Geography 140
Introduction to Physical Geography

Lecture: Glacial Processes

     G. Glacial processes
        1. Glacial processes entail the erosion, transportation, and 
           deposition of earth materials by moving ice.
        2. Glaciers presently cover about ten percent of the earth's land 
           surface. At times during the Pleistocene epoch (roughly 2 million 
           years ago to about 14,000 years ago), they covered up to thirty 
           percent!  You can see the extent of the Pleistocene glaciation and 
           its Holocene shrinkage here, as shown just for North America:

           

        3. Glaciers are classified according to size/shape/location and 
           temperature.
           a. Size/shape/location:
                i. Confined glaciers are those confined by valley walls to one 
                   extent or another.  
                   a. They are relatively small and found in mountainous 
                      terrain.  Confined glaciers, then, can be called alpine 
                      glaciers, mountain glaciers, or montane glaciers.
                   b. They can be found at sea level at high latitudes (as in 
                      Alaska, Scandinavia, or southern Chile) but they can 
                      also be found right on or near the equator (Mt. 
                      Kilimanjaro in Tanzania is glacier-covered, as are the 
                      Andes in Ecuador).  There is an inverse relationship 
                      between latitude and elevation of confined glaciers.
                   c. The smallest of these confined glaciers are cirque 
                      glaciers.  These are found in concave depressions on the 
                      higher sides of mountains, up near their crests.  These 
                      concavities are circular in shape (hence the French word 
                      cirque).  The cirques are produced by the weight 
                      and motion of these small glaciers. 

                       

                   d. Valley glaciers are montane glaciers that flow down a 
                      pre-existing valley.  They are confined by the valley's 
                      sides.  They grow as smaller tributary valley glaciers 
                      and cirque glaciers fuse together. 

                       

                   e. Branched-valley glaciers have one or more tributary 
                      glaciers flowing onto and into it (kind of like a 
                      higher-order stream has lower-order tributaries).

                      

                   f. Piedmont glaciers are valley glaciers that have spilled 
                      out of the mountain ranges that confine their sides out 
                      onto flat land, such as a valley, plain, or beach.  
                      Their ends, then, are unconfined.

                      

                   g. Tidewater glaciers are those that empty out directly 
                      into the sea.  They form at such high latitudes that 
                      they can reach down to sea level before ablating.

                      

               ii. Unconfined glaciers are glaciers that flow over a landscape 
                   and are not confined by it.
                   a. Continental ice-sheets are huge domes of ice burying an 
                      entire continent, a significant part of a continent, or 
                      a very large island. 
                      1. Only two exist today:
                         A. Antarctican ice sheet, burying virtually all of 
                            Antarctica, with the occasional mountain peak (or 
                            nunatak) sticking out of it here and there.  The 
                            Antarctican ice sheet is nearly 5 km thick in 
                            places (4,770 m)!
                         B. The Greenland ice sheet covers just about all of 
                            Greenland, the gigantic island to the northeast of 
                            Canada.  In places, this ice sheet gets over 3 km 
                            in thickness!  
                      2. Other continental ice sheets have existed in the 
                         past.  During the Pleistocene glaciation, ice sheets 
                         also extended out from the Rockies (Cordilleran ice 
                         sheet), Canada (Laurentian ice sheet), the 
                         northernmost Canadian islands (the Innuitian ice 
                         sheet), Scandinavia and Finland (the Fenno-Scandian 
                         ice sheet), and northern Russia (the Eurasian ice 
                         sheet).

                          

        4. Glaciers are formed when winter or annual snow accumulation exceeds 
           summer melt (ablation), sublimation (evaporation of ice directly 
           into vapor), and iceberg calving (breaking off of peripheral ice 
           into the sea) over several years' time.
           a. Each year's accumlation is packed down by the next year's, until 
              the pressure of its weight causes the snow to go through a 
              series of mechanical changes that eventuate in glacial ice.
                i. New snow is not very dense (~50-300 kg/cubic meter), 
                   basically consisting of the ice lattice of interlocked 
                   snowflakes with plenty of air and maybe water vapor or even 
                   liquid water.

