L3 The glacial history of Sweden: key to understanding surficial geology

AE1106 course

How much of Sweden has been covered by ice in the past?

At various times in the past 2.5 million years, nearly all of Sweden has been covered by ice sheets, most recently as little as 22,000 years ago.

Name three soil types that are linked to the glaciation of Sweden

• Till (Morän) –

  • The most widespread glacial sediment in Sweden, covering over 85% of the country.

  • Formed directly under or within the ice sheet.

  • Poorly sorted, ranging from clay to boulders

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• Glaciofluvial Sediments (Isälvsmaterial) –

  • Deposited by meltwater streams flowing from the ice.

  • Typically sorted sands and gravels, often forming eskers (rullstensås), deltas, and meltwater channels

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• Glaciolacustrine Sediments (Svallsediment/Svämsediment) –

  • Fine-grained sediments (clay and silt) deposited in ice-dammed lakes during and after glaciation.

  • Often found below the Highest Coastline in Sweden

Why is there only patchy/thin till (morän) cover in the Stockholm area?

because of the way the last ice sheet interacted with the underlying bedrock.

Fast ice flow lubricated by water led to high erosion rates under the ice along this part of the Swedish coastline. Therefore most soil/morän has been scrapped away by the ice

• Warm-based ice (temperate ice) eroded the landscape very efficiently. This type of ice is at the melting point at its base, meaning it slides over the bedrock and grinds away existing sediments.

• In contrast, cold-based ice tends to freeze to the ground and protect the underlying surface, preserving thicker deposits.

• Around Stockholm, the ice was mainly warm-based, which polished and scoured the bedrock surface, stripping away much of the till that might otherwise have accumulated.

How many times has Sweden been covered in ice over the last 1 million years?

at least 4 times but probably more

When was the “Last Glacial Maximum” in the northern hemisphere?

22 000 years ago (22 ka)

What drives the changes in Earth climate over the last 5 million years?

a combination of external (orbital) forcing and internal feedback mechanisms:

1. Orbital Forcing (Milankovitch Cycles)

   • Precession (23,000 years) – wobble in Earth’s axis.

   • Obliquity (41,000 years) – tilt of Earth’s axis.

   • Eccentricity (100,000 years) – shape of Earth’s orbit around the Sun.
     These cycles alter how much solar radiation reaches Earth and where it is distributed, which triggers glacial–interglacial cycles. Around 2.5 million years ago (Pliocene–Pleistocene transition), orbital forcing began to drive much stronger glaciations

2. Feedbacks and Internal Mechanisms

• Ice–albedo feedback: expanding ice sheets reflect more sunlight, cooling Earth further.

• Ocean circulation changes: shifts in heat and carbon storage influence glacial cycles.

• Atmospheric CO₂ variations: amplify warming or cooling.

• Most recently, humans have become a major climate driver through greenhouse gas emissions

3. Regional Evidence (Scandinavia/Sweden)

• Early hints of northern hemisphere glaciation appeared ~12–14 million years ago, but large-scale ice sheets expanded strongly from ~2.7 Ma.

• Sweden has been covered by multiple ice sheets over the last 2.5 Ma, with the largest at ~200 ka and the most recent at 22 ka (Last Glacial Maximum)

What are “glacials (glacigen)” and “interglacials (mellanistid)”?

• Glacials (glacigen) = cold periods when large parts of Earth are covered by continental ice sheets and mountain glaciers.

  • These are the ice ages within the Quaternary.

  • For example, the Last Glacial Maximum (22,000 years ago) when Sweden (and most of northern Europe) was under thick ice

    L3 The glacial history of Swede…

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• Interglacials (mellanistid) = warm periods between glacials, when the ice sheets melt back significantly or disappear.

  • During interglacials, like the present Holocene (last ~11,700 years), climate is relatively stable and warmer, with forests and ecosystems expanding in northern latitudes.

What is “ice rafted debris” and what is it’s significance in terms of the glacial history of Scandinavia?

Ice-rafted debris (IRD) refers to rock and sediment material carried by icebergs. When icebergs break off glaciers and drift into the ocean, they eventually melt and drop the entrained debris onto the sea floor. These sediments—often mixed sizes from clay to boulders—are called IRD or sometimes dropstones when individual large clasts land in fine-grained marine sediments

• The earliest hint of glaciation in Scandinavia comes from IRD found in Norwegian Sea sediments, dating to the middle Miocene (~12–14 million years ago)

• This shows that incipient glaciations were already affecting land in northern Europe long before the major Quaternary ice ages.

• A marked increase in IRD supply around 3.2–2.7 million years ago reflects the strengthening of Northern Hemisphere glaciations at the Pliocene–Pleistocene transition (~2.5 Ma).

• Thus, IRD provides direct geological evidence that ice sheets existed in Scandinavia and were large enough to calve icebergs into the ocean.

