Trail Ridge Road and Bear Meadows
Permafrost and Periglaciation
Meanwhile, when the ice age had covered such places as today's New York, Chicago, Minneapolis, Seattle, and much of Europe, with thick ice, what was happening in central Pennsylvania and other areas around that ice? The evidence is clear that the ice ages cooled all or almost all of the Earth. And, some of that evidence comes from central Pennsylvania, which was a real frozen tundra.
If you climb the ridges of Central Pennsylvania, perhaps up in the Seven Mountains just southeast of Penn State's University Park campus - go up Bear Meadows Road past the ski area, for a start - you may notice several interesting things geologically. Beneath the Pennsylvania forest, the soils, streams, and hillslopes have more in common with the high meadows of Trail Ridge Road in Rocky Mountain National Park, or with the regions around the ice sheet in Greenland, than with the modern climate of University Park. Trail Ridge Road crosses tundra, where small, hardy plants grow atop permafrost. Although the uppermost soil along Trail Ridge Road thaws during the brief summers, and the deep Earth is thawed by the heat of the Earth, the materials between are frozen year-round in permanent frost. (Such areas are sometimes called “periglacial,” because they may occur around the perimeter of a glacier, but sometimes they are far from glaciers, so "permafrost" is the better name.)
The Bear Meadows National Natural Landmark, just over the ridge from Penn State’s University Park campus, was recognized by the National Park Service in 1966 as a site that “possesses exceptional value as an illustration of the nation’s natural heritage.” Although many guidebooks somehow have decided that Bear Meadows is 10,000 years old, the Meadows are much older, having formed during the most recent ice age, roughly 20,000 years ago (and possibly earlier). Here, take a walk just above the Meadows, and learn why Pennsylvania hikers, like those in the high country of the Rocky Mountains, are wise to wear sturdy shoes. Then, see what this has to do with the Formation of the Meadows — they really are related.
Video: The Formation of Bear Meadows (1:56 minutes)
Dr. Richard B. Alley: So here we are at Bear Meadows, perhaps the biggest and best natural wetland in central Pennsylvania. Natural wetlands, lakes, bogs, are fairly rare in Central Pennsylvania. And that's because nothing has been making them recently. And nature fills them up. Rocks wash in streams. Trees fall and leaves fall. And wetlands fill up. So when you see a wetland, you have to say geology made this fairly recently. Or humans made it. And this one's natural.
If we were to go out into this bog and stick a pipe down in the mud about 20 feet and pull it up and split it open, the mud on top has sticks and leaves and twigs of things that live here today. At the bottom, it has remnants of things that live on the north slope of Alaska today. It has evidence of tundra. This formed during the Ice Age. Below that is rocks.
And so it's rocks and then Ice Age and then stuff that lives there today. So this formed when the climate was different. And it formed by those beautiful rivers the rocks that we were looking at just up the hill. When this was tundra, when this was the North slope of Alaska or the top of Rocky Mountain, the hillsides were creeping down in these great rivers of rocks. And one of those dammed the stream. And that made a lake. And since then the lake has been filling in to give us this beautiful wetland that's full of good things all year.
Back in Module 5, we saw how freeze-thaw cycles can break rocks. We also saw how ice crystals can grow under rocks that have been broken free and push them upward, allowing smaller rocks to fall underneath and thus raising the bigger rocks toward the surface. This behavior is especially common in permafrost regions, causing them to have broken-up big rocks sitting on top of soil or smaller rocks.
Ice under rocks very near the surface melts in summer. In warm places, as we saw in Module 5, the water drains down through spaces in the ground, with some of the water eventually reaching rivers. But, in permafrost regions, the deeper spaces are clogged with still-frozen ice. The upper layers then become saturated with water, which can lubricate the downhill motion of the rocks on top of the soil. Permafrost regions thus commonly have blockfields extending downhill from ridges, with big rocks on top of soil, often with the blocks tipped up on edge and pointing downhill. Such a moving mass that reaches a valley may dam a small stream to make a wetland or small lake, and then the mass may turn and move downhill, filling the former stream valley with rocks much too big for the stream to wash away.
Features such as this are still developing today in parts of Alaska, around the ice sheet in Greenland, and on top of Trail Ridge Road in Rocky Mountain National Park, where the role of permafrost can be documented in detail. And, features such as this are spread across the mountains of central Pennsylvania and into surrounding states. Hikers on the Appalachian Trail, Mid-State Trail, and other trails in Pennsylvania are wise to wear sturdy shoes, and often complain bitterly about “Rocksylvania”, where hiking boots go to die.