                   

               ii. As it's slowly compacted down and as the snow experiences 
                   both melting and sublimation, the original snowflakes lose 
                   much of their spikiness:  They become more like rounded ice 
                   crystals.  This can pack down more compactly and attain 
                   greater density (>500 kg/cubic meter).  This stuff is 
                   called "névé," and it normally forms each 
                   year from any snow that falls and persists throughout the 
                   winter.  So, you find this old snow in lots of places that 
                   don't have glaciers.

                     

              iii. If the névé survives the entire ablation 
                   season, packing down and becoming blunter all the while, it 
                   becomes "firn."  Each year's firn is packed down more and 
                   more tightly by the layers of firn forming above it, until 
                   most of the air is pressed out of it.  

                     

               iv. At this point, with densities getting above 850 km/cubic 
                   meter, the firn becomes true glacier ice, a process that 
                   may take from 25 to 100 years.  The ice continues to become 
                   denser through time as long as it exists in the glacier.  
                v. You can often see the separate layers of ice formed by the 
                   annual accumlation and ablation cycle inside crevasses or 
                   great cracks in the ice (which are extremely hazardous, by 
                   the way, because they're so hard to see until you fall in).

                   

           b. These ice accumulations begin to move and become true glaciers, 
              agents of erosion, transportation, and deposition, once they 
              become thick enough for gravity to press down on them so hard 
              that they begin to flow outward or downward away from that 
              pressure.
                i. In other words, the downward stress of gravity acting on 
                   the ice mass exceeds the shear strength (resistance to 
                   differential lateral flow) of the ice.
               ii. This exceedence causes plastic deformation, or squashing 
                   outward and downward.
              iii. For alpine glaciers, this usually occurs once the ice mass 
                   exceeds about 20 m in thickness.
               iv. This flow is outward in all directions (some of it is even 
                   upward on the upper slopes of the mountainside just above 
                   the glacier!), but the great preponderance of the flow will 
                   be downward for an alpine glacier, as slope angle biases 
                   the response of the ice mass to gravity.
           c. The ice moves forward through its own internal plastic 
              deformation and through basal sliding.  
                i. Basal sliding is movement of the lower part of the glacier 
                   as basal shear stress exceeds the resistance of the contact 
                   between ice and rock.  
               ii. Basal sliding is facilitated by readily deformable earth 
                   materials and by a glacier having a relatively warm base.  
              iii. If the temperatures at the bottom of the ice are close 
                   enough to the melting point, meltwater may form below the 
                   ice to lubricate its flow.
               iv. Basal sliding can facilitate glacier motion up to 50 m/day!
           d. Once the ice begins to flow, it will develop internal zones that 
              move at different rates of speed.  
                i. The flow is generally slowest along the bottom of the 
                   glacier, due to the frictional resistance of contact with 
                   the underlying rock.

                   

               ii. Flow is also slower along the sides of a confined glacier 
                   for the same reason.

                   

              iii. A glacier's middle sections flow fastest, especially near 
                   the top surface, because there is no friction from the 
                   ground to impede its movement and the ice molecules can 
                   shear over other ice molecules.
               iv. Extending flow develops when the internal flow lines in the 
                   glacier spread out a bit, perhaps as a valley becomes less 
                   confining, as the glacier spreads out on a piedmont, or 
                   where the ice starts to "fall" down a steeper part of a 
                   descending slope.  
                   a. There is rotational movement at the top of a 
                      concavity, produced by tension, resulting 
                      in extending flow.  This zone of extending flow is 
                      called an "icefall," kind of like a waterfall!
                   b. Tensional forces cause ice failure here, so you often 
                      see huge chunks of jumbled ice blocks marking this area.
                      

                v. Compressive flow forms wherever the flow lines of the ice 
                   are brought closer together by "pinching" as a valley 
                   narrows or wherever the slope of the landscape becomes 
                   shallower (or wherever there is a small rise on an 
                   otherwise descending slope).  This is often seen just
                   downglacier from an icefall.
                   a. These slope variations or concavities in slope create 
                      rotational flow at their upper and lower ends, with 
                      compressive flow developing at the lower end of the 
                      concavity where the ice tends to pile up.
                   b. The ice typically develops a bumpy, stepped appearance 
                      here, called ogives (oh, jives).  These look like arc-
                      shaped steps, with the center bulging downglacier.
                   c. There is often a train of them below an icefall.

