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#52524 - 01/14/02 09:48 PM Re: drumlin  
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Wordwind Offline
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Drumlins aside, even though this is where we've gone, I'm trying to picture things geological in my poor confused mind.

The glacier is ice, right? And let's go ahead and start at the beginning.

There's the Ice Age. And then there's some melting, right? And the great ice thing--the glacier--begins to move at the pull of gravity because of the melting. And, according to what Geoff has told me, there's a tremendous amount of water being melted, so this great ice thing is preceded by all this great rushing of water that washes out all kinds of things in its path, right?

So, you've got great rushing of waters beyond anything Hollywood has yet to capture, and you have this monolithic huge chunk of ice (the glacier) that follows the water and it carves what the water hasn't already taken out.

Is this, at this point in geology, what is happening? Great cutting down and about by water, and then even more intense cutting out by the glacier?

A single "yes" will set my heart at ease, drumlins and tills and all the other terms aside for the moment.

Best regards,
WordWonderer


#52525 - 01/14/02 10:25 PM Re: drumlin  
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I'm no geological expert, but I don't think there's much liquid water involved in the moving forward of the glacier. It slides down the mountains and then once it gets warm enough it starts to melt. The melting causes it to recede. It's not actually moving back, the front is just melting off. When the glacier forms rocks get stuck inside it and when it melts the rocks stay where they are. I believe a hill made of glacier debris is called a terminal moraine. Moraine, Ohio is named after this.


#52526 - 01/15/02 01:54 AM Re: drumlin  
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The glacier builds up from a great deposit of snow winter after winter,until it can be over a mile deep, with minimal summer melting. When it becomes over a hundred feet deep, the ice at the bottom cannot stay rigid any longer. As if it were wax in a warm room it begins to spread out, becoming less deep. As it spreada out, fissures develop into crevasses. In summer melting puts water into crevasses, and there may be outwash from the southern margin. The Finger Lakes in New York were created by massive melting behind a dam-like formation. Out West not far below the Border, a tremedous lake formed, until it was truly huge. When it finally overflowed the obstruction, it caused enormous erosion - a miniature Grand Canyon. Get stales to tell you about it.


#52527 - 01/15/02 05:55 AM Re: drumlin  
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It's only at the end of the Ice Age that we get the great floods - as the glaciers begin to thaw, refreeze, then thaw again, and ice dams break from time to time.


#52528 - 01/15/02 01:41 PM Re: drumlin  
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NYC is located at the souther edge of the last glacier to come this way. the giant ice sheet, advanced, and pushed before it, loose rocks and dirts, the same way a push brooms does. as it came to NY, is ground to a halt, and stopped. slowly, it receded, and melted. the land below was compressed and sunken as a result of the weight of the glacier. the ridge of rocks, sand, and dirt, (a moraine, and in this case, a terminal moraine, since it marks the end point of travel) acted as a dam for the melt water. a giant lake began to form. the weight of the melt water helped keep the land compressed. for several thousand years, the water melted, the lake grew, until finally, a weak point in the moraine failed. Today, this weak point is called "The Narrows"-- and it is the site of the Verranzano Narrows bridge, which marks the entrance into NY Harbor. on the brooklyn side, you have Bay Ridge, named for the characterist high rigde of land (the moraine!) on the Staten Island side, you can actally see the sheared moraine!

with the dam broken, the water poured out of the glacial lake. and slowley the land emerged. but technically, the Hudson river is a fjord-- a sunken river. the old river bed is several hundred feet below the surface. (how ever, the river has silted up, and the silt is hundreds of feet deep, and the Harbor nowdays required dredging to keep a deepwater channel open.) but at Bear mountain (a hours drive, and a state park,) the river is narrow, and runs swiftly, and the water is 265 feet deep, and the surface is about 100 feet below ground level, in a gorge. Even in NYC, the side of the Hudson are (starting at about 23rd street in Manhattan, but on the NJ side, and at 150th Street Manhattan side) high cliffs. the area in NJ is know as Palisades-- since it resembled one. (and some of us old folk remember the pop hit of the early 60's Palisade's Park, for an amusement park there)

the NY area still has pleanty of Kills-- a dutch word for meadow, marshy remnats of the old glacial lake Fresh Kills in Staten island is really a continuation of the NJ Meadowland-- home of a large sports complex. the same meadow continue (but in many places it has been "improved" out of existance!) to queens-- as "Fresh Meadows". Fresh Kills is now one of the highest point of land in NYC-- home to one of three "Mount Garbages" (the others being in Pelham (Bronx) and Flatbush (Brooklyn)) most of these areas are between the newest moraine, (North Hils Moraine,) and an older one--Ronkonkama Moraine.

