ENJOY THE EATHCACHE!!
Spearfish Canyon is often studied by geologists due to the extreme old age of the Precambrian rocks exposed by the creek bed. The canyon's high walls are of three dominant rock types. The Cambrian to OrdovicianDeadwood Shale at the bottom lies on an unconformity above Precambrian rock and can be identified by its brown color. It is multi-layered in appearance and ranges from 10 to 400 feet thick. Englewood Limestone in the middle is pink to red colored and is 30 to 60 feet thick. The PaleogenePaha Sapa Limestone, the top layer, is the thickest (300 to 600 feet) and is buff-colored and weathered gray in appearance. Caves and fossils are frequently found in the Paha Sapa Limestone.
The genesis of Spearfish Canyon spans a time interval twelve-times greater than that of the Grand Canyon, and because of the canyon’s rich vegetation, wears the mantle of being “the most magnificent canyon in the west”.
Evolution of Spearfish Canyon began about 62 million years ago (mya) although its present formation began only about 5 mya. At this later time, carving of the inner gorge, known today as Spearfish Canyon, began by the erosive power of the rushing stream. The high canyon walls and deep canyon floor are composed of sandstone, shale and, dominantly, limestone deposited in ancient oceans.
In this latest episode of development, the sculpting of the 1,000 foot deep Spearfish Canyon continued during the same time period that the Grand Canyon was carving a 5,000 foot deep gorge. Although of lesser depth, Spearfish Canyon has the distinction in history of development over a period twelve times longer than that of the Grand Canyon. The distinctive ledges seen in Spearfish Canyon, although substantially less thick than those of the Grand Canyon, consist of rocks of the same ages and similar types.
The “Rise and Fall” of the Black Hills
Three hundred to 500 million of years ago, long before the Black Hills rose, western South Dakota was part of a vast plain standing near sea level. At times, it lay below the sea level of repeated invasions of numerous shallow inland seas that divided the North American continent. Each of the seas deposited successive layers of sediments until about 7,500 feet of flat strata had accumulated.
The Black Hills dome started to rise around 62 mya, probably related in some way to the collision between the floor of the Pacific Ocean and the North American continent. Had the Black Hills retained its sedimentary overlay from deposits of the visiting oceans, they would today be a mountain range rising out of the prairie to an elevation of nearly 15,000 feet. But, because of erosion, about 7,500 feet of the mountains were removed, and part of the material now exists as the popular moonscape of the Badlands National Park. Within the mountain range, hard rocks form the peaks, ridges and plateaus, while softer rocks became valleys and gorges.
As the Black Hills began bulging upward above the surrounding plains, the area hosted a climate much like that of present-day Louisiana. At that time, molten magma from as much as 30 miles below the earth spud into the fractured rock rising into massive domes and along narrow dikes. Domes such as Terry Peak, Devil’s Tower and Bear Butte were created. The dikes were veins of gold that later fostered the Black Hills Gold Rush of 1876.
Most of the major streams that leave the Hills start as springs within the limestone plateau on the western side of the Black Hills and eventually join the Belle Fourche River to the north or Cheyenne River to the south before eventually joining the Missouri River. These streams, including Spearfish Creek, generally lose some or all of their water to the cavernous limestone-base of the creek beds as they cross the Madison (Pahasapa) Limestone.
Spearfish Canyon…A Work in Progress
West of Terry Peak turnoff, the road follows Icebox Canyon and drops 1,000 feet in 3 miles to Cheyenne Crossing. Here, the gorge turns north, where the Canyon’s Scenic Byway begins, and drops an additional 1,600 feet in 20 miles to Spearfish.
From Cheyenne Crossing to Savoy, the green shale of the Deadwood Formation (greater than 500 mya) is at the base of the cliffs, and the dominant tan-gray Madison limestone (320 mya) caps the top. For a mile between Elmore, an old lumber camp and switching station, and the mouth of Annie Creek, and again at the confluence of Rubicon Gulch, the highway cuts through the dark-gray igneous rock. Between Savoy and the mouth of the canyon at Spearfish, the sedimentary strata dip down to the north more steeply than the gradient of Spearfish Creek. One after the other, each of the formations exposed in the canyon walls disappears beneath the valley floor. At Rubicon, the tributary creek can not erode its valley fast enough to keep pace with the main stream, and the result is a hanging valley that ends in a cascade, known as Bridal Veil Falls. About a mile below Bridal Veil, the shale of the Deadwood Formation dips below the valley floor, followed by the orange-and-brown Whitewood Dolomite, then the tan and weathered-gray Madison Limestone, and finally, the upper yellow sandstone of the Minnelusa Formation (286 mya). At the very mouth of the canyon, the highway cuts through a narrow slot in the pinkish-red Minnekahta Limestone (230 mya) and emerges into the Spearfish Valley.
