Glaciers (Part 3)

Subglacial Landforms

flutes, drumlins, megaflutes, rogen moraines, murtoos

Flutes

Lines of poorly sorted subglacial sediments formed behind boulders/rocks, small features, fabric with clasts aligned with direction of formation, small ice features

Flute formation

Water saturated till squeezed into cavity, either cavity propagation (till freezes and becomes part of boulder and creates a bigger obstacle) or sediments move (freezes onto base of ice and moves with glacier)

Drumlins

sedimentary feature, looks like a rock drumlin, occur in fields of drumlins, varied composition (rock, till stratified sediments, core and carapace), unsure how they form, icesheet feature

Megaflute

elongated drumlins, associated with ice sheets, older than moraines

Rogen moraines

sinuous ridges with ripple form, fields of them, occupy lowland areas (lakes and swamps), made out of anything, horn shape is up-glacier, ribbed bedform with lakes and swamps in between

Boulton model of drumlin formation

Differential deformation of different materials due to hardness and water content, fine-grained is pulled away while more resistant material stays put (different degrees of deformation, resistant material forms the core of drumlins)

Rogen moraines and drumlins link

Rogan moraines are an end member of the continuum, Rogen moraine to drumlin to megaflutes (more deformation over time)

Shaw model

Subglacial flood creates ripple shaped cavities which fill with sediment (rogen moraines), can be both depositional and erosion (explains variation in types of rogen moraines)

Murtoos

meltwater feature, triangular shaped (points down-stream), might show amount and severity of floods (breaks apart ripples)

Eskers

sinuous, steep sided ridges, icesheet features, sand and gravel (well rounded), from subglacial tunnels (R-channels), closed tunnel (under pressure and can go uphill) vs open tunnel (can’t go uphill, low pressure), good source of water and gravel, can see quick changes in velocity with changes in sediment size

Moraines

ridges of sediment formed by glacier, marks the ice margin, classified by location and mechanism, formed from till, size determined by how much sediment it holds (big glacier doesn’t always mean big moraine)

Moraine location

terminal (ends of glacier), lateral (sides, right and left), medial (in middle), recessional (formed during retreat), can be overridden during advancement, when new they are very steep and sharp

Ice-contact slope

side facing the glacier, very steep when new, contains ice, slumps over time (rounds out)

Grounding line moraine

marine moraine, where the ice starts floating, rocky surface (currents take away fine sediments)

Push moraines

glacier acts as a bulldozer due to advancement, not preserved well (overrides previous), can be in retreating glaciers if winter advancement, ice distal slope (back side), irregular shape copying ice margin (distal is steeper), size (how much material and how strong)

Dump moraines

Formed by stuff falling off the ice at margin, very common in polar areas, amount of material carried and how long glacier is at that position (size)

Ice-cored moraines

debris protects ice from ablation (ice inside), core disappears overtime leaving little pile of debris

Glaciotectonic moraines

translocation of sediments by faulting or squeezing, caused by stress differences under and in front of glacier, common in areas with partially frozen ground, favored by pre-existing weaknesses, squeezes out sediments from fault, finer grained sediments, fabric, fewer boulders, pressure differences

Medial moraines

represent where tributaries come together or downstream of nunataks (rock falls off and is carried away)

Kame terraces

ice marginal features in valleys, form from streams from margin and valley wall, stream beds are left as ice leaves (up on valley wall), show position of past glaciers (fluvial, outwash), are not kames (kames are any lump that doesn’t fall under another category)

Polar sediments

where glacier is cold based, not sliding, frozen to bed, smaller more subtle, little to no striations or molding, debris=parent material, coarse grained (no rock flour), deposits of many ages, thin deposits (windows to underneath)

Polar landforms

no water formed features

Palimpsest landscape

stacking of previous glacial deposits, polar don’t destroy previous deposits (windows show previous)

Aeolian features and reworking

wind blows fine-grained away (katabatic winds), tightly packed coarse rocks

Kettle

buried ice, a pond from a buried ice block, if didn’t sublimate immediately, persists up to 8 million years

LGM deposits

No staining (not enough time exposed), goes from light deposits to darker old deposits, grey big rocks from LGM

Cavernous weathering

makes hollows in rocks (sand blowing against it)

Glaciocustrian

lakes (proglacial, ice-dammed, moraine dammed, kettles), pattern of sedimentation controlled by lake stratification dur to temperature, salinity, and sediment content (temperate=lots of sediment, polar=not many lakes, very small)

Sedimentation processes (plumes)

meltwater plumes, subglacial and supraglacial meltwater coming into lake, can get trapped seawater at bottom of coastal lakes, overflow (denser below), interflow (middle), underflow (lots of sediments), sediment rains out of plumes (gradient of sizes away from glacier)

