Earth Science Themed Essays
It has been about six years since the time I collected the rocks I will describe in this paper. It was summertime, and my family was driving from our home in Wisconsin to visit my grandparents living in western Montana. In a mistaken attempt at a shortcut we became lost somewhere between North Dakota and Montana. We decided to take a walk in the incredible landscape we found—like the South Dakota Badlands but with much greater variety of color in the rock bands. Not knowing exactly where the site was made ordering a topographic map difficult, and since I wouldn't really have been able to plot the rocks' locations anyway (the latitude and longitude of each would have been identical or close; I found them on a steep slope and so it is the depth rather than the horizontal distance that is important), I think it is better not to include the topographic map, even if it had arrived by now.
Because the strata are so important, I decided to include a layer map based on one from my journal made when I visited the site. The plants are most abundant in certain layers of the rock, so I tried to show this on there as well.
The rocks are mostly freshwater shale, though there is a good deal of sandstone and one band of durable slate. These rocks were formed when water carrying sand grains and smaller particles slowed down, possibly in a river delta or when it had flowed into the sea. When the water began to slow, it could no longer carry the heavier particles of rock (quartz), and these settled to the bottom. The more slowly the water moved, the smaller the size of the particles it could carry. This means that the sediments deposited at this place would have been beneath very slow moving water for most of the time as much of the rock is made up of clay-sized particles. Water that was moving more quickly dropped larger particles, which are now the colored bands of sandstone. When iron settled into the accumulating sediments, it colored the rock, creating a red or reddish-brown color. If the iron did not get a chance to mix with oxygen, the bands it became a part of would have been green, but there are none of these at this site.
Not all of the minerals in the rock were formed while the layers of sediment were settling out. Mineral-rich groundwater filtered between the clay and sand grains to replace the plant and animal remains that had been covered by the sediment. These fossils are made mostly of quartz, according to an information sign in Makoshika State Park (a park we found on a return visit to that part of the state and a place I suppose to be quite close to our collection site).
Petrification was not the only way plants and animal parts were preserved at this place, though. I picked up a small cube of bituminous coal there, one with distinct alternating bands of shiny and matte material, some of which still shows the original wood structure. The piece of coal is light and smooth, except for the few intact pieces of grainy carbonized wood. Thick layers of coal would suggest that there had been a humid jungle. However, layers of peat—the first step in the creation of coal—accumulate in the bogs of cold northern Europe as well. Once the wet plant remains become peat, they begin to form lignite coal as the moisture is squeezed out of them. There are such extensive layers of lignite in Makoshika State Park that when lightning causes a fire, the lignite may burn for decades before the supply is exhausted. There were small chunks of lignite in two other representative rock samples that I gathered from the site.
The first sample is a small, firm lump of freshwater shale. Embedded between the layers of clay are splinters of carbonized wood. When I first found this, I assumed that these must be the charcoal from a fire; pieces of burned logs that had fallen into water and become rock when the surrounding sediments hardened. Since then I have found fossils in which the plant was reduced to a film of carbon in the rock. I now think that this is what happened with the clay, because the bands of clay mixed with carbon are so thick and the carbon chunks so small. If there had been a fire I would expect to see whole charred trunks.
The second sample is similar to the first. It is also a solidified mass of clay and carbon, but this one is arranged in paper-thin layers so brittle that two-thirds of the sample had flaked off onto my hand before I was done drawing it. It comes from a layer at least 15 feet from the top of the landform (it was deeper originally, but the tops of the freshwater shale hills have weathered away) in a darkly colored level. The minerals in this band are different from those in the first. Though they are both mostly kaolin, the second has smudges of iron-coloring and lacks whatever was cementing the other together. I'm not sure exactly what sort of environment this rock came from.
My first idea is that it came from an inundated forest in which there was enough vegetal matter to accumulate into these small blobs of coal but in which there was also enough flowing water to lay down all of the clay, which makes up most of the rock's mass. Since the water would need to weave through the plants, it would certainly be slow moving, so the sediment it left would be composed of clay-sized particles. This, however, would not account for the size of the carbon pieces. Wouldn't there be large amounts of carbon where a tree had stood rather than small chunks spaced evenly both vertically and horizontally?
An alternative explanation for the creation of the layer is that an accumulation of vegetation immediately above or below levels of clay was broken up in a single violent event, such as a storm. The vegetation would be exposed long enough for leaves to rot, leaving only the hard wood of trees. This wood might have been smashed up in the storm, the result being small wood fragments scattered evenly throughout the clay (if the tree chunks were waterlogged they would have sunk). This second scenario fails to account for the bands in the sediment, though, and does not seem likely considering that a catastrophic storm would have disrupted the straight border between the clay-with-carbon layer and the next one down, since it would have had to mix in the carbon to the very edge of the border.
The coal and two samples are not the only rocks with plant remains that I collected. A piece of petrified wood knapped into a chopper or hand adze and used (the tip is dull from wear) by Native Americans may also tell a great deal about the climate of its time. But since it was a tool used by humans, it may have been taken from its place of origin. There was other petrified wood eroding from the bank above it, so it may have been made and discarded near where it was found. Even then, it would be much deeper and older than the previously described specimens.
This piece of petrified wood has no visible wormholes, unlike most of my collection, and it also has remarkably even growth rings. Tree rings are differences within the size of the wood cells as they grow at different times. During the spring melt, water swells all cells that are being made. The spring cell walls are spaced far apart, so the spring rings appear light colored, while the summer rings are dark in pattern. The even growth rings mean that the climate was also even, but it means more than that. It means that there were definite seasons, or at least regular changes in the amount of moisture available. Tropical trees, which live in places without seasonal change, do not have growth rings.
