Why Is There a Long Gravel Ridge Winding Across Flat Country? A River That Ran Inside Ice

Meltwater on and within a glacier collects into streams that find their way to the bed and flow towards the margin in tunnels melted through the ice. Those streams carry sediment exactly as any river does, sorting it by size and depositing sand and gravel along the tunnel floor where the flow slows. When the ice eventually melts away, the walls and roof that confined the channel disappear, and the deposit is left standing as a ridge on the landscape, preserving the winding course the tunnel followed. Because the water was under pressure inside the ice, the route could climb over rises, which is why these ridges sometimes run uphill in a way no surface river would.

Several features distinguish them from other ridges:

• •A long sinuous shape, winding rather than straight, running for kilometres
• •Composition of sorted and rounded sand and gravel rather than unsorted debris
• •Internal bedding visible in any cut face, showing water deposition
• •A course that ignores the present topography and may run uphill
• •Steep sides and a narrow crest, frequently only metres wide
• •Occurrence in swarms, several running roughly parallel

These ridges have been exploited for as long as people have lived on them. They supply well-drained routes across country that is otherwise boggy, which is why roads, tracks and railways across Finland, Sweden, Ireland and parts of Canada follow them, and why some of the oldest routeways in those countries do too. They contain enormous quantities of clean sorted aggregate immediately at the surface, which makes them the obvious source of building sand and gravel and which has destroyed a great many of them. They hold groundwater in usable quantities, so wells and water supplies are frequently sited on them. And their well-drained soils support distinctive vegetation in wet landscapes.

Meltwater leaves a family of landforms and separating them clarifies what each records. Outwash plains are broad flat spreads of sorted sand and gravel laid down by braided rivers beyond the ice margin, and they are the material an esker feeds into. Kames are irregular mounds formed where sediment accumulated in a hollow on or against the ice and collapsed when it melted. Kame terraces run along a valley side where a stream flowed between the ice and the slope. Kettle holes are depressions left where a buried block of ice melted out, frequently now lakes. Varves are annual layers deposited in a proglacial lake, each pair recording one year, which allows counting back through thousands of them.

The ridges record the drainage system of a vanished ice sheet, which is otherwise almost impossible to observe. Their orientation gives the direction water was flowing, which is broadly the direction the ice was flowing. Their continuity indicates that a tunnel remained stable in one position long enough to accumulate a deposit, which constrains how the ice was behaving. Beaded forms, where a ridge swells at intervals, are interpreted as recording annual pulses of summer meltwater, so counting the beads counts years. Their distribution shows where the bed was thawed rather than frozen, since a frozen bed permits no tunnels. Mapping them across a continent reconstructs how an ice sheet drained and therefore how fast it could flow.

Meltwater flowing in tunnels within a glacier deposits sorted sand and gravel along the tunnel floor, and when the ice melts the deposit stands as a winding ridge that ignores the present topography and can run uphill. Sorted rounded material, internal bedding and a sinuous course identify one. They carry roads across boggy country, supply aggregate, hold groundwater and record the drainage of vanished ice sheets.