Exercise #4: "Which Way is Up?"

The directions for this exercise are very simple:

1. Watch a movie and read a short article about determining basic geological ordering and layering below.
2. Answer 6 questions to see if you'd pass the "Careful Geologist" test.

This exercise is not terribly difficult and is graded out of 12 points. There is no time limit (except to submit by the due date) to worry about for this exercise, and it will be graded automatically. You have one chance to submit ...so take care to proof your answers carefully in order to get all of your points.

Onwards and Upwards!

Please note that this is similar in format to the "RockOn" quizzes so the same technical "cautions" still apply.

1. Once you access a quiz, do NOT open a new browser window to look something up elsewhere in Canvas. The system gets very confused when two active sessions are going on in the same Web browser, typically resulting in an "access denied" message once you try to submit the quiz.
2. Be sure to double-check your answers before you submit! Materials submitted online are graded in the same way as any other course submissions--we grade what you submit. If the answer is D but you chose C, the answer is wrong...whether you accidentally wrote C by hand, or you accidentally colored in the C circle on the scantron sheet with your #2 pencil, or you accidentally clicked on C with your mouse.
3. Do not use the mouse wheel when taking the quiz as it can cause your answers to change indiscriminately. Instead, use the scrollbar or arrows to view and review the page.

Now, if you think you understand everything, you may go directly to the exercise. If everything isn’t crystal-clear to you, read the following article about determining basic geological ordering and layering, and everything should clear up.

Determining Basic Geological Ordering and Layering

When rivers, wind, and other processes make layers of sediment on the Earth’s surface, the layers are almost always nearly horizontal. Try to make a steep pile of mud, and you will see that gravity quickly pulls the mud down into a nearly flat layer. You can make a steep sand castle at the beach, but watch what happens to the castle after the first waves hit it. The new mud layer, or the remains of your sand castle, won’t end up perfectly flat, but they will be pretty close to flat. However, after sediment is turned to sedimentary rock by squeezing or hard-water deposits or other processes, the layers may be tipped up on end or even turned upside-down by mountain-building processes without being flattened by gravity (see the pictures of folds just below). When geologists become forensic scientists, figuring out the history at the scene of the “crime”, the geologists need to learn which side of a layer was on top when the layer formed. Knowing which way was up originally lets the geologists put the rock layers in order from oldest to youngest, because younger layers are deposited on top of older layers.

Folded rocks seen in Waterton Lakes N.P., Canada. Rocks were deposited in beds before being folded into this huge “S”-shaped curve

Folded layered rocks from Grand Canyon National Park. The red item in the lower left is a pocket knife.

Mud Layers

Mud layers often are disturbed from above before they are hardened, and this happened in the past as well. A dinosaur may have walked by and made a footprint. Dinosaurs didn’t wriggle under the mud, lie on their backs, and make footprints in the mud above; the dinosaurs walked on the mud. (We do see traces in mud where worms wriggled through. We also see places where critters burrowed down into undersea mud to avoid being eaten by hungry fish cruising by.) So, a footprint goes down into mud. Mud often cracks as it dries in the sun, with the crack going down into the mud, and thick mud layers often have cracks that end within the mud. Raindrops may pock-mark the upper surface of mud. Mud cracks, dinosaur footprints, raindrop imprints and hiding-from-fish burrows are among the features that help us put rocks in order. To see examples of this, click on the slides below.

Sand Dunes

Sand dunes often give us “up” indicators. As we told you earlier, layers of loose sediment tend to be more-or-less horizontal, but not exactly horizontal. Some of the steeper natural layers are in sand dunes. As shown in the slides below, these layers usually curve, being steeper near the top of the sand dune and flattening out at the bottom where the sand grains bounce across the old surface. When a new dune is deposited on top of an old dune, the nearly horizontal layers (also called beds) at the bottom of the new dune will cross the steeper layers in the old dune, giving cross beds that tell us which way was up when the rocks were deposited.

Shells

Nature gives us many other “up” indicators. A living clam orients itself the way it wants to be oriented. But, after the clam dies, and the shells on the two sides of the clam come apart, those shells tend to be flipped by waves or currents to the stable frowny-face orientation (hollow-side down, or rocker-side up). Click on the diagrams just below for more details.

Lava

A thin frozen layer will form on top of a lava flow where cooled by the air, while the lava remains liquid beneath. Bubbles rise through the liquid lava, but are trapped by the frozen layer. Thus, one often finds more and bigger bubbles near the top of a lava flow, but not right at the top. It sometimes takes a little experience to recognize how thick the frozen layer was, but a good geologist can use this to learn which way was up when the lava was flowing.

Shadows, Hills and Holes

A word about shadows, hills and holes. Sometimes, it is hard to tell whether a picture is showing a hill or a hole. But, if you can see shadows, and if someone tells you where the main light source was when the picture was taken, you can always tell a hill from a hole in a picture. Take a look at the pictures of the moon snail shell by clicking on the slides just below. This shell was found on the Outer Beach of Nauset Marsh, Cape Cod National Seashore. After the snail died, some of the shell broke off in the surf. In A, you can probably tell that the shell sticks up towards you. But, if we tell you that the light was shining in the direction of the arrow, you can be sure. If the shell had been in the hollow-side-up configuration, it would have looked like B (A and B are the same shell). Notice the regions labeled “shadow” (which are in shadow) and “lighted” (which were in the direct light), which, together with knowing where the light was, tell you the orientation of the shell, as shown in the diagrams.

Review

Here's one more look at learning which way was "up" when a rock layer was deposited. Check out this extra Review Video and dance on down with the dinosaur. We’ll revisit these topics in Module 9. Everything you need to ace the exercise is in the instructions just above, but in case you really want to learn all the background and read more about these topics, you can take a sneak peek ahead to Module 9.

Now begin the exercise. Exercise #4 consists of 6 multiple choice questions in which your job is to identify which way was up when the rocks in the pictures were formed.