Lecture 4

Question 1

glacial till

Answer 1

Rock debris entrained, transported and deposited in direct contact with glacier
ice

Question 2

moraines

Answer 2

Rock debris entrained, transported and deposited in direct contact with glacier
ice is known as glacial till and creates landforms known as

Question 3

proglacial envvironment

Answer 3

beyond the ice margin

Question 4

Meltwater streams transport rock debris of all sizes from boulders to clay. Deposition of stratified _______ sand and gravel occurs within stream channels while finer silt and clay are deposited as ______ sediments on the bottom of ______ lakes.

Answer 4

Meltwater streams transport rock debris of all sizes from boulders to clay.
Deposition of stratified glaciofluvial sand and gravel occurs within stream channels
while finer silt and clay are deposited as glaciolacustrine sediments on the bottom of
proglacial lakes.
-These glacial, glaciofluvial and glaciolacustrine deposits constitute the
parent materials for many soils throughout the circumpolar North.

Question 5

Permafrost

Answer 5

Permafrost refers to a
thermal condition observed in soils, peat and rocks in which ground temperatures remain
below 0oC for two or more consecutive years.
-In regions where the depth of frost penetration into the ground during the fall and winter is greater than the depth of ground thawing in the summer, a zone of permanently frozen ground known as permafrost persists throughout the year.

Question 6

3 types of permafrost

Answer 6

1.Continuous permafrost refers to an environment where more than 80 percent of the ground surface is underlain by permafrost. The southern limit of continuous permafrost corresponds closely to the –8oC mean annual isotherm derived from air temperatures.

2. Discontinuous permafrost refers to an environment where 30 to 80 percent of the ground surface is underlain by permafrost. The southern limit of discontinuous permafrost corresponds closely to the –1
   oC mean annual isotherm derived from
   air temperatures.
3. Sporadic permafrost refers to an environment where less than 30 percent of the ground surface is underlain by permafrost.

Question 7

patterned ground

Answer 7

Landforms characterized by a variety of shapes
including circles, polygons and stripes are associated with the presence of ground ice in
periglacial landscapes.

Question 8

Glacier

Answer 8

A glacier can be defined as a large naturally occurring deposit of perennial ice formed
from the accumulation and recrystallization of snow, which is capable of flowing slowly
under the pressure of its own weight and the force of gravity.

Question 9

Glaciers cover approximately __ percent of the present surface of the Earth (Table 4.1). At the Last
Glacial Maximum (ca. 30,000 years ago) glaciers covered approximately __ percent of
the Earth’s surface.

Answer 9

10

30

Question 10

Area with largest current glacier areas

Answer 10

Greenland Ice sheet

Question 11

3 types of Glaciers

Answer 11

1. Alpine glaciers develop in highland regions and are
   constrained by topography, being confined to mountain valleys (Figure 4-1a, b).

2.Piedmont glaciers form when the lower reaches of alpine glaciers coalesce and spread over lowlands at the foot of a mountain range (Figure 4-1c). These two forms of glaciers cover an area ranging from 5 to 10,000 km2

3.Continental glaciers, on the other hand, cover a vast area exceeding 25,000 km2 . These large glaciers tend to inundate the underlying topography; however, the tallest mountain peaks may protrude through the
glaciers as nunataks (Figure 4-1a).

Question 12

nunatuks

Answer 12

tallest mountain peaks may protrude through the

glaciers

Question 13

Where does glacial ice come from?

explain steps

Answer 13

Glacier ice originates primarily as snow deposited on the glacier surface. Over time snow is gradually transformed into firn via the combined action of compaction and melting. Firn is eventually converted into glacier ice. The transformation of fresh snow into glacier ice involves the progressive compression of air bubbles and a reduction in porosity resulting in an increase in the density of the material

Question 14

Density of different types of snow:

Fresh Snow ___ – ___
Glacier Ice ___ – ___
Firn ___– ___

Answer 14

1. Fresh Snow 50 – 100
2. Firn 400 – 800
3. Glacier Ice 830 – 910

Question 15

Glacial Mass Balance:

Mass balance = _______ – ______

Answer 15

Mass balance = accumulation – ablation

Glacier mass balance refers to the annual variations in the mass and volume of glaciers
that result from the interaction of accumulation and ablation processes.

