Missouri River Coal Banks-Judith 5 EarthCache

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Hole in the Wall - A Tale of Weathering and Erosion

For this EarthCache you will be climbing up to the well known Hole in the Wall. The pulloff for the trailhead is located at N 47º49.150, W 110º03.012, about 0.6 miles beyond Hole in the Wall campground. Alternatively, if you are staying at the campground, you can walk through the gate at the back of the campground, and there is a small road that crosses a riverbed and then parallels the river. This road eventually meets up with the pulloff location, which is at an undeveloped campsite under a lone cottonwood tree. A scratched rock will inform you that you are at the trailhead. From here there is a clear footpath leading all the way up to Hole in the Wall. It is ~¾ of a mile with a steady incline, and a few spotty sections that require scaling 5-10 foot slopes of sandstone. It is not for the faint-hearted and small children may have trouble, but you can always turn back if you feel uncomfortable.

Previously in this EarthCache trail we have discussed the processes of weathering and erosion. For example, how erosion exposes igneous features or concretions that formed at depth, well below the Earth’s surface. Here too, good old weathering and erosion have worked together to form Hole in the Wall. To review, weathering is the breakdown of rock exposed to the elements, caused by wind, water, and biological activity. Erosion is the process by which weathered material is transported away from its source. When you look at a mountain and see piles of boulders and debris below, it is weathering that has loosed them from the solid rock of the mountain. When you dig in your garden or a farmer plants a field, the soil is a natural product of the slow process of weathering, combined with decomposed organic material.

There are two different types of weathering: physical weathering and chemical weathering. Physical weathering involves the breakdown of rocks through mechanical means, by actually breaking apart the rock into smaller pieces. Chemical weathering, on the other hand, involves chemical reactions on the molecular level that eat away at the surface of rocks.

Physical weathering occurs on both small and large scales, working individual sediment grains apart, but also affecting entire mountainsides. The most common form of physical weathering is one that occurs all the time: abrasion in rivers, windy environments, and underneath glaciers. Water, wind, and ice alone actually don’t have significant erosive power. However, each of these transportation mediums is constantly moving material, and this material, ranging in size from stones to smaller sand and silt particles, are the erosive agents at play. For instance, over the course of a day, hundreds, if not thousands of sand grains may strike a rock lying in a river, or a rock outcrop in a desert. These impacts wear away at the rock overtime, smoothing out angled edges and knocking off loose grains. It is for this reason that rocks in a riverbed are usually so well rounded—it is a reflection of the constant process of physical weathering that has shaped the rock. Similarly, in glaciers it is the material caught up in the ice, not the ice itself, that causes abrasion and grinds away at the bedrock below.

Frost weathering is another powerful mechanical weathering process. Daily temperature fluctuations allow water to trickle into preexisting cracks or microfractures in rock, and then freeze at night. When water freezes to ice, its volume increases by 9%. As the ice expands, it exerts stress on the surrounding rock, which can be enough to split it apart. This is called frost wedging. Constant freeze-thaw cycles can even break apart rocks completely, in a process known as frost shattering. Cracks can also be forced apart by plants through root wedging, or even the expansion of salt crystals left behind after naturally salty water evaporates in rock fractures.

A common large-scale physical weathering process is the development of joints, cracks that form when rocks are being stretched. One mechanism for the formation of joints is unloading. Unloading joints are common in granite, an igneous rock that originates in the Earth’s crust as a cooled magma chamber, called a pluton. When a rock is buried in the Earth’s crust, it is subject to incredible compressive stresses, mostly from the weight of overlying sediment, called overburden. Upon being uplifted and exhumed (exposed by erosion), the rock is no longer confined by the weight of the overburden, so it expands, and joints are created. Unloading joints can lead to a fascinating geologic phenomenon called exfoliation. Over time, weathering exploits the exposed surface joints, causing large sheets of rock to slide off along joint planes, as if the rock were shedding off layers. Exfoliation can also occur on a small scale when a forest fire heats a rock, causing the outer layer to expand and break off. See the diagrams below for an illustration of unloading and exfoliation.

