Why Are Those Hills All Pointing the Same Way? Ice Left Them Behind

The form is distinctive enough to be recognised immediately once known. Each hill is elongated, typically a few hundred metres to a couple of kilometres long, tens of metres high, steeper and blunter at one end and tapering at the other, with a smooth streamlined profile that has been compared to an upturned boat or the bowl of a spoon. The blunt end faces the direction the ice came from and the tapered end points the way it was going, which makes the whole field a direct record of ice flow direction. They occur in swarms rather than singly, with hundreds or thousands packed together over a wide area, all aligned, producing a landscape sometimes described as a basket of eggs.

The internal composition varies more than the external shape:

• •Most consist largely of unsorted glacial debris, a jumble of clay, sand and stones
• •Some are layered, indicating water-sorted material incorporated during formation
• •Some have a core of solid bedrock with a covering of debris shaped over it
• •Stones within them are frequently aligned with the long axis
• •Some contain material from earlier deposits reworked by later ice
• •This variety is the central problem, since one shape arises from several different fills

Explaining them has occupied glacial geologists for over a century without producing agreement, which is unusual for a landform this common. The deposition account has ice plastering debris around an obstacle until a streamlined mound builds up. The erosion account has ice carving into existing sediment and leaving the resistant parts standing. A sudden meltwater flood beneath the ice has been proposed, carving the whole field at once, which explains the regularity and demands an implausible volume of water. Instability in the deforming sediment beneath moving ice is the currently favoured family of explanations, in which small irregularities grow automatically because of how the sediment and ice interact. The varied internal composition suggests more than one process produces the same shape.

These hills belong to a family of features that together record what an ice sheet did. Eskers are winding ridges of sorted sand and gravel deposited by rivers running in tunnels within or beneath the ice, left standing when the ice went. Moraines are ridges of debris marking where an ice margin stood, either at its furthest extent or during a pause in retreat. Erratics are boulders carried far from their source and dumped on unrelated bedrock, which is how ice transport was first demonstrated. Striations are scratches cut into bedrock by stones held in the ice base, recording flow direction directly. Kettle holes are depressions left where a buried block of ice melted. Reading these together reconstructs a sequence rather than a snapshot.

Whatever formed them, the landforms carry information that is used extensively. Their alignment maps the flow direction of ice sheets that vanished thousands of years ago, and mapping thousands of them across a region reconstructs the pattern of an entire vanished ice sheet including where the flow converged into fast-moving streams. Cross-cutting fields with different alignments record changes in flow direction over time, which constrains how the ice sheet behaved as it grew and decayed. Their distribution indicates where the bed was soft and deforming rather than rigid, which matters for modelling how fast ice sheets can move. That last point connects directly to predicting the behaviour of the ice sheets that still exist.

Smooth elongated hills occurring in aligned swarms mark where an ice sheet flowed, with the blunt end facing the direction the ice came from. Their internal composition varies from unsorted debris to layered sediment to bedrock cores, which is why no single formation mechanism is agreed. Mapping them reconstructs the flow patterns of vanished ice sheets and indicates where the bed was soft enough to deform.