Question 16

Accumulation vs. Ablation

Answer 16

1.Accumulation: processes include direct precipitation as snow, the refreezing of meltwater and sublimation (i.e., the physical transformation of water vapour into ice).
These processes jointly serve to increase the mass and volume of glaciers.

2.Ablation: processes include the melting of snow and ice via insolation (i.e. solar radiation), friction associated with internal deformation of the glacier, the flow of geothermal heat (i.e., heat supplied from the interior of the Earth), sublimation (i.e., the physical transformation of glacier ice into water vapour), and the calving of icebergs (Figure 4-1b). These processes jointly serve to reduce the mass and volume of glaciers.

Question 17

The surface of a glacier can be split into three sections

Answer 17

The surface of a glacier can be divided into a zone of accumulation (i.e., positive mass balance) at higher elevations and a zone of ablation (i.e., negative mass balance) at lower elevations separated by the equilibrium line (i.e., accumulation equals ablation;
Figure 4-2).

Question 18

Positive vs Negative Mass balance

Answer 18

positive mass balance: The glacier
thickens and develops a steep surface slope, the glacier flows more rapidly and the
glacier margin advances.

negative mass balance: The glacier thins and develops a gentle surface gradient, glacier flow decelerates and the glacier margin retreats.

Question 19

Internal Deformation

Basal Sliding

Answer 19

Internal deformation involves the slippage of ice crystals past one another within the
glacier.

Basal sliding involves the glacier ice sliding along the underlying bedrock surface
on a thin layer of meltwater.

Question 20

Explain Difference between

1. warm-based or subpolar
2. Cold-based or polar glaciers

Answer 20

In warm-based or subpolar glaciers, ice temperatures near the bed are near the melting point of ice. Warm ice deforms easily resulting in a higher rate of internal deformation. Warm ice also permits meltwater generated at the surface or within the glacier to move through the glacier to lubricate the bed thus facilitating basal sliding.

Cold-based or polar glaciers, on the other hand, exhibit basal ice temperatures well below the melting point of ice and these glaciers are frozen to their beds. Motion of these glaciers is via internal deformation only.

Question 21

Average ice velocities range from ___ to
___ m annually, but can exceed ____ m annually in exceptional circumstances such as
glacial _____.

Answer 21

Average ice velocities range from 10 to
200 m annually, but can exceed 1000 m annually in exceptional circumstances such as
glacial surges.

Question 22

Abrasion

Plucking

Answer 22

Abrasion is a mechanical weathering process. Glaciers use rock clasts at their base as abrasive cutting tools. The rate of abrasion is influenced by variations in ice pressure and velocity, the concentration of rock clasts near the base of the glacier and the hardness of
the rock clasts relative to the bedrock substrate. Thick, fast-flowing ice armed with hard (e.g., igneous and metamorphic rocks) rock clasts promotes rapid and deep abrasion of the bedrock substrate.

Plucking is a mechanical weathering process that involves the removal of large blocks of rock from the glacier bed. Plucking occurs where a glacier is
forced to flow over bedrock obstacles on the bed of the glacier. In this situation, ice pressure exerted on bedrock obstacles is greater on the upstream side of the obstacle than on the downstream side causing the rock to fracture on the downstream side of the obstacle. Higher ice pressure on the upstream side of the obstacle promotes melting of basal ice and the generation of meltwater. Meltwater lubricates the glacier bed allowing the glacier to slide over the obstacle. On the downstream side of the obstacle the meltwater refreezes, a process known as regelation, and loosened blocks of rock are removed by freezing to the base of the sliding glacier.

Question 23

regelation

Answer 23

a type of plucking: On the downstream side of the obstacle the
meltwater refreezes, a process known as regelation, and loosened blocks of rock are
removed by freezing to the base of the sliding glacier.

Question 24

High rates of erosion via abrasion are most likely to occur during periods of high meltwater discharge in the _______(season)

Answer 24

summer

Question 25

Supraglacial environment?

Answer 25

supraglacial environment (i.e., on the surface of the glacier)

Question 26

Supraglacial rock debris is delivered by…2

Answer 26

rock debris is delivered via mass
wasting (i.e., avalanches, rockfalls, etc.) and aeolian (i.e., wind) processes onto the
surface of the glacier.

Question 27

Englacial environment?