                      

               vi. Where the ice is free of compression and extension, and ice 
                   flow lines are moving parallel with themselves, any shear 
                   stress may cause foliation.  This is a deformation of the 
                   ice in lines and planes parallel to the direction of flow, 
                   which form along shear planes separating ice flows moving 
                   at somewhat different speeds.

                   

           e. These varying rates of flow within the glacier exert tremendous 
              stresses on it from tension, compression, and shear forces. The 
              result can be ice failure in places, with cracks and fissures 
              and eventually great "crevasses" forming in the ice.
                i. Crevasses are most commonly seen toward the "snout" or 
                   outward limit of a glacier lobe, due to the tensional 
                   stress there produced by divergent or extending flow and 
                   because of the age of the ice and its history of repeated 
                   failure.
               ii. Because of the different rates of flow in the ice, however, 
                   the crevasses themselves are pulled out of alignment, 
                   forming lunar crescent shapes when seen from above.  The 
                   horns of the crescent point toward the source of the 
                   glacier, away from the direction of flow.
              iii. Crevasses are very serious hazards to anyone hiking up onto 
                   a glacier.  They are pretty much invisible until you fall 
                   into them, 
                   a. They are often hidden by a fresh snowfall or rock debris.
                   b. You may not see them, because you're a bit snowblind 
                      from the full-spectrum reflection of sunlight from the 
                      snow and ice.
                   c. And on cloudy days, there is less contrast to make out 
                      these openings in the ice.

                       

                       

           f. Glacier mass budget is a concept to help in understanding the 
              behavior of glaciers. 
                i. There are three components to glacier mass budget:
                   a. Accumulation of snow on the upper glacier, which has to 
                      do with the amount and type of precipitation falling on 
                      the source area, which relate to the absolute humidity 
                      in the air, the temperature of the air, the slope on 
                      which the ice accumulates, and winds (winds can scour 
                      snow from a slope and prevent its accumulation).
                   b. Forward movement of the glacier and
                   c. Ablation of the lower glacier through melting, calving, 
                      and even sublimation.  Ablation is governed by air 
                      temperature, reflectiveness of the snout's surface 
                      (whether it's covered with insolation-absorbing debris), 
                      type of precipitation in the snout area, wind speeds. Of 
                      these, temperature is the most important.
               ii. If all three are equal, then the glacier has gained and 
                   pushed ahead an amount of ice equal to the amount lost 
                   through ablation.
              iii. If more ice is accumulated than ablates, then the glacier 
                   grows, and its snout advances:  Sustained advances in most 
                   glaciers of the world mark ice ages.
               iv. If more ice ablates at the edge of the glacier than 
                   accumulates in its source, the snout retreats.  Throughout 
                   the process of retreat, ice continues to move downward and 
                   outward, away from the source:  It's just that it ablates 
                   faster than it accumulates and pushes ahead.  In other 
                   words, glaciers don't retreat by shifting into reverse 
                   gear! 
                v. So, while the mass budget is equal to net accumulation and 
                   net loss (with continuing forward movement throughout), the 
                   rate of flow is pretty complex because of the time lag 
                   between one season's accumulation and the corresponding 
                   movement of the glacier.
                   a. It takes decades for snowfall to evolve into glacial 
                      ice.
                   b. Similarly, it takes decades for a large glacier to 
                      respond to any one year's net accumulation.
                   c. As a result, it is not uncommon for glacier behavior to 
                      be out of phase with current climatic trends.
                   d. Sometimes glaciers engage in surges or pretty sudden 
                      extensions (at maybe 10 or 20 or even 50 m per 
                      day!  This may reflect a concentrated input of 
                      gravitational energy, such as a really snowy winter or 
                      run of snowy winters, many decades before, or the 
                      lubrication of meltwater underneath the glacier as 
                      temperatures from a long-ago hot spell diffuse there.
               vi. That said, although some modern glaciers are still 
                   advancing, the vast majority of glaciers are retreating at 
                   the present time.  So, their mass balances are negative.  
                   This could be because:
                   a. Less snow is falling and accumulating over the last 
                      several decades or centuries or
                   b. Temperatures have been warming in that timeframe or
                   c. Both
              vii. This could reflect the warming of the planet's atmospheric 
                   temperature with the ending of the Little Ice Age of the 
                   1700s and early 1800s (when a number of glaciers made 
                   strong advances and/or it could reflect human activity 
                   adding greenhouse gasses to the atmosphere.
        5. Glacial erosion mechanisms.
           a. Glacial ice by itself is not likely to erode most bedrock, 
              because the bedrock has greater shear strength than the yield 
              stress of the ice.  That is, the ice will fail a lot sooner than 
              the rock will.  Yet, glaciers are undoubtedly one of the 
              greatest erosive forces sculpting the high latitude and high 
              elevation parts of the land.  Glaciers accomplish this seemingly 
              contradictory trick through picking up rock materials and then 
              using them to abrade the surfaces over which they pass. 
           b. Plucking occurs when glaciers pick up and incorporate rock 
              fragments on their underside.  
                i. Bedrock usually is jointed and cracked.
               ii. Basal sliding generates friction, which often melts some of 
                   the ice on the bottom of the glacier.  
              iii. This water flows into those cracks and joints.
               iv. This water is also under tremendous pressure from the 
                   weight of the ice above, so it is injected with enough 
                   force to exert an upward pressure on the rock materials it 
                   flows into, essentially jacking them upwards, where they 
                   can be progressively lodged into the ice itself as the 
                   water refreezes whenever the glacier pauses. 
                v. Plucking often creates a pocked and pitted surface where 
                   rock chunks have been lifted up out of the bed.
           c. Scouring occurs when glaciers use this material to abrade the 
              land surface. Some of the material is enbedded within the ice 
              and some of it is carried along under the ice, meaning the 
              glacier sort of rolls along on the material lodged under it.  
                i. Abrasion lasts as long as the rock material survives.
               ii. As time goes on, each rock chunk is worn down in size and 
                   is able to produce only smaller and smaller scratches on 
                   the underlying rock surface.
              iii. Eventually, these rock materials are worn down to a clay-
                   sized powder which makes glacial meltwater look kind of 
                   cloudy or milky.  This fine rock material is called 
                   "glacial flour," and the cloudy meltwater is called 
                   "glacial milk."
           d. Alpine or valley glaciers often erode through undermining the 
              slopes above them, which results in a pile of unsorted debris on 
              their surfaces, which is carried along with the ice and 
              eventually deposited at the glacial snout.
           e. Some of the surface debris is actually material that originated 
              in the bottom of the glacier but was carried up along 
              deformation planes in the ice, as by the rotational flow that 
              produces ogives.
        6. Landscape features produced by glacial erosion vary depending on 
           whether we're talking about the smaller alpine, valley, or piedmont 
           glaciers or the huge ice sheet glaciers.
           a. Smaller glaciers.
                i. Striations are grooves or scour lines worn into bedrock by 
                   rocks entrained in the ice passing over it.  They typically 
                   become smaller in width and depth as you move downward in 
                   the direction the ice once flowed, which makes sense, given 
                   that this scouring will eventually wear the rock fragment 
                   into powder.

