otseng wrote:Not sure how this explains the lack of sedimentary rock on the Canadian shield...
Most sedimentary rock forms under water. There are volcanic ash deposits (e.g. the Morrison formation) and petrified sand dunes (e.g. the sandstones that make such marvelous cliffs in the southwest, such as in Canyon de Chelly), but most is aquatic deposition. To rephrase it, there's a lot of limestone and shale, and it's way easier to find fossils of marine organisms than of land animals. So, the rock types and their associated flora and fauna indicate that sedimentation from water is the main source of sedimentary rock.
The question then is: how can we submerge a continent enough to build up several hundred feet of sediment--or several thousand, in many cases? The continents float on the mantle, so they don't submerge easily--but they
can tilt. Push down one side, and you can get water to flow onto it.
The hardest part to submerge is the center of bouoyancy. The "stable craton" as this part is called is what is most stable, and least likely to be submerged, or subjected to tectonic forces. Tectonic forces act on the edges of the plate, not the center. Therefore this part shows up as having little sediment on top of the precambrian rock of which it is made.
In North America, the stable craton turns out to be in the northern midwest and south-central Canada, and is called the Canadian Shield.
Around the edges of the Canadian Shield, there's more sedimentary rock. As your elevation map shows, there's also likely to be more topographical relief--e.g. the mountainous islands in the north, and the mountains in the west. The mountains on the east (Vermont, for example) are older and not as high, but still count.
In any event, I pointed this out initially to contrast the small amount of sedimentary rock in the center of the country to the large amount in the western mountains. Yet, both can have drumlins
if they are in appropriate relationship to glaciers.
This was to address your suggestion that drumlins should be over precambrian rock, with little sedimentary rock beneath.
otseng wrote:How can it be explained that there are erratics at these locations, but no drumlins?
It's relatively easy to explain erratics, compared to drumlins. I think people tend to agree that they are rocks that were carried by glaciers, then left behind when the glaciers melted out from under them. As you have indicated as well, strong flood currents can also move huge rocks. Of course, the fact that a strong flood in Kansas can move a large rock doesn't indicate that the rest of the world was flooded at the same time.
I
think the consensus on drumlins is that we need lots of glaciation. A single small glacier can carry an erratic. Drumlins require more work.
otseng wrote:The map I quoted stated it was the "most extreme stage of the last glaciation". So, my suspicion is that geologists are not in a consensus of exactly what is the maximum extent of glaciation.
Jose wrote:You ask how the Ice Age theory would explain this region not being covered. I suggest that this is inverted reasoning. The data are the moraines, drumlins, erratics, scrape marks, kettle holes, etc.
I would assume that they determined the maximum glaciation through the data you described. But, since there doesn't seem to be agreement on exactly what areas were glaciated or not, then it's probably pointless to debate about specific maps.
Not quite pointless, but it does suggest that we need to be cautious about the positions of the borders of the glaciers. Your map specified 18,000 years ago. I couldn't find a date for mine. Nor could I find an indication of which part of the glacier was at its maximum--for example, I've read that the "Lake Michigan Lobe" of the Wisconsonian Glaciation began its advance 23,000 years ago, and began its retreat 12,000 years ago. This would put its maximum at 12,000 years.
It is nonetheless clear that all of the maps put a big chunk of ice over most of Canada, with the edges oozing into the US.
Part of the difficulty in determining the edges and the ages may be, I suspect, that the terminal moraines of one advance will be run over by a later advance. What's in the moraines isn't subject to standard rock dating, because the rocks are just being moved around by the ice. And, of course, if a glacier pushes debris into the ocean, it's likely to wash away--unless it's a whole lot of stuff and a relatively shallow bit of ocean (said to be the origin of Long Island).
You comment on the uncertainty of how drumlins are formed...this does seem to be a problem. One of the sources I read referred to moraines being over-run by later glaciers as "the way it happens." A more exhaustive discussion is
here. It may be that multiple mechanisms can create similar things. The current ideas all seem to require deformable debris (sediment, till, whatever you want to call it) that can be shaped by the glacier moving over it--or by meltwater that flows under the glacier.
Still, the Big Question is whether they are due to glaciers in some form, by some mechanism, or whether water alone is sufficient.
otseng wrote:Also, how are glaciers able to "carve" out a canyon?
I tink the short answer would be that the glacier slowly flows downhill, breaking off bits of rock as it goes. Once there are rocks in the ice, they scrape on the rocks over which the glacier moves, both digging the valley wider and deeper, and polishing the rocks over which they move. If you look
here, you'll see the upper end of Yosemite, where all of the mountain tops have been ground and smoothed. It is said that this occurred well over a quarter-million years ago, by the Sherwin glacier, which covered just about everything around:
The later, smaller glaciers then looked more like this:
The above pictures don't quite illustrate some of the other important features, so I'll add this photo of Yosemite:
It's pretty clear that the main valley goes off into the distance to the upper left. Coming in from the right is another U-shaped valley, out of which a waterfall cascades. The explanation for the upper valley being so high up, and "hanging" as it were above the main valley, is its glacial origin:
Farther south, and lower down, where it's not cold enough to form glaciers, the canyons typically come together at the same elevation as their streams flow together.
