ex4

You are a geologist. While walking in the fog one day, you bang into a cliff. After rubbing your sore nose, you inspect the cliff, and see what is shown in the picture, in a one-foot-square area. You recognize that this cliff is made of "fossil sand dunes", with wind-blown sand that was later glued together by hard-water deposits. You are accompanied by a student, who is carrying your tea and crumpets for you. You sketch four arrows on the cliff, label them as shown, and ask the student which of the arrows was pointing up when the loose sand was deposited. Your student is brilliant, and correctly tells you the answer. The arrow that was pointing up when the loose sand was deposited is the arrow that is closest to:

C The wind dumped sand in the lee of a dune, making the striped-looking pattern that you see behind the big A. Then the wind blew the top off, making a nearly horizontal erosion surface, followed by deposition of more sand. Cut ends face up, so the cut-off ends of the old dune were the upper side when the dune was deposited. So, the arrow at C is pointing in the original up direction.

You are still a geologist, still wandering around in a fog with a tea-and-crumpets-toting student, and you walk into another cliff. This one turns out to be a hardened lava flow. Again, you look at a one-foot-square region, sketch pink arrows with A, B, C, and D on that region, and ask the student which of the pink arrows was pointing up just after the lava flow hardened. To help the student, you draw four additional arrows on the cliff; these are light blue (turquoise) arrows, pointing at bubbles. (If you are not able to distinguish pink from light blue, the four pink arrows are very close to the four letters A, B, C, and D, and the four light-blue arrows are not close to the letters.) You suggest that the student consider the behavior of bubbles in a liquid. These bubbles are within the lava flow, and not in the crust on top of the flow that was chilled very rapidly by the air. Your student is brilliant, and correctly tells you the answer. The pink arrow (close to a letter) that was pointing up when the lava flowed in and slowly cooled is the arrow that is closest to:

D Below the frozen upper crust of a lava flow, bubbles tend to rise, and they tend to grow as they rise because less pressure squeezes them near the top of the flow, and because they pick up dissolved gas from the flow as they rise. So, you expect a big-bubble layer near the top and a little-bubble or even no-bubble zone near the bottom. Two of the blue arrows point to big bubbles on the "D" side, and two point to little bubbles on the "B" side, so you know that D must have been the top when the lava flow was cooling.

You walk down the beach on Cape Cod, where waves have been bouncing shells around, including pieces of moon-snail shells, as shown here. You find 10 identical shell pieces. 9 of the 10 shell pieces appear as shown in one of the pictures (either A or B), and the 10th shell piece looks like the other picture. It is very likely that the 9 shell pieces look like:

Image A The shell has a hollow side, shown turned upward toward you in B, while in A the hollow side is down. Waves tend to flip shells hollow-side-down, so moon-snail pieces on beaches usually look like A and only occasionally like B.

The picture shows hard stone that was once soft sediment, from the Tonto National Monument, Arizona. Examination of the sample tells a geologist that mud cracking was occurring where the sample formed. When the picture of this sample was taken, the light was shining along the arrow, as shown, making shadows, some of which are indicated by the arrows. Are you looking at the side that was down when the sediment was soft, or the side that was up?

Side that was down Mud cracks extend downward into soft sediment. When more sediment is washed in, this second layer will fill the cracks beneath. Later, after the layers have hardened, the rock may be turned upside-down and then the layers cracked apart (or, the layers cracked apart and then turned upside-down). If you see ridges in a mud-crack pattern, you are looking at the side of the second layer that originally was down. You can tell that this picture shows ridges, and not holes, by the shadowsridges have a light on one side and a shadow on the other, as seen here, whereas holes have light and shadow on the same side.

Dr. Alley took this picture in central Utah. The rock shown started out as soft sediment deposited in Lake Flagstaff, at about the same time and just a little north of the lake in which the limestones of Bryce were deposited. These lakes grew and shrank with changing climate, often forming muddy flats that dried and cracked to make mud cracks, which then were filled and covered by more sediment as the lake grew again. The pocket knife shows you that this sample is a foot or two in length. The sun was high and hot when the picture was taken, but slanting in from the left as shown, and we have provided arrows to direct your eye to a couple of shadows. Dr. Alley placed this sample so it could be photographed easily. Are you looking at the side that was down when the sediment was soft, or the side that was up?

Side that was up Mud cracks extend downward into soft sediment. When more sediment is washed in, this second layer will fill the cracks beneath. Later, after the layers have hardened, the rock may be cracked apart. If you see troughs in a mud-crack pattern, you are looking at the side of the second layer that originally was up. You can tell that this picture shows troughs, and not ridges, by the shadows—troughs have the light and the shadow on the same side, as shown here, whereas ridges have light and shadow on opposite sides.

Dinosaurs once stomped across much of the planet, sometimes leaving tracks in mud that were buried in more mud and later hardened to stone. This sample is from the Philmont Scout Ranch in New Mexico. It is a loose block that fell down from a cliff above. The footprint was made by a Tyrannosaurus rex. The print is 33 inches long (almost 3 feet for one foot!) and 28 inches wide, and extended 9 inches deep into the sediment. The sun was shining as indicated, and the black arrows point to shadows from two of the three toes of the track. Is the rock with the track now upside-down (you are looking at the side that was down when the sediment was soft) or right-side up (you are looking at the side that was up when the sediment was soft)?

Upside-down Footprints are pushed downward into soft sediment. When more sediment is washed in, this second layer will fill the print beneath. Later, after the layers have hardened, the rock may be turned upside-down and then the layers cracked apart (or, the layers cracked apart and then turned upside-down). If you see a footprint sticking up, you are looking at the side of the second layer that originally was down. You can tell that this picture shows a track sticking up by the shadows—ridges or other things that stick up have a light on one side and a shadow on the other, as seen here, whereas holes or troughs or other things going down have light and shadow on the same side. According to the United States Geological Survey, the T. rex responsible for this track was probably moving 6 or 7 miles per hour. A mature T. rex would have been about 60 feet long, two stories high, and 10,000 pounds or so, quite capable of making a 33-inch-long, 9-inch-deep track in mud.