                   

               ii. A plucked landscape is pitted and you often see the rock 
                   clasts that were in the process of being removed when the 
                   ice ablated and the ice age ended.

                   

              iii. Glacial polish is a smoothly polished surface that formed 
                   on resistant igneous rock (e.g., granite) that was scoured 
                   to nearly the point of being shiny by glacial flour.

                   

               iv. Glacial pavement is a rock surface from which all soil was 
                   carried away by ice.  It may show striations and polish, 
                   and there may be glacial erratics, or rocks stranded when 
                   the ice melted out from under it.

                   

                v. Roche moutonnée or "woolly rock" or "sheep-shaped 
                   rock" is a rock knob with one side scoured or even polished 
                   by materials compressed against against it and the other 
                   side plucked by the extending flow of ice pulling away from 
                   it.  The scoured side faces the direction the ice came from 
                   and has a gentle slope, and the plucked side faces the 
                   direction the ice was flowing towards and features a 
                   steeper slope.

                    



               vi. Cirques are circular or semi-circular gouges or concavities 
                   high on a mountainside produced by a cirque glacier. 
                   a. The head of the glacier will undermine the top of the 
                      slope by the formation of a crevasse or "bergschrund" 
                      each summer when there is summer melting and the glacier 
                      pulls away from the upper wall of the slope.  Then all 
                      sorts of debris and meltwater falls into the opening and 
                      refreezes there, squashing into the bedrock and 
                      undermining it.  
                   b. So what's left after the ice melts is this circular or 
                      semi-circular depression in the slope, which may contain 
                      a small lake, which is called a "tarn."