Turbidites

underflow builds up and falls over

Rhythmites

cyclical sedimentation, if annual=varves (coarse-grained lighter in summer and darker and fine-grained in winter), repeating layers of sedimentation

Ice Rafted Debris (IRD)

debris coming out of icebergs, results in coarse grained material farther from glacier

Lake-Ice conveyors

permanent ice cover, melts out at edge (solar radiation interacts with dark rocks), creates a moat (dump zone of ice rafted drift)

Fjord

loses contact with bed (creates lip), causes shallow pinning point when glaciers advance, moraines form on lip, restricts flow and berg movement

Calving

creates large waves, scouring on land when wave hits, stirs up sediment and makes an iceberg turbate

Overflow plume

meltwater (freshwater), less dense and goes to surface, sediments rain out, can create fans or deltas (debris flows if unstable)

Glacier environments

ice contact (touching glacier or subglacial), ice proximal (within a few 100 meters, very active, lots of resuspension, lots of sedimentation, fans and deltas), ice distal (less activity, far from glacier)

Ice contact delta

sediments come across top and cascades down creating topsets and foresets, boulders and kettles left behind after glacier retreats

Presumpscot formation

Maine, marine clays, blue-grey, sticky, created by rock flour that is suspended when glacier retreats, very poorly drained (not very strong when disturbed), results in slumps and landslides (destabilization)

Relative dating

gives floating age sequence, this is younger/older than this, stratigraphy, morphology (crosscutting, overlying), fossils, weathering of rocks (progression), lichen sizes, varves (annual), documents

Absolute dating

real numbers, radiocarbon dating

Radiocarbon dating

absolute, C14 made from cosmic rays bombarding nitrogen, 14CO2 reacts with oxygen (taken up by all living things), C14 decays back to nitrogen over time

Radiocarbon dating requirements

how much at the beginning, how much left, rate of decay, in a closed system

Closed system requirements

growth (bacteria, moss, etc.), recrystallization, water (carries away carbon), contamination during sampling

Beginning concentration

assume atmospheric C14 is same then as not (not true), altered by solar cycles (more activity causes less cosmic rays), burning fossil fuels (dilutes), nuclear testing (big C14 input), ocean circulation, geomagnetic field, need to use a calibration curve (good for 50,000 years)

Periglacial

form in areas with permafrost, continuous and discontinuous (patches)

Permafrost layers

Upper layer (active layer, lots of influence of frosting and thawing, poor drainage), permafrost table (not thawing, always frozen), isothermal permafrost, frost-free soil below

Freezing front

water is attracted to it and creates ice lenses (sticks to it), pulls rock upward and other stuff falls underneath, pushes rock up (frost heave)

Stone circles

in areas of humid environment and with continuous permafrost, sorted (finer in middle and coarser on outside), thought to be due to frost heave, as freezing front moves down into ground it is unequal (depends on how well drained, if poorly drained there is slower progression), rocks collect in other areas with rocks (good drainage)

Contraction cracks

ground contracts as it freezes, fills with sand or ice and rocks move into it sometimes

Thermokarst

melting or freezing of ice in active layer, creates streams along surface, disappear and reappear, collapsing of roofs of streams creates sinkholes (thermokarst)

Stone stripes

sorted (similar to circles), form with sloping ground, stone circle stretched along a slope and elongated, needs to be freeze-thaw

Mud boils

churning up/convection of fine grained and waterlogged sediments

Pingo

hills with a core of ice, sediment overtop, lake gets covered which cools forming ice and pushes it up, or slope pushes under at high pressure (bumps)

Palsa

made of peat (wetlands with uneven, thin snow cover), frost line penetrates deeper from less snow and pushes up

Rock glaciers

any mass of flowing rock and ice, frost heave moves them up while moving downhill, ridges and furrows, often start as glaciers with debris but melt ice to have more rock than ice

Glacial history of Maine

LGM (covered by Laurentide icesheet), came out to Georges Bank (outwash plain to ocean), ice stream cut Maine off from Laurentide (St Laurence River) which caused Maine to be covered by local ice cap (flow reversals)

Dating in Maine

radiocarbon from marine shells and/or organic carbon in lakes and bogs, recent surface exposure dating (cosmic rays make reaction on rock Be10), retreat happened very fast and readvanced

Katahdin

First place uncovered by Laurentide

Ocean inundation

accompanied deglaciation, shows grounding line moraines (push moraines and folds)

Pineo ridge

Big ice contact delta, ice reorganized and pushed forward again after retreat, created many deltas and eskers from high meltwater, very flat land, kame and kettle topography (bumpy), boulders left from ice contact slope

Shoreline notch

from changes in elevation, isostatic rebound causes water level to be lower than delta height which cuts out a shoreline