Glacial Erratics and Striations
A boulder the size of a house sitting in a flat meadow, composed of granite from a mountain range a hundred kilometres away, is one of geology's most legible clues. It did not roll there, nor was it carried by a river — rivers cannot move boulders of that mass across flat terrain. It was carried by a glacier, embedded in ice, and deposited when the ice melted. These displaced boulders are called erratics, and alongside the scratches and grooves called striations that glaciers engrave into bedrock, they form a record of ice movements that can be read millions of years after the ice itself has vanished.
The study of erratics and striations was central to the development of glacial theory in the nineteenth century. Before Louis Agassiz and his contemporaries proposed that large ice sheets had once covered northern Europe and North America, the standard explanation for scattered boulders and polished rock surfaces was the Biblical flood or iceberg transport during a period of higher sea levels. The coherent geographic patterns of erratics — consistent source-to-deposition paths that matched ice flow directions indicated by striations — ultimately convinced the geological community that continental-scale glaciation had occurred.
How Glaciers Transport Erratics
Ice at the base of a glacier is in contact with the underlying bedrock, and the combination of pressure, freeze-thaw cycling, and shear stress at the ice-rock interface can pluck fragments of bedrock and incorporate them into the basal ice. Plucking is most effective on the downstream side of bedrock knobs, where ice separation from the rock surface and refreezing occurs. Once embedded, these rock fragments travel with the ice at whatever speed the glacier moves, which ranges from a few metres per year for slow mountain glaciers to several hundred metres per year for fast-flowing outlet glaciers.
Supraglacial rockfall onto glacier surfaces also contributes rock material that eventually becomes englacial or subglacial debris as snow accumulates above it. In mountain glaciers with steep valley walls, avalanche and rockfall deposits can be substantial enough to form prominent medial moraines visible from the air. This supraglacially-derived material is typically angular and unmodified by ice grinding, whereas basal material is rounded, faceted, and sometimes striated on its own surfaces from contact with the bedrock below.
The Making of Striations
Striations form when rock fragments carried in basal ice are dragged across the bedrock surface under the weight of the overlying ice column. A hard rock clast — often quartz or feldspathic granite — cutting into a softer or equal-hardness bedrock surface under pressures of tens to hundreds of atmospheres produces a clean, linear scratch that can be millimetres to centimetres deep and extend for metres or tens of metres along the direction of ice flow. The walls of the groove are polished smooth. Multiple striations in parallel indicate sustained, directionally consistent ice flow. Curved or crossing striations indicate that ice flow direction shifted over time, which can happen as ice sheets thin, as topographic barriers are overridden, or as flow reorganises during deglaciation.
On surfaces where glacial erosion has been intense, striated bedrock is accompanied by a smooth, curved topography called roche moutonnée — a French term for a sheep-shaped rock, referring to a bedrock knob that has been streamlined and polished on its upstream face by abrasion while its downstream face is steepened and roughened by plucking. Roches moutonnées are directional indicators as reliable as compass needles: the gentle, striated upstream slope points into the former ice flow, and the steep, blocky downstream face points in the direction the ice was moving.
Reading Erratic Provenance
Erratics are geologically identified by comparing their composition to potential source outcrops. In the Alps, for example, pink Aar granite erratics sourced from the Aar massif in the central Swiss Alps are found as far away as the Jura Mountains and the Swiss Plateau, hundreds of kilometres from their origin. In Scandinavia, distinctive Norwegian larvikite — a blue-grey syenite with characteristic iridescent feldspars — appears in erratics across Denmark, northern Germany, Poland, and the eastern Baltic states, tracing the path of Scandinavian ice sheet lobes during multiple Pleistocene glaciations.
The Bowlder Train concept formalises this: a linear chain of erratics from a single identifiable source rock, narrowing downslope to indicate the ice-flow path. One famous example in North America is the Jasper Erratics Train in Alberta, where quartzite boulders from the Athabasca Pass area are scattered in a narrow band across the plains east of the Rockies, deposited by a lobe of the Cordilleran Ice Sheet.
Striations and Ice Sheet Reconstruction
Systematic mapping of striation orientations across formerly glaciated landscapes has been a primary tool for reconstructing the geometry and flow patterns of Pleistocene ice sheets. The Geological Survey of Canada mapped thousands of striation sites across the Canadian Shield through the twentieth century, and the resulting flow indicators — supplemented by the alignment of drumlins, fluting, and other subglacial bedforms — allowed researchers to reconstruct the pattern of ice dispersal from the Laurentide Ice Sheet's central domes over Hudson Bay and Keewatin.
In Fennoscandia, the compilation of striation data from Norway, Sweden, Finland, and the Baltic states by researchers including Risto Aario and Per Lundqvist produced detailed reconstructions showing multiple cross-cutting flow phases, corresponding to the changing geometry of the Scandinavian Ice Sheet as it grew, reached maximum extent, and then progressively disaggregated during deglaciation. The youngest striations in any location record the final flow direction of ice as it thinned and disappeared.
Cosmogenic Dating of Erratic Deposition
A technique developed in the 1990s and refined since then, cosmogenic nuclide surface exposure dating, allows the deposition of an erratic to be dated directly. When an erratic is deposited on a stable surface and its upper face is exposed to cosmic ray bombardment from that moment onward, rare isotopes including Beryllium-10 and Aluminium-26 accumulate in the crystal lattice of quartz grains at known rates. Measuring the concentration of these isotopes in a sample from the erratic's surface gives an exposure age — effectively the date the glacier dropped it there.
Cosmogenic dating has dramatically refined our understanding of the timing of deglaciation across different regions. Erratics in Patagonia have been dated to match the global Last Glacial Maximum around 18,000 to 20,000 years ago, while erratics in parts of Scotland preserve ages corresponding to a late glacial readvance, the Younger Dryas, around 12,500 years ago. The technique has also been applied to recently deglaciated terrain around modern glaciers — dating erratics deposited during the Little Ice Age maximum in the Alps or during Holocene neoglaciations in Alaska — providing context for the longer retreat record.
Where to See Erratics and Striations Today
Polished and striated bedrock surfaces are exposed wherever glaciers have retreated recently enough that the surface has not been obscured by weathering or vegetation. The forefields of retreating Alpine glaciers — Mer de Glace in France, Gornergletscher in Switzerland, Pasterze in Austria — expose fresh striated surfaces within years of deglaciation. In these settings, the features are often visible at close range with no special equipment, the scratches running in consistent directions across pale grey gneiss or schist.
Erratics of spectacular size are found across the northern European plain — the Findling Boulders of Mecklenburg in northern Germany include examples weighing hundreds of tonnes — and across the Canadian prairies, where isolated granite boulders sourced from the Canadian Shield dot agricultural fields for hundreds of kilometres. Iceland provides a particularly clear setting where active ice meets accessible terrain: the margins of Vatnajokull and Langjokull expose both active striation formation and recently deposited erratic material.
Explore the map to find glaciers and glaciated landscapes where erratics and striated bedrock are accessible, from the forefields of Alpine glaciers to the formerly ice-covered lowlands of northern Europe and North America.