An erratic is a rock resting on a landscape where its composition is completely different from the underlying geology. These displaced stones can be found across continents, often far from their geological source. They serve as tangible evidence of massive geological forces that shaped the Earth’s surface long ago. The presence of these rocks poses a fundamental question about how they traveled across hundreds of miles and over existing topography.
Defining Glacial Erratics
A glacial erratic is defined by the striking lithological mismatch between the rock’s composition and the native bedrock upon which it sits. For example, a boulder of hard, crystalline granite might rest on soft, layered limestone, clearly indicating a distant origin. Geologists study the composition of both the erratic and the local stone to confirm this difference.
These transported rocks range dramatically in size, from small pebbles to massive boulders, sometimes described as being “as big as a house.” The largest of these, termed mega-erratics, can weigh thousands of tons. The rock types that survive long-distance transport are typically hard, resistant igneous or metamorphic rocks, such as granite or gneiss, which are less prone to shattering and abrasion than softer sedimentary stones.
The Mechanism of Transport
The formation of erratics is directly tied to the immense power of continental ice sheets during past Ice Ages. As a glacier moves, it erodes the bedrock beneath it through a process called glacial quarrying or plucking. This process involves the ice freezing into cracks and joints in the bedrock, then pulling large pieces of rock away as the glacier flows forward.
Once plucked from its source, the rock fragment becomes embedded within the moving ice mass. The glacier acts as a vast conveyor belt, transporting the rock over hundreds of miles, sometimes for thousands of years. The rock is carried either subglacially (at the ice base) or englaciually (within the ice mass itself). When the climate warmed and the ice sheet finally melted, the entrained rock debris was released and deposited onto the new landscape, far from its origin.
Geological Significance
Glacial erratics were instrumental in establishing the theory of continental glaciation in the 19th century, serving as evidence that huge ice sheets once covered vast expanses of the Earth. Before this understanding, scientists proposed that these displaced boulders were carried by gigantic floods or ice rafting. The presence of these foreign rocks confirmed the extent and reality of past Ice Ages.
Today, geologists use erratics to reconstruct the flow path and maximum extent of ancient ice sheets. By identifying the specific rock type of an erratic and tracing it back to its known source outcrop, scientists can map the precise direction of the ice flow. This technique helps reveal complex flow patterns across the landscape. Erratics are often found alongside other debris in glacial deposits, such as moraines or glacial till.