                      

              vii. An arête is the sharp, toothy ridge produced when 
                   adjacent cirques cut back into one another.
             viii. A horn is the sharp, spire-like peak that's left of a 
                   mountain when cirques have undermined much of the highest 
                   part of the mountain.  The spire has arêtes radiating 
                   out from it, bordering the cirques.  In this photograph, 
                   you can see a horn with two spires, two cirques, and three 
                   arêtes.

                    

               ix. U-shaped valleys are created when an alpine glacier flows 
                   down a pre-existing stream valley and gouges at its sides 
                   and bottom and undermines its sides until the V-shaped 
                   cross-section usually associated with mountain streams 
                   becomes U-shaped.  
                   a. Hanging valleys are small U-shaped valleys created when 
                      a tributary glacier merges and flows directly onto a 
                      larger glacier.  The tributary glacier, with its smaller 
                      mass and erosional ability, creates a small U-shaped 
                      valley that ends abruptly over the edge of the much 
                      larger U-shaped valley created by the larger valley 
                      glacier.  The larger valley glacier was able to excavate 
                      its valley much more effectively than the small 
                      tributary glacier due to its much greater weight.  As a 
                      result, the larger valley suddenly cuts off the smaller 
                      valley, which is left suspended high over the main 
                      valley.  Once the ice melts, the streams that flow in 
                      these tributary valleys end in spectacular water falls, 
                      like you see in Yosemite Valley.

                      

                   b. Fjords are U-shaped valleys produced by tidewater 
                      glaciers.  Since the glacier reached the sea at the end 
                      of these valleys, the ending of the ice age and the 
                      associated rise in sea levels allowed the sea to invade 
                      these valley to form fjords.  You see these a lot on the 
                      coasts of Alaska, British Columbia, Norway, New Zealand, 
                      and Chile.

                      

           b. The huge continental-scale glaciers have such tremendous erosive 
              power that they tend to obliterate much of the underlying 
              landscape, making the erosional features associated with them 
              rather indistinct.
                i. Glacial pavement, indeed vast plains of exposed bedrock or 
                   bedrock covered with the thinnest and most skeletal soils 
                   that have managed to develop over the last 10-15 thousand 
                   years.  An example would be the Laurentian Shield of 
                   eastern Canada, which is a huge craton worn smooth by the 
                   Laurentian ice sheet.
               ii. Deranged drainage, which means that the varying weight of 
                   the great ice sheets, coupled with variations in the 
                   underlying bedrock and haphazard deposition as the ice 
                   melted, created irregular patterns of minor highs and lows 
                   on the surface.  After the ice melts, the resulting low 
                   spots drain poorly and fill with lakes and wetlands and the 
                   streams that do drain them often take the "scenic route" in 
                   getting from Point A to Point B.  The stream system simply 
                   hasn't had enough time to organize the drainage into an 
                   orderly pattern leading water from higher spots to lower 
                   base level.  Now, you know why Wisconsin is called the                    
                   "Land of 10,000 Lakes"!  

                   

              iii. Large lakes often form where a continental ice sheet flowed 
                   off a crystalline shield or craton onto weaker sedimentary 
                   rock beds. Examples are the Great Lakes and Finger Lakes of 
                   the northern United States and southeastern Canada, which 
                   actually continue in the system of large lakes in central 
                   Canada (e.g., Lake Winnipeg in Manitoba, Great Slave Lake 
                   and Great Bear Lake in the Northwest Territories).    
               iv. Nunataks are mountains sticking out of the ice.  When the 
                   ice melts, they will be grossly eroded on their steep 
                   sides. Nunataks are pretty much the only source of rock 
                   debris that can be found on the surface of the kilometers-
                   thick continental ice sheets.

