moraine

Moraine enters scientific English from French moraine, which in turn was borrowed from the Savoyard dialect word morena or moréna, referring to the debris deposited by Alpine glaciers. The Savoyard word is of disputed derivation: proposed etymologies include a connection to Latin murus (wall) — perhaps referring to the ridge-like form of a lateral moraine — or a Celtic root related to the Welsh morfa (flat land, seashore). The word entered French scientific vocabulary in the eighteenth century as naturalists began studying Alpine glaciers systematically, and from French it passed into the geological literature of all European languages. The Alpine origin is appropriate: the people who lived in glaciated mountain valleys had named what they saw long before scientists arrived to describe it, and their dialect word was more precise than anything the scientific vocabulary had available.

Glaciers are rivers of ice, and like rivers they transport material. As a glacier moves downslope, it quarries rock from the bedrock beneath it (plucking) and abrades the rock surface (creating the fine powder called glacial flour that gives glacially fed rivers their milky color). It incorporates rockfall from the valley walls above. All of this material — from house-sized boulders to clay particles — is carried within and on the glacier and eventually deposited when the ice melts. The spatial geometry of this deposition is precise: material deposited along the sides of a glacier forms lateral moraines (long ridges parallel to the glacier's direction of flow); material deposited at the glacier's terminus (snout) forms terminal or end moraines (arcuate ridges marking the furthest extent of the ice); material deposited beneath the glacier forms ground moraines (sheets of till covering the landscape). Each type of moraine tells a different story about ice movement and retreat.

The moraines of the last glacial maximum — the period between roughly 26,000 and 19,000 years ago when ice sheets covered much of North America and northern Eurasia — are now the landscape of everyday life across vast regions. The rolling hills of the Midwest's glaciated terrain, the kettle lakes of Minnesota and Wisconsin (formed where blocks of ice were buried in till and subsequently melted), the drumlins of Ireland and New York State, the morainic ridges of Long Island and Cape Cod — all of these landforms are glacial deposits, moraines in the broadest sense. New York City is built partly on the terminal moraine of the Wisconsin glaciation: the ridge that runs through Brooklyn and Queens from Brooklyn Heights to Flushing marks the line where the ice sheet stood for a long time before retreating northward. The subway riders of Brooklyn travel across ice-age geography without knowing it.

The study of moraines played a central role in establishing the reality of past ice ages — one of the most important scientific revolutions of the nineteenth century. Louis Agassiz, the Swiss naturalist, famously argued in 1837 that the boulders scattered across the Swiss landscape (erratic boulders, transported far from their source rock) and the scratched, polished bedrock surfaces of valleys were the work of ancient glaciers that once extended far below their current limits. His evidence was in large part the moraine system: the terminal and lateral moraines that preserved the former extent of Alpine glaciers with the precision of a geological photograph. The word moraine, and the landforms it describes, became the key evidence that Earth's past climates were radically different from the present — that the world had been, not once but repeatedly, deeply frozen.