Chemical weathering breaks down rocks one atom at a time by dissolving minerals and exchanging ions (atoms with a charge). Water is the driving force of all chemical weathering. It acts as a potent solvent, a liquid capable of dissolving other substances. Additionally, it acts as a transportation medium, carrying away the products from chemical reactions.

Dissolution is a type of chemical weathering that occurs when rocks and/or minerals are dissolved by water. It involves the breaking apart of chemical bonds, a process called dissociation. When you dissolve salt in a glass of water, that is dissolution in action. In technical terms, upon being dissolved in water, solid table salt (NaCl) dissociates into two ions: Na+ and Cl-. Another example of dissolution is when slightly acidic rainwater eats away at limestone, a carbonate rock. When rainwater passes through the atmosphere and soil on the surface, it picks up carbon dioxide, forming a weak carbonic acid (H2CO3). This acidic rainwater dissolves limestone rocks, creating spectacular karst environments filled with caves, towers, sinkholes, and disappearing streams.

Plants also contribute to weathering through what is called biological weathering. We have already discussed how plant roots can wedge fractured rocks apart. Furthermore, as plants decay, they produce organic acids that facilitate chemical weathering. In fact, rainwater becomes more acidic by picking up carbon dioxide as it flows through soil with abundant organics.

Hydrolysis is a chemical weathering process similar to dissolution, where water breaks apart chemical bonds. During hydrolysis, hydrogen ions (H+) are exchanged from water to the mineral surface. This process is key to breaking apart silicate minerals and producing phyllosilicates, sheet-minerals that include clays and micas. It is hydrolysis that transforms ancient ash beds deposited during volcanic eruptions into bentonite.

Bentonite, a clay made from altered volcanic ash, can absorb many times its volume in water. When wet, it swells and becomes sticky. In the Breaks, bentonite and other fine clays make a notoriously think, gluey mud known as “gumbo,” which makes dirt roads impassable after rainstorms. Bentonite’s swelling and absorptive properties have led to many commercial uses, including well sealant, cement and adhesive mixer, drilling lubricant, and even cat litter. Fun fact: bentonite is also the mud of choice for the pig wrestling event at the Chouteau County Fair in Fort Benton, MT!

The last type of chemical weathering is oxidation and reduction, collectively known sometimes as redox. They are often combined into this term because they occur together—you can’t have one without the other. Redox reactions involve the transfer of electrons. Oxidation is the loss of electrons to an oxygen ion, while reduction is the gain of electrons from an oxygen ion. More familiarly, redox is the set of chemical reactions responsible for rusting of metal. In nature, some rocks contain iron-bearing minerals that undergo oxidation when exposed to oxygen at the surface. When this occurs, they obtain a red or pink tint to them, which is responsible for many of the red rock layers in areas such as the Grand Canyon, the South Dakota Badlands, and the Upper Missouri’s own Judith River Formation. Environments with a lack of oxygen, such as the deep ocean, lakes, and bogs, are under anoxic conditions. In these places, redox reactions cannot occur because free oxygen is not available like it is at the surface. As a result, rocks in these locales (particularly shale, which is common in deep water environments), are grey in color.

Both physical and chemical weathering work together continuously to break apart and erode rocks. Sometimes unique formations like Hole in the Wall can be created when the conditions are just right.

To claim this cache: Answer the following questions and send the answers using Geocache's messaging tool.
Q. What sort of weather conditions and processes do you think formed Hole in the Wall?

Q. On the top of the ridge as you approach Hole in the Wall, there are two clearly distinct rock types beside each other. Describe them as best you can (color, grain size, overall appearance, any unique features or crystal grains visible?) and hazard a guess at what they could be. Do you think they differ in origin?