                   

        7. Transportation by glaciers.
           a. Glaciers incorporate plucked material along their bottoms and 
              sides as lodgement till (lodged under the glacier).
           b. This material may be brought up to the surface if there are 
              deformation planes or crevasses that reach to the bottom of the 
              glacier.
           c. Material carried on the surface of the glacier (from the 
              undermining of the slopes that confine a glacier or from 
              nunataks or from dust blown onto the surface of the glacier):  
              Alpine glaciers are much cruddier than continental ice sheets 
              because they have so many more sources of debris available to 
              them.
                i. Some of this surface debris forms lines along the sides of 
                   the glacier.
               ii. Where two glaciers come together, these lateral lines of 
                   debris often join into a single streak running down the 
                   middle of the glacier.
           d. Surface materials may fall into the interior of a glacier along 
              crevasses (including hapless hikers!).
        8. Deposition by glaciers.
           a. Glaciers push a lot of material within and atop the ice to the 
              snout.
                i. If the ice snout is on a land surface, the debris is 
                   unceremoniously dumped as the ice melts out from under it.  
                   This forms a heap of unsorted debris at the outer edge of 
                   the ice.  These heaps will form wherever the edge of the 
                   ice is stable for a long time.
               ii. If the glacier terminates in the sea, the ice will continue 
                   to spread out on the surface of the sea well beyond its 
                   "grounding line" (where the ice loses contact with the 
                   ground surface, at a depth approximately 90 percent the 
                   thickness of the ice).  The ice sheet thins outward due to 
                   heat flux up from the ocean water.  The ice sheet is 
                   buoyant, but the inevitable tension and compression due to 
                   tides on its leading edge will lead to ice failure and the 
                   calving off of icebergs -- which include not just the ice 
                   but the rock debris on and in the ice.
              iii. Some of the finest glacial flour will be blown off the 
                   snout of the glacier, too.
           b. In addition to straight ice deposition, there is also fluvial 
              deposition associated with glaciers.  This is called 
              "glaciofluvial deposition."
                i. Glaciers extending into temperate climate areas often 
                   experience melting on their surfaces, which creates ponds 
                   on the surface.
               ii. Some of this water flows horizontally across the top of the 
                   glacier just like a regular stream, and pours off the end 
                   of the glacier's snout, where it deposits a texture-sorted 
                   fan of debris, just like the alluvial fans deposited by 
                   ordinary streams pouring out of canyons.
              iii. Some drop vertically into the glacier along "moulins" or 
                   crevasses.
               iv. Some water is formed by frictional heating underneath the 
                   glacier, which I mentioned earlier in connection with 
                   plucking. The water inside the ice is often under 
                   tremendous pressure and this alone can make it pretty 
                   erosive.  Some of the water under the glacier forms 
                   channels of its own and, like any stream, will deposit rock 
                   material in the stream beds. 
        9. Landscape features produced by glacial deposition.
           a. Terminal moraines are the heaps of unsorted debris deposited on 
              the land surface at the outermost extent of a glacier's farthest 
              advance.  The ridge to the upper right near the road is a 
              terminal moraine, the ice having come up from the lower left of 
              the image.

              

           b. Recessional moraines develop as the glacier experiences negative 
              mass balance and begins to melt back.  Since the process of 
              retreat is very uneven and may even feature re-advances of the 
              ice, the moraines often have an opportunity to pile up in 
              ridges, just like the terminal moraine.  You can see a few of 
              them in the image above.
           c. Ground moraine also forms during the process of melting back, 
              particularly during relatively even phases in the melt back.  It 
              consists of the till carried below the glacier, which is often 
              reduced to powder (clay).  In among the clay will be larger 
              rocks, which were dropped before the glacier had a chance to 
              pulverize them completely.  I've heard ground moraine (as in 
              ground-up) described as "boulder-studded clay."
 
             

           d. Lateral moraines mark the sides of a confined glacier.  They are 
              derived from the debris undermined from the sides of the 
              mountains confining them.  Lateral moraines join up with the 
              terminal moraines more or less at right angles to form chevron-
              shaped heaps at their intersection.

             

           e. Medial moraines are made of the debris carried atop the middle 
              of a valley glacier when two tributary glaciers came together 
              and their lateral loads linked up.  Like the lateral moraines, 
              they join up with the terminal moraines more or less at right 
              angles to form chevrons of debris.

             

           f. Erratics are rocks or boulders that had been carried along on 
              the top of the glacier but which were dumped when the ice 
              ablated out from under them.  They often are made of material 
              quite foreign to the area they were dropped, which tells you 
              they rode quite a long way.
 
             

           g. Perched rocks are erratics stranded on a precarious perch.
 