Answer 27

This transport of rock debris through the glacier occurs

within the englacial environment. englacial environment is seemingly the inside of the glacier

Question 28

In the _____ zone, supraglacial rock debris moves down through the englacial environment to the surface to act as new cutting tools, whereas in the ____ zone supraglacial rock is delivered at the surface to the glacial margin

Answer 28

accumulation

ablation

Question 29

Subglacial environment?

Answer 29

subglacial environment (i.e., at the base of the glacier)

Question 30

T OR F

The englacial transport of subglacial debris away from the bed coupled with the melting of glacier ice via solar radiation serves to decrease the quantity of supraglacial debris present in the ablation zone.

Answer 30

F-Increase the quanitity rather than decrease

Question 31

Lodgement

Answer 31

Lodgement occurs when the drag
force cannot overcome friction between a rock clast and the underlying bedrock surface
beneath thick ice and the clast comes to rest on the glacier bed.

Question 32

Where are these features found? Mountainous terrain or lowland landscapes?

roches moutonnees

aretes

cirques

rock basin

horns

glacial troughs

hard crystalline rock

Answer 32

In mountainous terrain: glacial erosion modifies pre-existing river valleys to produce classical alpine topography characterized by cirques, arêtes, horns and
glacial troughs.

Glacial erosion in lowland landscapes: creates features such as roches
moutonnées and rock basins in the hard crystalline rocks of continental shields.

Question 33

Roches Mountonnees

Answer 33

Roches moutonnées are asymmetric, streamlined bedrock forms produced by the
combined action of abrasion and plucking (Figures 4-5 a & b). The gently sloping
upstream side of these features is the product of intense abrasion. The steeply sloping
downstream side bears evidence of plucking in the form of fractured rock and rugged
topography. These features are oriented parallel to the direction of ice flow and provide a
sense of ice motion across the landscape.

Question 34

Deranged drainage

Answer 34

Note that the lake basins are not linked to one another by stream channels: a drainage
pattern known as deranged drainage that is typical of glaciated landscapes

Question 35

cirques

Answer 35

Cirques are amphitheater-shaped depressions excavated in bedrock through the combined action of glacial abrasion and plucking, and the frost shattering of rock (Figure
4-6).

Question 36

Cirque growth may progress over time to the point where narrow ridges of rock known as _____ separate adjacent cirques

Where cirque glaciers extend
into adjacent alpine valleys, glacier erosion will widen and deepen the valleys to create
glacial ______

Deglaciation of the coastline allows
the sea to flood into and partially drown the glacial troughs to create ______

Answer 36

Cirque growth may progress over time to the point where narrow ridges of rock known as arêtes separate adjacent cirques

troughs

fjords

Question 37

3 types of moraine

Answer 37

1. Lateral moraines are composed largely of coarse, angular rock debris delivered to the glacier margin or surface via mass wasting (Figure 4-7a). Till is deposited largely by ablation of the glacier surface to form ridges along each side of a glacier.
2. If adjacent alpine glaciers coalesce, their lateral moraines join to form a medial moraine.

3.Rock debris transported supraglacially and englacially and deposited via melt-out in distinct
ridges at the margin of glaciers forms end moraines (Figure 4-7b). End moraines mark the maximum extent of glaciers in present-day and formerly glaciated landscapes.

Question 38

In contrast to the till that composes lateral moraines, rock clasts present in the till that composes end moraines exhibit a greater degree of _______. The greater degree of _______ is the product of abrasion in a subglacial environment.

Answer 38

roundness

rounding

Question 39

What does a lateral moraine look like in comparison to a medial moraine?

Answer 39

Coarse, angular rock debris produced by frost shattering on the valley walls and transported by mass wasting accumulates against the margin of the glacier to form a lateral moraine (right foreground).
-larger more jagged

The ‘stripes’ of rock debris on the glacier surface are
medial moraines (left foreground).
-smaller material

Question 40

Describe Hummocky and Ground Moraine

Answer 40

Hummocky moraine develops where supraglacial debris is deposited over stagnant ice at the glacier margin. Stagnant ice refers to glacier ice that has become so thin via ablation that it is no longer capable of flowing. Slow and uneven melt out of the buried stagnant ice creates a landscape characterized by rolling topography associated with till ridges interspersed with shallow, water-filled basins known as kettle lakes (Figure 4-8).