Spearfish Canyon is a work in progress. Much of it is the soft, erosive Limestone. Rain washes mud from the hill sides and the creek attacks its banks and bed just as the rains and streams did 50 million years ago. The present view of the canyon is not the last frame in the moving picture show of the beautiful feature.
Weathering describes the breaking down or dissolving of rocks and minerals on the surface of the Earth. Water, ice, acids, salts, plants, animals, and changes in temperatureare all agents of weathering. Once a rock has been broken down, a process called erosiontransports the bits of rock and mineral away. No rock on Earth is hard enough to resist the forces of weathering and erosion. Together, these processes carved landmarks such as the Grand Canyon, in the U.S. state of Arizona. This massive canyon is 446 kilometers (277 miles) long, as much as 29 kilometers (18 miles) wide, and 1,600 meters (1 mile) deep. Weathering and erosion constantly change the rocky landscape of Earth. Weatheringwears away exposed surfaces over time. The length of exposure often contributes to how vulnerable a rock is to weathering. Rocks, such as lavas, that are quickly buried beneath other rocks are less vulnerable to weatheringand erosion than rocks that are exposed to agents such as wind and water. As it smoothes rough, sharp rock surfaces, weathering is often the first step in the production of soils. Tiny bits of weathered minerals mix with plants, animal remains, fungi, bacteria, and other organisms. A single type of weathered rock often produces infertile soil, while weathered materials from a collection of rocks is richer in mineraldiversity and contributes to more fertile soil. Soils types associated with a mixture of weathered rock include glacial till, loess, and alluvial sediments. Weathering is often divided into the processes of mechanical weathering and chemical weathering. Biological weathering, in which living or once-living organisms contribute to weathering, can be a part of both processes. Mechanical Weathering Mechanical weathering, also called physical weathering and disaggregation, causes rocks to crumble. Water, in either liquid or solid form, is often a key agent of mechanical weathering. For instance, liquid water can seep into cracks and crevices in rock. If temperatures drop low enough, the water will freeze. When water freezes, it expands. The ice then works as a wedge. It slowly widens the cracks and splits the rock. When ice melts, liquid water performs the act of erosion by carrying away the tiny rock fragments lost in the split. This specific process (the freeze-thaw cycle) is called frost weathering or cryofracturing. Temperature changes can also contribute to mechanical weathering in a process called thermal stress. Changes in temperature cause rock to expand (with heat) and contract (with cold). As this happens over and over again, the structure of the rock weakens. Over time, it crumbles. Rocky desert landscapes are particularly vulnerable to thermal stress. The outer layer of desert rocks undergo repeated stress as the temperature changes from day to night. Eventually, outer layers flake off in thin sheets, a process called exfoliation. Exfoliation contributes to the formation of bornhardts, one of the most dramatic features in landscapes formed by weatheringand erosion. Bornhardts are tall, domed, isolated rocks often found in tropical areas. Sugarloaf Mountain, an iconic landmark in Rio de Janeiro, Brazil, is a bornhardt. Changes in pressure can also contribute to exfoliationdue to weathering. In a process called unloading, overlying materials are removed. The underlying rocks, released from overlying pressure, can then expand. As the rock surface expands, it becomes vulnerableto fracturing in a process called sheeting. Another type of mechanical weatheringoccurs when clay or other materials near rockabsorb water. Clay, more porous than rock, can swell with water, weathering the surrounding, harder rock. Salt also works to weather rock in a process called haloclasty. Saltwater sometimes gets into the cracks and pores of rock. If the saltwater evaporates, saltcrystals are left behind. As the crystals grow, they put pressure on the rock, slowly breaking it apart. Honeycomb weathering is associated with haloclasty. As its name implies, honeycomb weathering describes rock formations with hundreds or even thousands of pits formed by the growth of salt crystals. Honeycomb weathering is common in coastal areas, where sea sprays constantly force rocks to interact with salts. Haloclasty is not limited to coastal landscapes. Salt upwelling, the geologicprocess in which underground salt domesexpand, can contribute to weathering of the overlying rock. Structures in the ancient city of Petra, Jordan, were made unstable and often collapsed due to salt upwelling from the ground below. Plants and animals can be agents of mechanical weathering. The seedof a tree may sprout in soil that has collected in a cracked rock. As the roots grow, they widen the cracks, eventually breaking the rock into pieces. Over time, trees can break apart even large rocks. Even small plants, such as mosses, can enlarge tiny cracks