The Earth’s surface records profound climate shifts, particularly those caused by massive ancient ice sheets. These glaciers, including those from the Pleistocene Epoch and older global glaciations, actively reshaped the land. As these ice masses advanced and retreated, they left permanent geological signatures on the bedrock and landscape. By interpreting these marks, geologists can map the former extent of ice, revealing glaciated regions across every continent, spanning from recent ice ages to events hundreds of millions of years ago.
Reading the Landscape: Markers of Past Ice Sheets
Geologists use distinct physical clues to determine where ancient ice sheets once flowed. The most direct sign is the presence of glacial striations, which are parallel scratches or grooves gouged into the underlying bedrock. These marks are created as moving ice drags embedded rock fragments, abrading the surface and indicating the glacier’s direction.
Depositional evidence is created when the glacier melts and releases its load of debris. This material, known as glacial till, is an unsorted mixture of clay, sand, gravel, and boulders deposited on the land or piled up at the ice margin. When lithified into rock, this unsorted sediment is called tillite, providing proof of past glaciation.
Moraines are significant landforms composed of glacial till, marking the former boundaries of the ice sheet. A terminal moraine, for example, indicates the glacier’s maximum advance. Large, isolated boulders, known as glacial erratics, are often composed of rock types different from the local bedrock. These erratics were transported great distances by the ice and dropped when the glacier retreated, allowing scientists to trace the ice flow path.
Glacial History of North America and Eurasia
North America and Eurasia hold extensive evidence of the most recent glacial period, the Pleistocene Ice Ages. In North America, the vast Laurentide Ice Sheet covered nearly all of Canada, extending deep into the northern United States to the Missouri and Ohio River valleys. The weight and movement of this ice sheet gouged out the massive Great Lakes basins, which filled with water upon melting.
The southernmost limit of the Laurentide ice is delineated by terminal moraines in the northeastern U.S. Long Island and Cape Cod are prominent examples, built up by sediment deposited at the edge of the ice sheet’s maximum advance around 22,000 years ago. These moraines represent distinct positions of the ice margin during this period.
Northern Europe was largely covered by the Fennoscandian Ice Sheet, which scoured the bedrock of Norway, Sweden, and Finland. The ice sheet’s erosional power is evident in the fjords of Western Norway, which are deep, U-shaped mountain valleys flooded by the sea. Large moraine systems like the Salpausselkä ridges in Finland and the Middle Swedish Moraines also mark the successive retreat stages of this enormous ice mass. Erosion across Fennoscandia and the Canadian Shield removed significant soil, contributing to the flat, lake-dotted landscapes seen today.
Tracing Ancient Ice in the Southern Continents
Evidence of glaciation is not confined to northern landmasses or the recent Pleistocene; geological records show much older ice sheets covering the Southern Hemisphere. This evidence points to the ancient supercontinent of Gondwana, which included present-day South America, Africa, India, and Australia. During the Late Paleozoic Era, approximately 300 million years ago, vast ice sheets covered large parts of Gondwana when it was positioned near the South Pole.
Tillites and striated pavements from this ancient glaciation are found in South America, specifically in regions like the Paraná Basin in Brazil. These Paleozoic glacial deposits are also found in equatorial Africa, such as the Great Karoo Basin of South Africa. This location is far too warm for ice sheets today.
The presence of same-age tillites and matching glacial striations across modern-day Africa, India, Australia, and South America was a primary line of evidence for the theory of continental drift. Furthermore, evidence of glaciation is also found in high-altitude regions closer to the equator, such as the high peaks of East Africa like Mount Kilimanjaro, which hosted smaller, localized glaciers during the Pleistocene. The distinctive tillites found in India and Australia support the historical connection of these landmasses to the former supercontinent.