The resistant sandstone layers that form the backbones of many ridges in Pennsylvania were broken by freeze-thaw cycles and produced blockfields that crept downhill and into the valleys during permafrost times of the ice ages, damming a small stream to make Bear Meadows, and giving the large blocks that fill other stream valleys. It takes a little investigation to find a road cut or other excavation cutting through one of these block fields, but if you do, you will see that the big sandstone blocks are on top, with smaller pieces of soil below, and then a different type of bedrock (commonly shale) below that. Note that the trees growing on top of the blockfields stand straight and tall, and roads bulldozed through the blockfields are not being buried or carried downhill—the motion largely or completely stopped when the ice age ended, waiting for the next occurrence of permafrost.
Permafrost produces many other features that are easily recognized. In nearly flat places, wintertime cooling often causes the ground to contract so much that it breaks into patterns, with summertime meltwater flowing into the cracks and filling them. Such ice wedges are still present in modern permafrost, and ancient ones can be recognized where they cut across other layers and then filled with things washed in as the ice melted. Such former ice wedges have been found in parts of Pennsylvania.
Also on nearly flat places in permafrost regions, the freeze-thaw processes combined with such ice wedging may sort the larger and smaller rocks into patterns, and such patterns have been found in Pennsylvania. (Note that many of these features, such as those on Big Flat near Bear Meadows, were described by geologists during times when logging and fire had removed the thick vegetation; the features are hard to see and almost impossible to photograph today but can be found during careful bush-whacking.) But if you're not an expert walking in thick vegetation on uneven blockfields, we recommend you just take our word for it.
Bear Meadows even records the history of warming from the ice age. A core collected from the sediment in the bog has silt with little evidence of vegetation in the oldest layer at the bottom. Above that, pollen and other remains of cold-weather plants appear, dating to the first bit of warming from the ice age, followed by a progression to warmer-weather types and on to the modern, productive bog with its blueberries and bears and other interesting plants and animals. A nearly barren tundra of the Trail Ridge Road type, with a creeping permafrost lobe that dammed a stream, followed by warming, would have produced the sediments we see.
The conclusion is nearly inescapable—Trail Ridge Road in Rocky Mountain today is an excellent picture of what Pennsylvania looked like during the ice age. Permafrost is common across much of northern Alaska, Canada, and Siberia, around the coast of Greenland, and in high-altitude regions. Permafrost poses grave problems for construction — the heat of a building can melt permafrost beneath, causing uneven settling that breaks the building. Permafrost also records the climate changes that have come to central Pennsylvania and other regions.
Video: Rivers of Rocks and Permafrost (2:30 minutes)
Dr. Richard B. Alley: So why do hikers in Central Pennsylvania carry so many ace bandages? And the answer is that there's rocks on top of everything. All the trails in Central Pennsylvania are covered with rocks that are sitting up on a edge like this on top of the dirt. Why do the rocks get on top of the soil? And that story's sort of interesting. If you ever have a cat and you buy a bag of kitty litter, and you shake the bag and then you open it, you'll find the big pieces are on top.
You may find this in cereal boxes too that you'll get the big pieces floating to the top. And that's linked to a very simple geometric fact which is that little pieces can fall under big ones. And big ones cannot fall under little ones. If you want to find things like this that are happening today you won't find them here. These trees are not being rolled over by rocks that are moving. Our trees are perfectly happy here.
To find places where things like this are really moving today, you go to the top of Trail Ridge Road in Rocky Mountain National Park. You go to the North slope of Alaska. And there the ground is permanently frozen at some depth. And the rocks are slowly creeping down in the summer on top of that, lining up and turning up as the freezing and thawing move things around.
Here if you thaw the snow, it just soaks down through the rocks. It goes through the spaces. It goes down the river and it's fine. If it's frozen underneath, it can't soak down. And so you get soft mud that's full of water. It can't get rid of its water. It's sitting on top of slippery ice. What's it do? It slides downhill slowly. And so you go to the North slope of Alaska. You go to the top of Rocky Mountain. And all the hillsides are moving. And they're tipping the rocks up on edge and they're lining the rocks up in the direction they're going. And they're making things that look just like this without the trees. And so what we see here is a route of the Ice Age.