An important point, of course, is that glaciers carve out canyons only where there are mountains with a steep enough gradient for them to flow in relatively thin ribbons downhill. Probably, they follow water-cut canyons that were present in warmer climates, before the glacier formed (this is the thinking for Yosemite).
So, we would not expect to find a glacially-carved canyon in the Canadian Shield. It's a better image to picture the snowcap getting thicker and thicker, and eventually starting to squish out to the sides as its weight pushes on the bottom layers.
otseng wrote:How can it be explained for areas that have both terminal moraines and drumlins?
Drumlins are thought to be the result of deformation/deposition under the glacier as it moves. Movement is pretty much necessary to get the tapered shape of a drumlin, and the alignment of the drumlins in a drumlin field. Moraines, on the other hand, are the debris piles at the ends of glaciers (or also on the sides, if it's a "tongue" shaped glacier.) Hence, the names "terminal moraines" and "lateral moraines." The ones south of Indianapolis are interpreted as terminal moraines.
otseng wrote:If a glacier "grinded" over a hill, shouldn't it be expected to flatten it rather than leave other hills?
Yes--if it had enough time. Look back at my photo of
the top of Yosemite--the peaks of the Sierra Nevada were pretty well ground down and polished into rounded humps. In time, the glacier could have flattened the whole mountain range. It didn't have enough time.
Nor did the ice age glaciers have enough time to flatten the White Mountains of New Hampshire. But, they sure could drag boulders up and over Mt. Kearsarge to create the grooves that come up from the north, over the top, and then down toward the south.
You'd think moraines would be easier to flatten than granite, so if glaciers can do what they've done to the Sierras and the White Mountains, it would seem that glaciers should be able to push previous glaciers' moraines ahead of them. Maybe they do. Or, maybe they flatten softer hills, rather like spreading peanut butter. This would create a bed of loose debris underneath the glacier. Or, it would "overload the glacier with sediment," to quote from your reference. It's a little hard for me to tell...uhh, not having been under a glacier lately
. Probably, the underside of a glacier is somewhat turbulent, on a slow time scale. Maybe drumlins are a result of this sort of slow turbulence, and maybe they occur only as the glacier is retreating and periodic floods of meltwater flow along underneath them. I'm guessing here, of course.
otseng wrote:What would determine if a glacier would carve out a lake/canyon/etc versus forming moraines/drumlins?
It would carve a canyon if it has a steep enough gradient and an initial channel to get started in. That is, if it starts in mountains that have previously had a normal snowmelt in the summer, and have carved some canyons already--and then the weather gets colder and the snow doesn't all melt in the summer and the glacier forms. Then the glacier follows the river canyon, but carves it from V-shape to U-shape.
To make lakes, such as the Kettle Holes of "The Land of Lakes" or the Great Lakes themselves, two ideas are proposed. Kettle Holes are explained as the places that great hunks of glacier sat and melted after the overall flow had stopped. These were big hunks of ice, which pushed down on the mud their meltwater made. Displacing the mud left depressions that are now lakes. The Great Lakes are thought to be partly the result of the depression of the center of the continent--the Canadian Shield--due to the weight of all that ice. It is said that the continent is still rebounding from the ice sheet's weight. It's a slow process. Rocks don't flow very fast.
So, to answer your question with a list of what seems reasonable to me from what I've read:
- glaciers carve canyons by flowing down mountains, probably widening canyons that had already been started by water
- glaciers create lakes by leaving great hunks of glacier lying around after the glacier has stopped moving forward; or, as may be the case for Yosemite, by carving a bit more deeply uphill than downhill, so that water pools behind a ridge. (Yosemite valley is said to be so flat because the lake filled entirely with sediment.)
- moraines are the debris that is carried by the glacier, and is deposited at the front as the glacier melts. If the glacier is pushed forward by addition of ice at the cold end, and melting at the warm end is about the same rate as formation at the cold end, the moraine will build up at the warm end.
- drumlins have something to do with what happens under the glacier near the warm end.
Since drumlins only exist in proximity to cold places with many other features of glacial activity, it seems that they have to be the result of glaciers. There are lots of places in temperate and tropical regions where the topography broadly matches the regions where drumlins are found in colder regions. But in these warmer places, there are no drumlins. If water is the explanation,
especially if a world-wide flood is the explanation, then there should be no correlation with regional climate and temperature.
----
By the way, since the latest migration, phpBB behaves relatively normally with Safari, but is very weird with Firefox, which I usually use. Any thoughts?
