Glacial Deposition: Till and Fluvioglacial Sediments

🔍 Glacial Deposition: Till and Fluvioglacial Sediments

Eventually, all the material a glacier has eroded and transported must be deposited. Glacial deposition happens in two very different ways, producing two very different types of sediment — and the distinction between them is one of the most testable aspects of this topic.

Till: Deposited Directly by Ice

Till (also called boulder clay, though this term is now less preferred) is sediment deposited directly by ice — without any sorting by water. The defining characteristic of till is that it is completely unsorted: boulders the size of a house can sit directly next to clay particles the size of a bacterium, with no grading or layering between them. This is because ice, unlike water, has no capacity to sort by particle size — it simply drops everything it is carrying when it melts, regardless of how big or small the particles are.

Till also typically contains:

• Erratic boulders — rocks that are completely different from the local bedrock, having been transported hundreds of kilometres from their source. Erratics are powerful evidence of past ice extent. For example, Norwegian granite boulders found on the Yorkshire coast were carried there by the last ice sheet.
• Striated clasts — particles with scratches on their surfaces, confirming subglacial transport.
• Mixed lithologies — rocks from many different source areas mixed together, as the glacier picked up material along its entire path.

Fluvioglacial Sediments: Deposited by Meltwater

Meltwater flowing from, within, or beneath a glacier carries and deposits sediment in the same way as any river — and this produces fluvioglacial (or glaciofluvial) sediments. In contrast to till, fluvioglacial sediments are:

• Sorted by particle size — larger, heavier particles are deposited first (where water slows), finer particles are carried further.
• Rounded — water transport rounds the angular edges of particles.
• Stratified (layered) — deposited in distinct horizontal or cross-bedded layers corresponding to different flow events.

Fluvioglacial sediments form distinctive landforms including eskers (ridges of material deposited in subglacial meltwater tunnels), kames (mounds deposited in lakes or depressions at the glacier margin), and outwash plains (broad spreads of sorted sediment deposited by meltwater rivers beyond the glacier snout).