The Age of Nittany Valley
If you visit Happy Valley, home of Penn State's University Park campus, you will be in a valley, but there used to be a mountain right over your head. Many other valleys (although not all) in the folded Appalachians are similar. Mount Nittany is the "bottom" of a fold, and Happy Valley is the "top" of a fold (as seen in the diagram below).
Most of the rocks in Pennsylvania are sandstone (which erodes slowly in Pennsylvania's climate), or shale and limestone (which erode rapidly in Pennsylvania's climate). The limestone, most of the shale and some of the sandstone were deposited in a shallow sea, while some of the shale and sandstone were deposited near the sea. When the proto-Atlantic closed, with Africa and Europe crashing into North America about as rapidly as your fingernails grow, the rocks in the collision zone were bent (geologists call this folding). If you try to bend your pencil, you'll find that it tends to break at the peak of the bend. The same was true of Pennsylvania's rocks - breaks or cracks formed at the tops of the folds.
Streams formed in these cracks and cut downward. Then, landslides and other mass-wasting processes widened the stream valleys. Often, the streams cut through slow-to-erode sandstones and into the faster-to-erode shale or limestone below. Once that happened, the streams cut down quickly, the sandstone blocks fell off the cliffs left behind, and what had been a mountain peak became a valley. As a new valley formed in a place that had once been a mountain crest, the adjacent regions that had once been valleys were being eroded more slowly, often eroding down to the resistant sandstones and then eroding more slowly. Eventually, the former valleys were left higher than the new valleys, so that the former valleys became the new mountains. Today, in central Pennsylvania, you will find that the mountains have hard-to-erode sandstone on top, while the rocks beneath the valleys are easy-to-erode limestone or shale. The shape of the landscape has long ago "forgotten" the mountains built during the great collision, and is now controlled mostly by the resistance of the rocks to erosion.
The easy-to-erode rocks under the University Park campus, the rest of Happy Valley, and many other valleys in Pennsylvania are limestone (or a close relative of limestone called dolomite, but we'll just call it all limestone to keep it easy for you). The limestone often contains caves. The caves are spaces that were left when the rocks dissolved in water in the ground. Eventually, that water flowed out to creeks and down the creeks to the sea, where shelly critters grabbed the dissolved limestone and used it to form shells, the raw material for new limestone. As caves formed and their roofs collapsed, and as rocks were dissolved along small cracks or right beneath the soil without forming caves, the valley floor was lowered.
This exercise is much easier than it may seem at first! Don't panic until you've really tried it. (Don't panic even then; panic doesn't help anything.) Please begin by studying the diagram below. Please also note that your neighbor may be doing a slightly different calculation than you are--we have made many versions of each question in this exercise, with slightly different numbers. The techniques, the difficulty, and the learning outcomes are all the same, and the numbers are similar enough to allow you to learn the material, but different enough to encourage you to master the exercise yourself. (Copying from a neighbor is also prohibited because it is academic dishonesty.)
You will only get one chance to submit this exercise so be sure to review your answers carefully before submission. You can, however, save your answers as long as you do not submit them first. Do not forget to hit the submit button when you are finished. This exercise will NOT automatically submit since there is no time limit (except to submit it by the due date shown on the calendar). This exercise will be graded automatically.
