Geology at Black Hawk State Historic Site EarthCache
Geology at Black Hawk State Historic Site
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Black Hawk State Historic Site is a wooded, steeply rolling 208-acre tract that borders the Rock River in Rock Island County. You will follow well-defined trails with some stairways and hills. You should be able to complete this cache in about one hour.
The Black Hawk State Historic Site in Rock Island, Illinois, recently voted one of the Seven Wonders of Illinois, occupies much of the historic site of the village of Saukenuk, the home of a band of Native Americans of the Sauk nation. The state park is located on a 150-foot bluff overlooking the Rock River in western Illinois. It is most famous for being the birth place of the Sauk warrior Black Hawk. The disputed cession of this area to the U.S. Government was the catalyst for the Black Hawk War.
The bluff below the Watch Tower in Black Hawk State Historic Site is an excellent place to observe three distinct types of sedimentary rock and a type of deposit formed during three different geological periods: LIMESTONE from the Devonian period (roughly 416 to 359 million years ago); SHALE and SANDSTONE from the Pennsylvanian epoch (roughly 325 to 299 million years ago); and LOESS from the Pleistocene epoch (1,808,000 to 11,550 years ago).
Three types of sedimentary rock are exposed at Black Hawk:
Limestone is composed largely of the mineral calcite, the primary source of which is most commonly marine organisms. Pure limestone is almost white. Because of impurities, such as clay, sand, organic remains, iron oxide and other materials, many exposures of limestone exhibit different colors, especially on weathered surfaces.
Shale is a fine-grained rock whose original constituents were clays or mud. It is characterized by thin, often splintery layers. Shale is the most common sedimentary rock.
Sandstone is a rock composed mainly of sand-size mineral or rock grains. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any color, but the most common colors are tan, brown, yellow, red, gray and white. (Wikipedia.com)
In addition to the three types of sedimentary rock, loess is exposed in the bluff at Black Hawk:
Loess, one of the classifications of soil types, is a fine, silty, windblown type of unconsolidated deposit. (The term sometimes refers to the soil derived from it.) It is derived from glacial deposits, where glacial activity has ground rocks very fine (rock flour). After drying, these deposits are highly susceptible to wind erosion, and downwind deposits may become very deep, even a hundred meters or more, as in areas of China and the mid-western United States. (Wikipedia.com)
Parking: There is a large parking area at N 41° 27.870 W 090° 34.480, south of Black Hawk Road west of the Watch Tower Lodge and Museum, near the trailhead.
Trailhead: From the parking area, proceed a short distance up the blacktop path toward the Watch Tower to the wooden stairway at N 41° 27.852 W 090° 34.460. Walk down the stairway and take the trail to the left at the bottom of the stairs.
Stop 1 – Sandstone: Proceed east on the trail to N 41° 27.831 W 090° 34.386. Note the sandstone to the north of the trail. You can see the layers in which it was developed, some of which have broken off.
Stop 2 – Loess: Proceed east on the trail to N 41° 27.826 W 090° 34.369. Look up toward the top of the bluff, and you will see a large area of loess, at least 25 feet thick with holes in it, that is exposed. Sandstone: This is the beginning of a long layer (about 8 feet thick and 125 feet long) of sandstone to the north of the trail. Again, notice the many layers, or strata, in the sandstone. Also notice the color of the soil just above the sandstone. At the end of the layer of sandstone, look up, and again you will see a large, very thick area of exposed loess with holes in it near the top of the bluff, just under the top layer of soil.
Your first question: How does the color of the loess compare with the color of the soil just above the sandstone developed during an earlier period? _______________________________
Note 7-4-11: You can no longer (at least not at this time)continue east on the trail from the bottom of the wooden steps mentioned below: There is now a fence across the trail at that point (possible because of erosion). So instead of continuing east, you need to return to your starting point and access the rest of the areas from the east. You can either get into your car and drive east on Black Hawk Road to the parking lot on the far east side near 17/24th street, or you can walk along the walkway up past the lodge and down to the east side of the park.
Stop 3 – Limestone: Proceed east on the trail, following down the wooden stairs and continuing to the east to N 41° 27.826 W 090° 34.248. You will see the river through the trees along this portion of the trail. At the coordinates you will arrive at the Observation Deck over the limestone in and just above the river. The upper surface of the limestone that you see there is very irregular, displaying numerous sinkholes, caves, and other phenomena.
Your second question: Estimate the height of the limestone above the water level: ___________
Shale: You will see dark shale on top of the limestone, filling some of the holes. Higher up, away from the water, you will see more shale. Turn around and face the bluff. In front of you, slightly to the right, is a wooden barrier. Look over it, and you will see a large area of shale. Especially notice the two small gullies a few inches wide, one on each side, from water erosion. In those gullies you can clearly see the strata, or layers, of shale.
Stop 4 – Sandstone: Proceed east on the trail to N 41° 27.862 W 090° 34.140. There you will see another, much thicker, sandstone layer.
Your third question: Estimate the height (thickness) of this layer of sandstone: ____________
Stop 5 – Top of Bluff: Proceed to N 41° 27.839 W 090° 34.376 to the top of the 150 foot bluff at the Watch Tower. You may do this in one of several ways.
1) You may return along the trail you just took back up the initial wooden stairway to the blacktop path, and take it to the right up to the Watch Tower.
2) You may continue east on the trail you are on a short distance until you come out of the woods at the picnic area and take the trail that goes up the hill to the left, and then follow the path to the left up to the Watch Tower.
3) You may continue east a little farther to the stone stairs just behind the picnic shelter, and take that path up the hill and continue on it to the Watch Tower.
At the top of the bluff behind you is a statue of Black Hawk. You are looking out over the river just as Black Hawk did, possibly as he observed the corn fields below. But you do not see corn fields out over the tree tops. Nor do you see cows grazing on the island, as I did as a child.
Your fourth question: Which of the rocks/soil you observed in the park today are you observing when you look out over the tree tops from the Watch Tower? ______________
Thank you for your participation in this earthcache. You may e-mail your answers to me (please do not log them online), and then log in online for completion of this cache. You may, but it is not required, upload one or more photos of your visit to the park.
References:
R. C. Anderson: “Notes on the Geology of Black Hawk State Park,” flyer obtained from the Indian Museum at the Black Hawk State Historic Site
Wikipedia.com
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