An Important Aside: Is This Storytelling or Science? (Hint: Science...)
Much of the science we have covered so far in this course is based on measurements taken today or very recently. The slow motion of drifting continents really can be measured with various techniques such as GPS, landslides are obvious to people whose houses are carried away, and the silt on the teeth of anyone who foolishly drinks untreated water from many rivers will prove that sediment is being moved.
But, much of our science involves history. For example, humans were not around with GPS receivers and satellites when Africa crashed into North America to raise the Appalachian Mountains. We are increasingly moving into subjects that involve “historical” geology, and reconstructing the events before modern humans were measuring the motions and writing down the results. We have presented a little of the evidence documenting past ice ages in this Module, in part to get ready for historical parts in future Modules.
We have focused on the scientifically accepted answers, but consider how scientists got those answers. If you go up to Bear Meadows and look around carefully, you will see the blockfields of big rocks extending down from ridges, sitting on soil, and then shale bedrock.
Many hypotheses could explain this observation—space aliens dropped the big rocks, or bulldozers pushed the rocks into place; or, the rocks slid down in a catastrophic, fast-moving giant landslide; or, they came creeping down slowly in permafrost; or, … you could probably think of others. Each hypothesis leads to predictions. If a bulldozer pushed the big rocks into place, we should find the bulldozer tracks, and we should be able to trace back in historical records to who was driving the bulldozer, and why. The first settlers, who arrived before bulldozers were invented, should have found hillslopes free of big rocks. Landslides start with big falls or slumps from particular places, so a landslide should have a big scar at its head where the rocks started, whereas creep slowly collects rocks as they are worked loose and carries them along, lining them up as they go.
So, scientists have looked for evidence that supports or refutes each hypothesis. The early settlers complained about the big rocks, and old cabins were built on the big rocks, so the bulldozer hypothesis wouldn’t work. There is no evidence of a landslide scar anywhere at the heads of these features, despite evidence for lots of different “stripes” of big rocks extending downhill from a ridgetop source where the bedrock of the same type as the big rocks sticks out.
You could follow the earlier scientists and quickly come to the realization that the rocks look like soil-creep deposits extending down from the ridge crests; the predictions from the other hypotheses fail, but each of the predictions from the soil-creep hypothesis is supported by additional data collected for testing purposes.
You can also note that the material is not now creeping—roads and trails are not being slowly buried by big rocks today, the trees are not knocked over, etc. Tree roots hold many of the rocks in place and prevent motion. So geologists looked for a time in the past when tree roots were not holding the rocks in place. The geologists collected the additional information given above (and much more!) - the big rocks are on top of smaller rocks and soil, not on the bottom, the big rocks are often standing on the edge, the rocks show patterning of coarse and fine, etc. Other geologists were scanning the whole planet, laboring over centuries, and learning the conditions of creeping hillslopes in the tropics, the deserts, the temperate zones, and the poles. By talking to other geologists, reading the literature, and devoting careers to careful study, they learned that the things you can see today on the slopes of Central Pennsylvania resemble features of permafrost and not features of any other modern setting.
But, if you are correct and these are permafrost features, there should be other evidence of cold conditions in the past, at the time that these features were active. Taking a core from the bog showed that the bog started in a very cold time (the deepest pollen is from plants that today are found only on the tundra), and the bog seems to be dammed by one of the soil-flow lobes, linking the soil-flow lobes to the time of the tundra cold. (It's true, no one has used a backhoe to take the dam apart to look for buried space aliens, but we'll stick to vaguely plausible things here).
Next, scientists ask whether this makes sense. Scientists have tentatively concluded that the hillslopes of Pennsylvania recorded cold conditions at a particular time in the past. Is there a reason why cold should have been here at that time? Well, just to the north, glaciers were pushing up moraines at the same time. Astronomers making orbital calculations find that the high northern latitudes were receiving about 10% less sunshine than today during that glacial age. Climate modelers who test whether such a drop in sunshine would be sufficient to grow glaciers and make conditions very cold find that cold indeed makes sense, especially when the modelers include the effects of the drop in atmospheric CO2 levels that was triggered by the change in the sunshine and that is recorded in ice-core bubbles from the time.
Now, a modern geologist who tells this “story”—Pennsylvania hikers risk twisting their ankles on permafrost deposits—actually has a lot more evidence than the little sketch provided here. For example, hypotheses often suggest new measurements that have never been made before but that can be used in further testing. Penn State researchers have even measured the concentrations of rare isotopes that are produced in the rocks by cosmic rays only very near the surface where abundant cosmic rays penetrate, showing the increase in erosion and transport caused by the onset of ice-age conditions. A vast amount of information collected by centuries of Earth scientists is combined in our modern understanding.