Glacial Processes Lab

Jagged or smooth

Around 27,000 to 21,000 years ago, at the height of the last glacial period, ice over a kilometre thick lay across much of Scotland and reached well down into England and Wales. It was gone from most of Britain by roughly 15,000 years ago, apart from a final cold snap about 12,900 to 11,700 years ago, and everything it did to the rock is still there to be read. Two processes did most of the damage, and they are the ones examiners most often find swapped. Plucking happens where meltwater seeps into cracks in the bedrock, refreezes, and welds the ice solidly to the rock; when the glacier moves on it tears whole blocks away with it, leaving a jagged, broken surface. Abrasion happens where rock fragments already frozen into the underside of the ice are dragged across the bedrock like sandpaper, grinding and polishing it and cutting long scratches called striations, leaving a surface that is smooth. That gives you a test you can apply to any description or photograph: look at what the surface is like. Jagged means the rock was pulled apart; smooth and scratched means it was ground down. Both processes usually act on the same ice mass in different places, which is why the test works better than trying to remember which one happens where. This module covers those two, the weathering that prepares the rock for them, how ice carries what it has torn loose, and what it leaves behind. The landforms all of this produced, and the upland where you can go and look at them, belong to the sibling modules; here the subject is the machinery itself.

Five glacial terms

The first prepares the rock, the next two break it, and the last two are what happens to the pieces.

Order the freeze-thaw cycle

These six stages break solid rock apart on a face above the ice. Put them in order.

• Rain or meltwater collects in a crack in exposed rock
• The temperature falls below freezing overnight and the water turns to ice
• The ice takes up about a tenth more space than the water did, pushing outwards on the crack
• The crack is forced a little wider than it was before
• Temperatures rise again, the ice melts and more water works its way into the enlarged crack
• After many cycles the rock breaks apart into angular fragments

Which process left this?

A geologist examines two surfaces beneath the same former glacier. One is smooth and polished, with long parallel scratches running in the direction the ice travelled. The other is broken and jagged, with sharp steps and hollows. What happened?

• The smooth, scratched surface was abraded: fragments held in the underside of the ice were dragged across it, grinding it down and cutting the striations as they went. The jagged surface was plucked: meltwater froze into joints in the rock and welded the ice to it, so blocks were torn away bodily as the glacier advanced. Both processes were acting on the same ice mass at the same time, which is why the state of the surface identifies the process more reliably than its position does.
• The smooth surface was plucked and the jagged surface was abraded.
• Both surfaces were produced by freeze-thaw weathering acting at different rates.
• Both were cut by meltwater streams flowing beneath the ice.

What each stage of the machinery does

Read the columns as three jobs: preparing the rock, breaking it, and moving what has broken. The bottom row of each is the thing to say if you want to show you can tell one process from another.

Label the processes

The cross-section shows a valley holding a mass of ice, with bare rock above it and loose material beyond its end. Tap a label, then tap the pin marking where that process is acting. Two of the labels name real glacial features that this diagram does not show.

Match each observation to its cause

Name the process, not the place

Questions on this element almost never ask you to list the glacial processes. They describe a surface, a deposit or a photograph and ask which process produced it, which means the useful knowledge is a set of recognition tests rather than a set of definitions. Three tests will carry you through most of them. Jagged or smooth? A broken, stepped face was pulled apart by ice frozen onto it; a polished, scratched face was ground down by debris dragged across it. Angular or rounded? Material carried by ice keeps its sharp edges, because it rides along rather than being rolled and knocked; material carried by water is rounded, because it is constantly colliding as it travels. Jumbled or sorted? Ice releases everything it holds at the same instant, so its deposits mix clay and boulders together; running water lets go of the heaviest first and carries the finest furthest, so its deposits are graded into layers. The second and third of those are the ones that let you tell a glacial deposit from a river one, and questions on this node often turn on exactly that, because meltwater flowing beyond the end of a glacier lays down sorted, rounded material only a short distance from unsorted, angular till left by the ice itself. Two deposits, two processes, a few metres apart. Finally, keep weathering and erosion apart. Freeze-thaw breaks rock where it stands and moves nothing; plucking and abrasion both require ice in motion. Writing that freeze-thaw erodes the valley is a small slip that tells an examiner you have not separated the two ideas.