All of Long Island is Moraine.. the fish shape, with the split tail at the east end, are remnants of the moraine. North Hills make the southern tail (Montawk Point, and crosses the Ronkonkama moraine in queens. it then runs diagonal through queens, to Brooklyn, and staten island.

any one who has flown into NYC, and gone to Manhattan, will have noticed that the trip to Manhattan is through a necropolis-- the moraines are not good farm land, and large parts of them became cemetaries.

i have take many local tours with geologists from AMNH http://www.amnh.org/ -- and learned many details about NY. i know the same glacial mass covered most of Michigan-- and formed gorges in upstate NY, (watkins glenn, etc). but mostly i know NYC geology.

Visiters to NY can see, in central park, the bedrock of Manhattan (schist) and in many places, it is scarred, and bears deep parallel groves, as if some giant claw dug in and gouged the rock. the "claw" was the last glaicer!

glacial erratica-- about all over the area. some even remain!


#52529 - 01/15/02 09:49 PM Re: drumlin  
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I haven't LIUed this, but wasn't Lake Missoula a giant drain-off from the shield glaciers over Canada which covered most of the north-western US? I remember hearing or reading that the lake was held back by an ice plug, and when that gave the lake emptied in a matter of a few days, scouring the bedrock through Washington State up to 200 feet deep on its way to the sea.



The idiot also known as Capfka ...
#52530 - 01/15/02 09:54 PM Re: drumlin  
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wasn't Lake Missoula a giant drain-off from the shield glaciers over Canada which
covered most of the north-western US?


Right, CK! The Columbia Gorge and much of the topography of Southern Washington/Northern Oregon is as a result of the Missoula Floods. Google on it and you'll get lots of info - ESPECIALLY you, Dub Dub!


#52531 - 01/16/02 09:56 AM Re: drumlin  
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Dear wwh,

You wrote:

When it becomes over a hundred feet deep, the ice at the bottom cannot stay rigid any longer.

Why not? Is the mystery of glacial movement tied into this 100-foot depth? Avalanches make a lot more sense than this. I was about to give up on glaciers till I went back and read what you wrote above.

Sorry to be so thick here, but the glacier in my mind must not be at 100 feet yet and, therefore, not ready to slide on out.

Best regards,
DubDub
PS: To Helen: What a profusion of facts you have in your head! Astonishing!


#52532 - 01/16/02 02:29 PM Re: drumlin  
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wwh Offline
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Dear WW: Water has many marvelous properties. You must have heard that skating is made possible by the fact that a lot of pressure over a small area covers the ice into a film of water that lubricates the movement of the skate runner. That plane Glacier Girl sank two hundred and fifty feet into Greenland ice in fifty years. And its area was so large that the pressure per square inch would have been far smaller than that under a skate runner. I would have expected ice to crush it to junk. Amazing that the ice must have filled up the inside fast enough to prevent crushing.


#52533 - 01/16/02 03:57 PM Glacial Movement  
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stales Offline
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Although I've seen a glacier first hand for about 30 minutes of my life (the JungFrau in Switzerland), I think the following may tidy up some of the Q's going on here...

The primary force behind glaciers moving downhill is gravity.

Increased pressure on ice decreases its melting point, thus liquefying it at the pressure point IF there is sufficient pressure - the skating reference is very apt. It may well be that an ice sheet has to be 100' thick for this to happen - I don't know.

Consequently, a glacier's downhill velocity will not only be a function of the slope it's on, but also the thickness of the ice (and hence the volume of water/lubricant at its base. It's conceivable that the downhill velocity may change from season to season, but predominantly (and this is a wild guess) as a function of the amount of ice being deposited upon the head of the glacier - its nevee field. More ice on the nevee field, bigger push from the top, therefore greater velocity in winter????????? A counter argument would be that the toe of the glacier melts more in summer, therefore causing an imbalance between head and toe, and therefore a greater velocity in summer. Either way, there WOULD be a degree of backwards and forwards movement with the toe between the seasons - couple with increased meltwater at the toe druing summer. This will add to the erosion of the boulders in the morraine, but bear in mind they have already travelled many kilometres during which they were being constantly ground.

Ice itself is not hard enough to grind the rocks within the glacier or the mountains and valleys surrounding it. The carving is caused by rock acting upon rock - at all scales. The ice or meltwater carry the agents of abrasion into contact with each other and carry away the products. Nevertheless, sundry processes such as frost wedging would apply.

Another thing to think about is the sheer weight of great ice fields such as those in Antarctica and Greenlan. In both cases the ice is several kilometres thick and has thus downwarped the Earth's crust underlying it. As the ice melts (as one day it will - though we wont see it), the rock will rise up again - like when you poke a sponge cake then take your finger away.

I've gotta say I'm sceptical that the aircraft "sank" 250' into the ice. I'm sure it sank some, but I reckon 240' of snow and ice have fallen on it since it landed there 50 years ago. Happy to be wrong on this though!!

stales





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