as they grow. Animals that tunnel underground, such as moles and prairie dogs, also work to break apart rock and soil. Other animals dig and trample rock aboveground, causing rockto slowly crumble. Chemical WeatheringChemical weathering changes the molecularstructure of rocks and soil. For instance, carbon dioxide from the air or soil sometimes combines with water in a process called carbonation. This produces a weak acid, called carbonic acid, that can dissolve rock. Carbonic acid is especially effective at dissolving limestone. When carbonic acidseeps through limestone underground, it can open up huge cracks or hollow out vastnetworks of caves. Carlsbad Caverns National Park, in the U.S. state of New Mexico, includes more than 119 limestone caves created by weathering and erosion. The largest is called the Big Room. With an area of about 33,210 square meters (357,469 square feet), the Big Room is the size of six football fields. Sometimes, chemical weathering dissolveslarge portions of limestone or other rock on the surface of the Earth to form a landscapecalled karst. In these areas, the surface rock is pockmarked with holes, sinkholes, and caves. One of the world’s most spectacular examples of karst is Shilin, or the Stone Forest, near Kunming, China. Hundreds of slender, sharp towers of weathered limestonerise from the landscape. Another type of chemical weathering works on rocks that contain iron. These rocks turn to rust in a process called oxidation. Rust is a compound created by the interaction of oxygen and iron in the presence of water. As rust expands, it weakens rock and helps break it apart. Hydration is a form of chemical weathering in which the chemical bonds of the mineral are changed as it interacts with water. One instance of hydration occurs as the mineralanhydrite reacts with groundwater. The water transforms anhydrite into gypsum, one of the most common minerals on Earth. Another familiar form of chemical weatheringis hydrolysis. In the process of hydrolysis, a new solution (a mixture of two or more substances) is formed as chemicals in rockinteract with water. In many rocks, for example, sodium minerals interact with water to form a saltwater solution. Hydration and hydrolysis contribute to flared slopes, another dramatic example of a landscapeformed by weathering and erosion. Flared slopes are concave rock formations sometimes nicknamed “wave rocks.” Their c-shape is largely a result of subsurfaceweathering, in which hydration and hydrolysis wear away rocks beneath the landscape’s surface. Living or once-living organisms can also be agents of chemical weathering. The decaying remains of plants and some fungi form carbonic acid, which can weaken and dissolve rock. Some bacteria can weather rock in order to access nutrients such as magnesium or potassium. Clay minerals, including quartz, are among the most common byproducts of chemical weathering. Clays make up about 40% of the chemicals in all sedimentary rocks on Earth. Weathering and People Weathering is a natural process, but human activities can speed it up. For example, certain kinds of air pollutionincrease the rate of weathering. Burning coal, natural gas, and petroleum releases chemicals such as nitrogen oxide and sulfur dioxide into the atmosphere. When these chemicals combine with sunlight and moisture, they change into acids. They then fall back to Earth as acid rain. Acid rain rapidly weathers limestone, marble, and other kinds of stone. The effects of acid rain can often be seen on gravestones, making names and other inscriptions impossible to read. Acidrain has also damaged many historic buildings and monuments. For example, at 71 meters (233 feet) tall, the Leshan Giant Buddha at Mount Emei, China is the world’s largest statue of the Buddha. It was carved 1,300 years ago and sat unharmed for centuries. An innovative drainage systemmitigates the natural process of erosion. But in recent years, acid rain has turned the statue’s nose black and made some of its hair crumble and fall.
Igneous rocks are formed from melted rock deep inside the Earth.
Sedimentary rocks are formed from layers of sand, silt, dead plants, and animal skeletons.
Metamorphic rocks formed from other rocks that are changed by heat and pressure underground.
QUESTIONS **Text VIA GEOCACHING
1. ESTIMATE THE DEPTH OF THE CANYON
2. Observe the canyon walls. List the different layers of sediment from the top of the canyon to the bottom. Describe each one.
3. Based on what you see, what type of rock do you think the canyon is made up of.
4. Based on your surroundings, What caused the erosion that led to todays canyon. Describe.
5. Walk down the trail anout 50 feet and look in the creek, how do think these rocks got here?
6. Briefly explain the route of the water through the canyon during rain fall, and tell how this could impact the canyon over time.
7. Post a photo at GZ!!
Send the answers to my geocaching account!
SOURCES
https://en.m.wikipedia.org/wiki/Spearfish_Canyon#Geology
http://www.spearfishcanyon.com/landscape/geology.html
https://www.amnh.org/explore/ology/earth/if-rocks-could-talk2/three-types-of-rock
https://education.nationalgeographic.org/resource/weathering