             

           h. Here's a shot of a moraine at the snout of a surging glacier, 
              which is actually in the process of bulldozing some trees, 
              caught in the act by Scott McGee, who runs the crevassezone.org web site!

              

           i. Drumlins are mounds of debris formed from deformed sheets or 
              beds of lodgement till.  They typically are teardrop-shaped and 
              run parallel to the ice flow. 

             

       10. Fluvioglacial depositional features.
           a. Kettle lakes are depressions that were occupied by chunks of ice 
              during the process of recession and then buried or partly buried 
              by fluvioglacial debris.  When the ice chunks finally do melt, 
              the exposed depression is filled by lakes.
           b. Kames are conical fans of texture-sorted material created by a 
              stream flowing off the top of a continental glacier and over 
              moraines.  They can occur in lines during the process of 
              recession and so form terraces of fused kames.  They can also 
              form from the debris trapped in crevasses toward the glacier's 
              snout, which, when exposed by recession, then collapse into 
              small hills and ridges.

              

           c. Related to kames but on the alpine scale are valley trains, 
              which are the reworked and texture-sorted debris created by a 
              stream flowing off the top of the glacier and over the moraine 
              at its snout and down the valley below the glacier's terminus.  
              They are analogous to alluvial fans produced by regular mountain 
              streams flowing out onto a flatter valley floor.
           d. An outwash plain is a larger texture-sorted feature fed by the 
              outwash from several supraglacial streams (kind of like a 
              bajada).
           e. Eskers are the bed deposits of streams that developed under a 
              glacier.  The result is a 
              winding ridge of bed deposits built up over the surrounding 
              ground moraine surface when the ice melts back.
  
               

       11. Glacio-æolian erosion and deposition.
           a. Glaciers are often associated with strong winds.
           b. These will often deflate or pick up glacial flour -- clay sized 
              clasts -- or silt-sized particles.
           c. These winds will then carry the fine material far from the 
              glaciers, depositing it elsewhere to build up thick beds of 
              something called "loess." 
           d. Loess forms a very crumbly and high quality soil, much desired 
              for agricultural purposes.  There is a lot of it in the American 
              Midwest and in China.

               

       12. Glacial trends today.
           a. Because of the lag between snow accumulation and glacial motion, 
              depending on local climates, we can find glaciers that are 
              surging or advancing quickly and glaciers that are retreating in 
              the world today.
           b. The Taku Glacier up in Alaska is advancing (remember the picture 
              of it pushing over some trees?), which is unique among the other 
              Juneau Icefield glaciers.  Arctic glaciers, in general, are 
              retreating but more slowly than is the case in the rest of the 
              world.
           c. Most places, however, glaciers are in rapid retreat:
                i. The glaciers in the Alps have lost about 30-40 percent of 
                   the area they cover and about 50 percent of their volume 
                   since 1850.
               ii. The glaciers in New Zealand have lost about 25 percent of 
                   their area in the last 100 years.
              iii. Glaciers in Central Asia have been declining since the 
                   1950s.
               iv. Glaciers in East Africa on Mt. Kenya and Mt. Kilimanjaro 
                   have lost about 60 percent of their area over the last 
                   century and are doing so at a faster rate recently.
                v. Glaciers in the Andes are now in accelerated retreat, too.
           d. The question is whether the retreating glaciers are responding 
              to the world-wide recovery from the Little Ice Age of the 1700s 
              or to human-induced increases in carbon dioxide or both.  
              Whether or not they are responding to human-induced climate 
              change, humans may need to reduce greenhouse gas production if 
              for no other reason than to offset the warming associated with 
              the end of the Little Ice Age.  Given the lag in glacial 
              response to temperature and humidity inputs, however, we may 
              well see significant glacial melting over the course of our 
              lifetimes, with a consequent increase in global sea level.  A 
              rise in sea level is very problematic, given that the world's 
              population tends to concentrate in low lying areas not far from 
              the coast.  Unfortunately, the earth sciences do not yet have 
              enough data to answer the critical question of the meaning and 
              the rate of glacial change.  Perhaps some of you might be 
              inspired to take on the training in the various earth sciences 
              necessary to put more trained people on the frontlines of 
              answering this question?

Document and © maintained by Dr. Rodrigue
First placed on web: 05/13/01
Last revised: 07/09/07