Complete the glacial process facts

Freeze-thaw weathering works because water expands by roughly _____ per cent when it freezes, levering cracks wider each time the temperature crosses zero. It needs repeated crossings rather than constant cold, which is why it is most active in spring and autumn. Ice then erodes the rock in two ways. _____ tears whole blocks away where meltwater has refrozen and welded the ice to the bedrock, leaving a jagged face, while abrasion grinds the rock with the debris held in the base of the ice, leaving a smooth surface marked with scratches called _____. Material dropped directly by the ice is called till, and it is recognisable because it is angular and _____. Beyond the end of the ice, _____ sorts what it carries into layers, so the two deposits look completely different only metres apart.

Spot the three accurate glacial claims

Select the THREE statements that are true.

• Freeze-thaw, plucking and abrasion work together rather than separately, because the fragments loosened by the first two become the tools that the third uses to grind the bedrock.
• A deposit of jumbled clay, pebbles and boulders was dropped by ice, while sorted layers of rounded gravel nearby were laid down by meltwater flowing beyond it.
• Freeze-thaw requires temperatures to cross zero degrees Celsius repeatedly rather than simply to remain cold, so it is most active in spring and autumn.
• Freeze-thaw weathering erodes the valley by carrying broken rock downhill.
• Material carried by a glacier becomes rounded over time, in the same way that river load does.
• Plucking leaves a smooth polished surface behind it.

A 6-mark process answer, marked

Explain how glacial processes erode the landscape. [6 marks] Erosion by ice depends first on rock being weakened, and that is the job of freeze-thaw weathering. Water collects in cracks in exposed rock, and because water expands by roughly 9 per cent when it freezes, each freezing forces the crack a little wider; repeated as temperatures cross zero degrees Celsius, which happens most often in spring and autumn, this breaks the rock into angular fragments and opens the joints that erosion will exploit. The first erosional process is plucking. Meltwater at the base and sides of the glacier seeps into those joints and refreezes, welding the ice firmly to the bedrock, so that when the glacier moves forward it pulls whole blocks away with it. This is why a plucked surface is jagged and stepped, and it also supplies loose rock to the underside of the ice. That supply matters, because the second process, abrasion, depends on it: fragments already frozen into the base of the glacier are dragged across the bedrock as the ice moves, grinding and polishing it in the manner of sandpaper and cutting long parallel scratches called striations in the direction of travel, while producing a fine powder known as rock flour. The two processes therefore reinforce one another, since plucking and freeze-thaw provide the tools that abrasion uses, and together they can wear a valley down over tens of thousands of years even though a glacier typically moves only metres to a few hundred metres a year. What earned the marks: the sequence running from weathering into erosion rather than treating them as a list; both erosional processes named and explained by mechanism; the tell-tale evidence given for each, the jagged face and the striations; and the link between them made explicit, which is what turns three separate paragraphs into an explanation of how a landscape is worn down.

Find the two signs of abrasion

Five observations made beneath a former glacier. Two of them are evidence of abrasion specifically. Tap those TWO.

• A rock surface polished smooth enough to reflect the light, with no loose material on it at all.
• A stepped face with sharp corners where blocks appear to have been lifted out bodily.
• Long parallel grooves cut into the rock, all running the same way down the valley.
• A fan of angular fragments lying at the base of a cliff well above where the ice reached.
• A jumbled mound of clay and boulders left where the ice once ended.

Build the deposit sentence

Assemble a sentence that distinguishes a glacial deposit from a meltwater one.

Four things found in the field

You are shown four observations from a formerly glaciated area. Say what made each one.

• A slab of bedrock in the valley floor is smooth to the touch and carries fine parallel scratches, all running along the axis of the valley.
• High on a rock face, well above the level the ice reached, there is a fan of sharp-edged fragments piled at the foot of the cliff.
• A low ridge across the valley contains clay, sand, pebbles and large boulders all mixed together with no arrangement of any kind, and the boulders are angular.
• A hundred metres further down the valley, a bank of gravel is neatly layered, with the coarsest material at the bottom and finer sand above, and the pebbles are well rounded.

Explain the processes

Explain how weathering, erosion, transport and deposition by ice shape a glaciated landscape. [9 marks]

• Freeze-thaw is weathering: it breaks rock in place rather than moving it
• The tell-tale surface each erosional process leaves behind
• Fragments loosened by weathering and plucking become the tools abrasion uses
• Why glacial material is angular and unsorted
• A chain, not four separate paragraphs