The petrified wood, chosen by the Native American flintknappers for its glassy conchoidal fractures, has been filled in with agate. Quartz-bearing water percolates through the soil until it finds a hole to fill, such as a cavity like the open cell in the petrified wood or just a space in the surrounding stone. An example of this is one of the other rocks I picked up, a piece of Montana moss agate. It is waxy and smooth on the inside, and, like the petrified wood, shows a conchoidal fracture, but the fracture surfaces are slightly lumpy (and lack the glassy sheen of the wood) because the stone does have a few of its own fractures. This might be because of mineral impurities, because its microcrystals are slightly larger than in the other, or because their sizes are uneven within the rock.
As the quartz crystallizes, the chemicals around the growing agate change, and these changes are reflected in the agate as bands. In addition, Montana moss agate has inclusions of manganese in the form of pyrolusite dendrites along cracks in the agate.
The agatizing water that ran between the sand grains in the sandstones to petrify wood and create agates also works on a much smaller scale, filling in areas between the sand grains to cement the sandstone into concretions. The concretions in Makoshika sometimes had the bands of color like the surrounding unconsolidated sandstone, showing that the colored sediments had been deposited before the concretion was glued together. The material of the concretions are the same as the surrounding rocks, apart from the minerals that glue them together, possibly quartz or calcite. My small concretion did not dissolve when I dunked it in a dilution of hydrochloric acid, as I have heard that calcite would. Since concretions often form around a hard nucleus such as a fossil, I smashed mine after I had drawn it. Disappointingly, all that fell out was a little pebble.
The site was not without interesting fossils, however. The percolating mineral waters preserved two bones that I found. The first is broken into twelve pieces that do not fit back together. They are part of the rim of a turtle's carapace—a turtle that was at least 3-feet long when it died. The shell is nearly an inch thick near the edge and might have been thicker in the center, which I did not find. The inside of this gritty white bone has projections where the ribs attached to the shell. Each of the ribs must have been at least three-quarters of an inch thick.
The presence of a turtle means that there would have been a body of water, but something like a river or an area along the coast of a sea; the turtles would not have lived in deep water far from shore. One of the fragments from that spot also has rodent chew marks. Since the fragment was found with the other related bone pieces, the carapace lay there in one piece. Because the turtle shell is held together with soft tissues, and is not a single bone, it will fall apart when the animal decays. This means that the turtle bone had lain in that spot long enough for the soft parts to rot away (it was chewed along the fissures where it would have been attached to other parts of the bone) but not so long that it became scattered. As it came from stone deposited by water, but shows evidence of being chewed by a land animal, the ground on which it lay was probably dry first, then inundated soon after it had been chewed. If it had washed into a body of water, the pieces would be distributed separately. The land was a floodplain.
The second of the bones I found is a heavy, grainy, water- rounded piece that I cannot identify. Its contortions and cavities suggest to me that it came from a pelvis or skull, but it could be from a marine animal or a terrestrial animal, or any other kind of animal. As I found it on the side of the slope, it could have eroded from any layer above it and does not really tell me anything about the land's previous climates.
The only two rocks that are left I could not weave into the other series of events in this place. They are my two anachronisms; one too old, the other too young to be from the rock in which I found them.
The first is a smooth and heavy hunk of fossilized coral from the landform's base. Coral grows in shallow seas, along the edges of islands and of coasts. I identified the piece I collected as something called "heliolites." It would have come from Silurian or Devonian deposits of rock. The rocks from this place are Eocene or Paleocene Continental, according to one of my books and a sign in Makoshika State Park.
The second fossil is a grainy piece of bone that an archaeologist once identified as a tusk. The only animal she knew of in this area that had tusks was the mastodon but this also would be very young for this type of rock, especially considering that the inside is still unpetrified bone. I found it on the top of the rock formation, near to the road, and I am open to the possibility that it was dropped by some other fossil collector. If not, since I associate mastodons with very cold temperatures, it might mean that this area had been covered by sediment following an ice age.
Even after the Guffey Volcano's demise, dramatic volcanic activity persisted in the Front Range. During the Miocene, an uplift of the Pikes Peak region shifted Four-mile Creek's southerly direction to its current northward course.
Cvancara, Alan M. A. Field Manual for the Amateur Geologist. New York: John Wiley and Sons, Inc., 1995.
Feldman, Robert. The Rockhound's Guide to Montana. Helena: Falcon Press Publishing Co., Inc., 1985.
Hotton, Nicholas III. The Evidence of Evolution. New York: American Heritage Publishing Co., Inc., 1968.
Roberts, David C. Peterson Field Guides: Geology-Eastern North America. New York: Houghton Mifflin Company, 1996.
Shaffer, Paul R. and Zim, Herbert S. A Golden Guide: Rocks and Minerals. New York: Golden Press, 1957.
Sorrell, Charles A. Rocks and Minerals: A Guide to Field Identification. New York: Golden Press, 1973.
Thompson, Ida. The Audubon Society Field Guide to North American Fossils. New York: Alfred A. Knopf, Inc., 1982.
Less than 1 period
Supplement a study of geology with an activity drawn from this winning student essay.
Send students to this online article, or print copies of the essay for them to read.
Divide the class into small groups, and give each one a rock sample (or photograph), telling them where the sample was found.
Have the groups try to deduce when and how the samples were formed.