being settled out slowly in water. The composition of till can vary dramatically depending on the bedrock the glacier passed over, sometimes containing rock types that originated hundreds of kilometers away from the deposition site.
The Glacial Deposition Process
Glacial till is created through erosion, where a glacier scrapes and plucks material from the underlying landscape, and is deposited when the ice loses its ability to carry the load. The material is transported either along the bottom (subglacial transport) or carried within and on top of the ice mass. The eventual release of this sediment occurs through two primary mechanisms that create distinct types of till.
Lodgement till is formed beneath the glacier when the weight and movement of the overriding ice causes debris to be plastered, or “lodged,” onto the ground surface. This type of till is often dense and rich in fine materials like clay and silt because it was deposited under immense pressure directly from the base of the ice. The elongated clasts within lodgement till frequently show a preferred alignment, indicating the direction of the past ice flow.
Ablation till, in contrast, is deposited as the ice melts away, typically at the glacier’s terminus or surface. This sediment was carried on top of or suspended within the ice and is simply let down onto the ground as the ice ablates. Ablation till is less compacted and may have a sandier texture because the meltwater often washes away some of the finer clay and silt particles.
Major Landforms Created by Till
The accumulation of glacial till creates some of the most recognizable landforms in formerly glaciated regions, with moraines being the most common and widespread features. A moraine is essentially a ridge or mound of unsorted till deposited at the margins of a glacier. Terminal moraines mark the farthest point a glacier advanced, forming an arc-shaped ridge at the snout, while lateral moraines form ridges of till along the sides of a valley glacier.
Ground moraine is a widespread sheet of till left behind as the entire ice sheet retreats, often creating a gently rolling landscape with low relief. Recessional moraines are similar to terminal moraines but mark positions where the glacier paused during its retreat, forming a series of ridges behind the outermost terminal feature.
Another distinct landform composed primarily of till is the drumlin, an elongated, streamlined hill shaped like an inverted spoon. Drumlins form beneath the ice and are oriented parallel to the direction of ice movement, providing clear evidence of the glacier’s flow path. They often occur in large clusters called drumlin fields, which can cover vast areas and create a unique “basket of eggs” topography.