Medial and Lateral Moraines
Look down on the lower Baltoro, Tasman or Mer de Glace and you notice dark stripes running along the ice surface. They are moraines — concentrations of rock material carried by the glacier. Their geometry is anything but random; it records the topology of the ice flow, the structure of the catchment, and sometimes the glacier's history. With a little practice you can read a glacier from a viewpoint just by looking at its stripes.
What is a moraine?
In the broadest sense, a moraine is rock material transported by a glacier. Outside the ice, "moraine" usually means a deposited ridge or sheet of glacial sediment. On the ice surface, we distinguish lateral, medial, supra-, intra- and englacial moraines.
Lateral moraines
Lateral moraines run along the sides of a glacier. They are fed by:
- Rockfall from valley walls, which falls onto the glacier edge and is carried downstream.
- Lateral abrasion, where the moving ice scrapes the bedrock and incorporates fine material at the margin.
Lateral moraines often form distinct ridges of debris on the glacier surface, sometimes several metres above the surrounding ice because the surrounding ice has melted faster than the debris-protected ice underneath.
Medial moraines
A medial moraine appears where two glacier tributaries merge. The lateral moraine from the right side of one tributary and the lateral moraine from the left side of the other are pressed together and continue down the trunk as a single stripe. So: count the medial moraines and you can count the tributaries.
On the Baltoro you can identify five or six clear medial moraines descending from Concordia — each marking a distinct contributing glacier.
Englacial and basal moraines
Material can also be transported inside the ice, particularly along shear planes between different ice masses, or at the bed itself. When that material reaches the surface (because the ice above has melted away), it is exposed as a new debris band. This can produce moraines that "appear" in the middle of an otherwise clean glacier.
What moraines reveal
- Catchment geometry: the number of medial moraines tells you how many tributary glaciers feed the trunk.
- Flow direction: moraines bend at confluences, recording the relative speeds of the merging streams.
- Glacier history: older lateral moraines deposited outside the current ice mark previous maximum extents — the most famous ones date from the Little Ice Age.
- Mass balance: if a lateral moraine ridge stands tens of metres above the glacier surface, the ice has thinned that much since the moraine was active.
Reading a glacier from above
From a typical viewpoint you can:
- Count medial moraines from the headwall down to identify the tributaries.
- Trace each one back to its source confluence.
- Observe how the moraine stripes bend or thicken — uneven flow or unequal supply often shows.
- Look at lateral moraine height relative to the current ice surface for a quick read of thinning.
Moraine deposits and landscape
Once a glacier retreats, the lateral and end moraines remain as ridges on the landscape — long after the ice is gone. The Little Ice Age moraine ridges of Alpine valleys are still clearly visible. They are how we know the historical maximum extent of ice in valleys that today have only small glaciers, or none.
See them all in one view
The interactive map highlights particularly clear medial-moraine systems on photographed glaciers — useful for training your eye before a Baltoro or Tasman trip.