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In order to count this Earthcache as a find, you must complete the following tasks and email the answers to me.
1. What is the elevation?
2. How many spires of the Cathedrals can you see from this location?
3. Describe the appearance of the Cathedral Spires.
4. Can you see any remnants of the much earlier sediments or has it all been eroded away? If earlier sediments exist, where can they be seen?
5. In what year was this area dedicated as a National Natural Landmark?
This Earthcache is located at the Cathedral Spires Trailhead, along Needles Highway in Custer State Park. A daily or annual park sticker is required to enter the state park and visit the lake. There is a small, but adequate parking lot nearby. The Cathedral Spires has been listed on the National Natural Landmark registry and serves as an excellent and rare example of joint-controlled weathering of granite. The Spires also contain a disjoined area of limber pine. This limber pine, South Dakota’s only known stand, is a relic of ice age flora. These pines are about 200 years old and reach heights of fifty to eighty feet. The granite of the Cathedral Spires, which towers about 1000 feet high, was formed similar to the outcrops of granite found near Sylvan Lake. However, the Spires and the rest of the Needles are sharper and more pointy than the rocks around Sylvan Lake because the Cathedral Spires were formed as closely spaced joints. The rounded forms of granite surrounding Sylvan Lake, on the other hand, were produced by widely spaced joints. The Cathedral Spires area was the original site proposed for the Mount Rushmore monument. However, Gutzon Borglum, the sculptor, rejected the area due to the poor quality of the granite and the fact that the granite spires were too thin to support the sculpture. Enjoy!
The geological history of the Black Hills is long and complex, filled with a history of vast oceans, uplifts, and volcanoes. These natural events, combined with millions of years of erosion and weathering, sculpted the Cathedral Spires--and the rest of the Needles--into their present shapes and elevations.
South Dakota’s oldest rocks are located in the heart of the Black Hills, the Cathedral Spires being one example. These rocks consist of granites and metamorphic rocks that were formed more than two billion years ago during the Precambrian Era. The granite that makes up the Cathedral Spires was created far beneath the earth’s surface from magma that slowly cooled and solidified.
During the end of the Precambrian Era and into the early Paleozoic Era (about 500 million years ago), the first sea to advance over South Dakota’s Precambrian surface (granite and other metamorphic rock) covered the western part of the state. For the majority of the Paleozoic time (about 250 million years), a series of inland seas advanced and retreated. In the process of traveling to and from the ocean, these inland seas left behind limestone, shale, and sandstone, which was deposited on top of the granite.
Following this period of inland seas, at the beginning of the Mesozoic Era (240 million years ago), South Dakota went through a stage of erosion. This was followed again by a continental sea that once again submerged parts of western South Dakota. During the Cretaceous period of the Mesozoic Era (140 million years ago), all of South Dakota lie under a large, inland sea that stretched north to south. It was during this time, that thick deposits of marine sediments were deposited. This meant more limestone, shale, and sandstone, as well as redbeds, chalk, and clay. The final retreat of the inland seas marked the birth of the Black Hills. As the Black Hills area was domed upwards, erosion actively attacked the soft shales and finally the harder, much older sediments. Erosion during this time period removed more than 6000 feet of sedimentary rock layers from the central part of the Black Hills. During the later stages of uplift that this time period saw, great volumes of molten rock were forced upwards, forming many of today’s valuable mineral deposits.
As the Tertiary Epoch began (55 million years ago), erosion continued to me the dominant force. By the early Oligocene period of the greater Tertiary Era (38 million years ago), stream gradients were so reduced that streams could no longer carry away their erosion products. This led to deposition of these sediments on the plains next to the Black Hills. Gradually, as the sediments piled up, the lower two-thirds of the Black Hills became buried by light-colored clays and sands. Volcanic activity near Yellowstone Park contributed large volumes of windblown volcanic ash to the piles of sediments. Further uplift of rock caused a renewal of erosion. The soft, unconsolidated sediments that were recently deposited were attacked first, and gradually, the lower part of the Black Hills were unearthed.
Thus, the Black Hills today looks very similar to how it appeared forty million years ago. The Black Hills are sort of a mountain range in reverse. The granite core is really ancient rock--the oldest in the area at nearly two billion years old. It was covered up by limestone in shallow seas about 350 million years ago, and later by sandstone. When the range uplifted into a dome sixty million years ago, at a height of 15,000 feet, the top, younger limestone and sandstone eroded first. The core, the granite, was the last sediment to be exposed and remains the highest in the area, while the lower sections of the dome on the edges contain the youngest materials.
Since the granite has been exposed, it has been victim to gradual weathering and erosion, which has shaped it into the sight you see today.
NOT A LOGGING REQUIREMENT: Feel free to post pictures of your group at the area or the area itself - I love looking at the pictures.
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