Why Is That Boulder in the Wrong Place? Rocks Carried by Ice

An erratic is a rock resting on bedrock of a different type, which means it must have been transported rather than weathered out of what lies beneath it. The mismatch is the whole of the evidence and it can be striking, with blocks weighing thousands of tonnes sitting on entirely unrelated rock, sometimes perched on high ground where nothing could have carried them uphill. Because rock types can be matched to their sources, an erratic can frequently be traced to a specific outcrop, which gives a direction and a distance of transport. Collections of erratics from a distinctive source form a trail spreading outward from it, and mapping those trails reconstructs the direction ice flowed and how far it reached, which is how the extent of former ice sheets was established.

Glaciers carry material in several positions and each leaves different evidence:

• •On the surface, where rockfall from valley sides rides along untouched and stays angular
• •Within the ice, incorporated by burial or by ice flowing over debris
• •At the base, where rock is dragged along and becomes scratched and faceted
• •Frozen into the bed, where plucking removes blocks from bedrock directly
• •In meltwater, which sorts and rounds material and carries it beyond the ice
• •On floating ice, which drops rock into the sea far from any land, producing dropstones in marine sediments

Erratics were the central evidence in one of the great scientific disputes of the nineteenth century. The prevailing explanation attributed scattered boulders to a great flood, or to icebergs floating over a drowned landscape and dropping their loads, which is why the deposits were long called drift. Swiss naturalists familiar with living glaciers argued instead that ice had once extended far beyond its present limits, pointing out that glaciers demonstrably carry and deposit exactly such material, that scratched bedrock matched what glaciers produce, and that the distribution of erratics traced coherent flow patterns rather than random scattering. Louis Agassiz presented the argument forcefully from 1837 and it was resisted for decades before becoming accepted, and the resulting concept of ice ages reorganised the understanding of recent geological history.

Erratics are one part of a distinctive assemblage and the rest is worth recognising. Till is unsorted material dumped directly by ice, containing everything from clay to boulders jumbled together, which distinguishes it instantly from anything deposited by water, since water sorts by size. Moraines are ridges of that material marking where an ice margin stood, and mapping them traces the retreat stage by stage. Striations are parallel scratches cut into bedrock by rock held in the ice, giving flow direction directly. Roches moutonnées are bedrock knobs smoothed on one side and plucked steep on the other, which also records direction. Outwash plains of sorted gravel lie beyond the ice limit, deposited by meltwater. Together these features let the extent, direction and timing of vanished ice sheets be reconstructed in considerable detail.

Large erratics attracted attention long before anyone understood them and the responses are recorded in place names and folklore. Many carry names attributing them to giants, to the devil or to saints throwing rocks, which is a recurring explanation across northern Europe and reflects a reasonable inference that something enormous put them there. Some were incorporated into monuments, used as boundary markers or as meeting places, and a number of prehistoric structures were built from or around them. In a few documented cases erratics were the source stone for standing stones and for building. They are now protected in many places as geological monuments, and several are among the most visited natural features in their regions, which is an unusual afterlife for a rock that is simply in the wrong place.

A rock resting on bedrock of a different type must have been transported, and matching it to its source gives a direction and distance. Trails of erratics from a distinctive outcrop reconstruct the flow and extent of vanished ice sheets. They were the central evidence in the nineteenth century argument that established ice ages, against explanations involving a flood or floating icebergs.