Ground moraine is composed of poorly sorted glacial till deposited beneath a glacier by a combination of lodgement and basal melting of the glacier. The till is deposited in layers of variable thickness over broad areas of present-day and formerly glaciated
landscapes (Figure 4-9).

Question 41

Stream Load

Answer 41

Stream load refers to the quantity of sediment that a stream is capable of transporting at a given discharge. Rock debris of all sizes from boulders to
rock flour is entrained and transported by streams during these periods of high
discharge.

Question 42

eskers

braided streams

Answer 42

The capacity of streams to transport sediment decreases contributing to the deposition of glaciofluvial sediments in both subglacial and proglacial environments. Deposition of sand and gravel within channels at the base of the glacier forms eskers

Beyond the glacier terminus deposition of sand and gravel within proglacial stream channels creates bars that split the stream into numerous small channels to produce an intricate channel pattern known as a braided stream

Question 43

The finer silt and clay transported by meltwater streams are eventually deposited
as ______ sediments on the bottom of ______ lakes

Answer 43

The finer silt and clay transported by meltwater streams are eventually deposited
as glaciolacustrine sediments on the bottom of proglacial lakes

Question 44

A zone of permanently frozen ground known as permafrost persists throughout the year in regions where the depth of _____ penetration into the ground during the fall and winter is greater than the depth of ground ______ in the summer

Answer 44

A zone of permanently frozen ground known as permafrost persists throughout the year
in regions where the depth of frost penetration into the ground during the fall and winter
is greater than the depth of ground thawing in the summer

Question 45

The upper surface of the permanently
frozen ground is known as the ______ _____. The ground above the permafrost table
is referred to as the ______ layer, which thaws in the summer. Seasonal variations in
ground temperatures decrease with depth towards the level of ______ ____ ______
that defines the permafrost table (Figure 4-12).

Answer 45

permafrost table

active

zero annaual amplitude

Question 46

The __-shaped figure above is a geotherm, illustrating how temperature changes with
depth

Answer 46

Y
-The geotherm divides at the Level of Zero Annual Amplitude (LZAA).
-Below the LZAA, ground
temperature is unaffected by seasonal temperature changes at the ground surface.

Question 47

talik

Answer 47

permanently unfrozen ground, a talik, exists where the

geotherm crosses to the right of 0oC.

Question 48

Ground temperatures do not fluctuate at greater depths, but they gradually increase with depth, i.e., __ degree C per __ metres, in response to the flow of geothermal heat towards the ground surface.

Answer 48

1

50

Question 49

what does the prescence of talik within or below permafrost mean?

Answer 49

Taliks are areas of unfrozen ground that are situated within and/or below the base of the permafrost layer. The presence of taliks allows for the movement
of groundwater through permafrost to lakes and stream channels

Question 50

Mass Movement

Answer 50

Mass movement is the downslope movement of rock and soil in response to the force of
gravity

Question 51

Movement of material occurs

via two processes

Answer 51

1. Frost creep refers to the downslope movement of particles in response to the expansion and contraction of surficial materials associated with frost heaving of the ground surface. Materials moving in this manner can
   be displaced downslope several millimetres to several centimetres each year.
2. Little water can seep into permafrost soils. Thus, water from rain, melting snow, or melting ice in the soil pores is trapped in the active layer of the soil. On sloping land, the weight of the water-saturated soil causes the active layer to slowly flow downhill when thawed in summer;
   - depends on slope and type of vegetation
   - a few cm per year

Question 52

solifluction lobes

Answer 52

The combination of frost creep and gelifluction of surficial materials produces distinct lobate landforms known as solifluction lobes

Question 53

sorted circles

Answer 53

Common features observed in periglacial landscapes are circular patches of bare, finegrained soils that are bordered by coarser rock clasts and tundra vegetation known as sorted circles (Figure 4-16). They may occur singly or in groups and commonly vary from 0.5 to 3.0 metres in diameter. These landforms are common on upland surfaces where the soil contains large quantities of coarse sand and gravel in a fine-grained matrix. The origin of these periglacial landforms remains unclear, although numerous theories have been proposed to account for their development.

Question 54

Frost Heaving

cryoturbation

Answer 54

Freezing of groundwater within pores in soil contributes to the formation of segregated ice crystals. The growth of segregated ice within the active layer results in frost heaving of the ground surface. Vertical displacements up to 20 cm can be achieved in this manner.