Sweden has been covered by ice that was “warm” (ice contains some liquid water) in some areas and “cold” (no liquid water) in others. Give an example of a landform or soil types that correspond to coldbased ice and warm-based ice

cold-based: preserved pre-glacial land surfaces and weathered bedrock in northern Sweden, older landforms (>22ka) that survived later glaciations because cold-based ice acted like a protective blanket

warm-based: glacially polished and smoothed bedrock surfaces, till, drumlins, eskers and De Geer moraines (streamlines or depositional features formed beneath or at the margin of warm ice.

Name three physical/sedimentological characteristics of till (morän)

1. Poorly sorted: contains a very wide range of grain sizes, from clay to silt to gravel, cobbles and even large boulders 

2. Subrounded to subangular clasts (stones)

3. Tend to be very consolidated (hard in comparison to other soils)

Till is often described as “non uniform” because it has variable sedimentological properties (e.g. grains of many different sizes). Why is till so variable? (hint: think about the different ways that ice sheet and glaciers can erode, transport and deposit soils/sediment)

1. Different modes of erosion 

   • Glaciers pluck large blocks of bedrock, grind fine rock flour by abrasion and entrain pre-existing sediments

   • Produces a wide grain-size spectrum (clay → boulders) within till

2. Transport with the glacier

   • Sediment may be dragged at the ice base (producing highly compacted subglacial traction till)

   • or may be carried within or on top of the ica and released later with minimal deformation (melt-out till)

3. Deposition process

   • At the glacier bed: strong shearing leads to consolidated, structureless till 

   • melt-out of stagnant/debris-rich ice →less compacted, more variable till

   • deformation of pre-existing sediments under ice can create glacitectonite with mixed structures 

4. Local geology and hydrology 

   • Type of rock that erodes (granite, sanstone, limestone etc) strongly influences till composition

   • Meltwater action can partially sort some sediments, adding more variability 

Till is so variable because glaciers act as both bulldozers and conveyor belts, eroding bedrock, mixing materials of many sizes, transporting them in different ways (at the base, inside, or on top of ice), and then depositing them under contrasting conditions (shearing, melt-out, deformation).

What are “turbidites (turbidit)”? why are they important?

• Turbidites are deposits formed from turbidity currents — underwater flows of sediment-laden water that move downslope, often triggered by slope failure, earthquakes, or rapid sediment loading.

• They are dilute, gravity-driven flows where fluid turbulence keeps sediment in suspension.

• When the current slows down, the sediment settles out in a graded bed: coarser material at the bottom, finer at the top.

Importance: 

• They provide an important record of past ice sheet advances into marine environments.

• Many oil and gas reservoirs are found in ancient turbidite deposits because their sands can have high porosity and permeability.

• Modern turbidity currents can damage submarine cables and infrastructure, so understanding them is important for engineering.

What are the “Sweden Lake Stages”? what types of sediments are associates with these lakes?

These are sequential lake phases that filled low-lying parts of Sweden when the ice margin was still nearby, and later when land was depressed by ice load and the sea/lakes flooded the basins. The main stages include:

• Baltic Ice Lake

• Yoldia Sea

• Ancylus Lake

• Littorina Sea

Types of sediments associated with these lakes

1. Glacilacustrine sediments (svallsediment / svämsediment)

   • Deposited in ice-dammed lakes while the ice sheet was still present.

   • Typically clay- and silt-rich, often forming varved (annually laminated) clays.

   • Can occur on hillsides where former lakes were ponded.

2. Lacustrine sediments (lakustrina sediment)

   • Deposited in postglacial lakes after ice retreat.

   • Usually fine-grained (clay to fine sand), sometimes with organic layers.

   • Commonly found below the highest shoreline in Sweden.

3. Postglacial clays (postglacial lera)

   • Widespread in basins and depressions below the Highest Coastline.

   • Important because they can act as aquitards (low-permeability layers) and may contain contamination.

Where would you find “varved clay” in Sweden?

varved clay consists of annual layers (varves) - alternating light (summer, coarse silt/sand) and dark (winter, fine clay) layers deposited in lakes

Below the highest coastline and in valleys in the mountains that were contained ice-dammed lakes during ice sheet melting

What is “quick clay”?

marine/brackish clays deposited with salt particles after the last glaciation that can liquefy when disturbed, common in coastal/low-lying areas of Sweden.

How are eskers (rullstensås) formed? What type of sediments are found in eskers?

Formed by meltwater rivers flowing in tunnels under the ice.

Composed of sorted sand, gravel, and rounded cobbles.

Name two types of moraines (morän not “till”)? What type of sediment do you find in moraines?

1. De Geer moraine: series of separate, narrow ridges tredning prallel to a former ice front.

2. Rogen (ribbed) moraine: formed below ice, but still debated exactly how

sediment: till

What is a drumlin (drumliner)?

• A streamlined hill of till/bedrock shaped beneath fast-flowing ice.

• Long axis aligned with ice flow direction (looks like a teardrop in map view)

• formed below the ice (subglacial) by a successive build of sediment (till) to create the hill (deposition) or pre-existing sediments may have been depleted in places leaving residual hills (erosion)

• depositional drumlins: aquifers, source of aggregates

• glacial landfrom