Subsequent thawing of segregated ice within the active layer results in subsidence of the ground surface. The mixing of soil that accompanies the seasonal freezing and thawing of the active layer is referred to as cryoturbation. This process contributes to the movement and sorting of stones within the active layer. The depth of mixing affects the rooting zone for
vegetation and nutrient cycling within soils in periglacial landscapes (see Module 6).

Question 55

can transform stone circles present on upland surfaces into stripes on
hillslopes as mass movement becomes more important on steeper slopes

Answer 55

Gelifluction

Question 56

Ice-Wedge Polygons

Answer 56

-Ice-wedge polygons are common landforms observed in periglacial landscapes
-occurs in poorly drained lowlands within continuous permafrost zone
steps:
1. initiated by prolonged freezing of ice-rich soil at
temperatures below –15oC during winter. In response to this intense freezing the ground contracts and cracks, a process known as frost cracking. The cracks outline the borders of polygons visible on the ground surface.

2.In the following spring and early summer meltwater infills the cracks and freezes creating narrow, wedge-shaped veins of ice that penetrate to depths below the active layer. As the active layer develops over the summer the soil warms and expands. However, soil
expansion is constrained by the presence of the ice wedges so that the soil is forced to expand upward forming a slightly raised rim of soil adjacent to the ice-filled frost cracks.

3. The ice wedges present within the frost cracks exhibit lower strength than surrounding icerich
   soil which form lines of weakness within the ground. In the following winter frost cracking occurs preferentially within the ice wedges and the cycle is repeated year-afteryear progressively enlarging the ice wedges so that they become wider and deeper. As the ice wedges enlarge the soil adjacent to the frost cracks continues to be thrust upward to form distinct ridges.

4.The end result is a polygonal landform characterized by a marginal trough occupied by ice wedges and a raised rim of soil and vegetation surrounding a central depression. This landform is referred to as a low-centred ice-wedge polygon
-The central depression is often waterlogged in summer and inhabited by wetland vegetation dominated by mosses and sedges (Carex spp.). The rim exhibits better drainage and is inhabited by a variety of plants
better suited to drier soils, such as dwarf willow (Salix spp.), dwarf birch (Betula spp.), crowberry (Empetrum nigrum), bearberry (Arctostaphylos uva-ursi), bilberry (Vaccinium uliginosum), grasses and lichens.

5.Over time low-centred ice-wedge polygons are transformed into high-centred ice-wedge polygons. Several processes operating simultaneously serve to bring about this transformation. Continued enlargement of ice wedges at the margins of these landforms
serves to gradually raise the ground surface in the interior of the polygons. At the same time, the gradual accumulation of plant debris and aeolian sediments serves to raise the ground surface within the central depression above the surrounding terrain. As the ground surface rises drainage improves and the soil becomes progressively drier. Plants inhabiting
the rim of the polygon expand and displace the wetland vegetation at the centre of the polygons. Water passing over this landscape is progressively diverted into marginal troughs. The flow of water within the troughs, as well as heat stored in the water during the summer season, serves to thaw the upper surface of ice wedges and erode the margins of the polygons resulting in the progressive widening and deepening of the troughs. The end result produces a dome-shaped polygon referred to as a high-centred ice-wedge polygon

Question 57

Periglacial landscapes are closely linked to regions characterized by …(4)

Answer 57

Periglacial landscapes are closely linked to regions characterized by low annual
temperatures, low annual precipitation, intense frost action and the presence of
permafrost

Question 58

DO YOU KNOW WHAT THESE MEAN ASSHOLE?

1. Arête:
2. Braided Stream:
3. Cirque:
4. Cryoturbation:
5. Eskers:
6. Fjord:
7. Firn:
8. Frost Cracking:
9. Frost Creep: .
10. Frost Heaving:
11. Gelifluction:
12. Geotherm:
13. Geothermal Energy:
14. Glacial Till/Diamicton:
15. Glacial Trough:
16. Glaciofluvial:
17. Glaciolacustrine:
18. Horn:
19. Ice-wedge Polygons:
20. Kettle Lake:
21. Level of Zero Annual Amplitude:
22. Lodgement:
23. Moraine:
24. Nunatak:
25. Roche Moutonnée:
26. Rock Basin:
27. Stone Circles:
28. Stone Stripes:
29. Talik:

Answer 58

1. Arête: A sharp, narrow rock ridge commonly found above the snow line in mountainous areas that have been sculpted by glaciers.
2. Braided Stream: A river channel that consists of a network of small channels separated by small and often temporary islands called bars.
3. Cirque: A steep bowl-shaped hollow occurring at the upper end of a mountain valley, especially one forming the head of a glacier.
4. Cryoturbation: In rock, sediments or soils affected by the presence permafrost, cryoturbation (or frost churning) refers to the mixing of materials due to freezing and thawing.
5. Eskers: A long, narrow, sinuous ridge of coarse gravel deposited by a stream flowing in or under a decaying glacial ice sheet.
6. Fjord: A long narrow inlet of the sea between high steep cliffs of a mountainous coast formed by glacial erosion.
7. Firn: Granular, partially consolidated snow that has passed through one summer melt season but is not yet glacial ice.
8. Frost Cracking: Contraction of sediments and ice at extremely low ground temperatures; commonly associated with low air temperatures and a lack of snow and vegetation cover.
9. Frost Creep: The downslope movement of debris, firstly through the growth of needlelike ice which lifts a thin surface layer of particles at right angles to the ground followed by thawing, which allows the loosened debris to slip slowly downslope.
10. Frost Heaving: The uplift and cracking of a ground surface through the freezing and expansion of water underneath.
11. Gelifluction: The slow, downhill movement of saturated rock, sediments or soil in areas typically underlain by permafrost.
12. Geotherm: A line or surface within or on the earth connecting points of equal temperature.
13. Geothermal Energy: Energy derived from the heat in the interior of the Earth.
14. Glacial Till/Diamicton: Glacial till consists of coarse-grained, extremely heterogeneous sediments deposited directly by glaciers.
15. Glacial Trough: A deep U-shaped valley with steep valley walls that was formed from glacial erosion.
16. Glaciofluvial: A term pertaining to streams fed by melting glaciers or to the deposits and landforms produced by such streams.
17. Glaciolacustrine: A term pertaining to lakes fed by melting glaciers or to the deposits and landforms located within them.
18. Horn: When there are three or more cirque headwalls and arêtes joined together to form a single pyramidal shaped peak with very steep walls called a horn.
19. Ice-wedge Polygons: A 3- to 6-sided polygon of ice wedges with straight to gently curving sides formed by ice segregation and the freezing and contraction of sediments.
20. Kettle Lake: A water-filled depression left in a mass of glacial till formed by the melting of an isolated block of glacial ice.
21. Level of Zero Annual Amplitude: The distance from the ground surface downward to the level beneath which there is practically no annual fluctuation in ground temperature.
22. Lodgement: Deposition of sediment by plastering of glacial debris from a sliding glacier bed.
23. Moraine: A mass of glacial till deposited in the form of mounds or long ridges.
24. Nunatak: An isolated bedrock hill or peak projecting prominently above the surface of a glacier and completely surrounded by glacial ice.
25. Roche Moutonnée: An elongate mound of bedrock worn smooth and rounded by glacial abrasion. A roche moutonnée has a long axis parallel to the direction of glacial movement, a gently sloping, striated side facing the direction from which the glacier originated and a steeper side facing the direction of glacial movement.
26. Rock Basin: A depression formed in bedrock by abrasion performed by the sharp-edged boulders that are transported at the bottom of a glacier.

27.Stone Circles: Stone circles range in size from a few centimeters to several meters in diameter. Circles can consist of both sorted and unsorted material and generally occur with fine sediments in the center surrounded by a circle of larger stones. Unsorted circles
are similar, but rather than being surrounded by a circle of larger stones, they are bounded by a circular margin of vegetation.

28. Stone Stripes: Stripes are lines of stones, vegetation and/or soil that typically form from transitioning steps on slopes at angles between 2° and 7°. Stripes can consist of either sorted or unsorted material. Sorted stripes are lines of larger stones separated by areas of smaller stones, fine sediment or vegetation. Unsorted stripes typically consist of lines of vegetation or soil that are separated by bare ground.
29. Talik: A Russian term applied to permanently unfrozen ground in regions of permafrost; usually applies to a layer which lies above the permafrost table but below the active layer, that is when the permafrost table is deeper than the depth reached by winter freezing from the surface.