Chert Within the Castle Walls 🏰 EarthCache

Clark Tower:

The Clark Tower is a castle-like structure located in Madison County. The tower was constructed with limestone bricks to remember Caleb and Ruth Clark, who were stonemasons. To get to the tower, you can either hike to drive on the gravel road to the GZ. Please pay attention to the road signs, as many of the roads in the park are one-way.

Madison County’s Bedrock:

Madison County is located on the East-Central Iowa Drift Plain. This region has an abundance of limestone and shale. Glaciers once covered this area 10,000-12,000 years ago, and its loess hills are evidence of deposited moraine or glacial sediment. Creeks and rivers have carved the landscape in recent years, running through the loess hills and shaping valleys. Layers of limestone and shale formed millions of years ago when the entire Midwest was submerged in the Carboniferous Sea. As sediment was deposited to the bottom of this sea, it began to lithify through cementation and compaction, forming sedimentary rock. In sedimentation, organic matter (like shells, corals, etc.) can become entrapped in sediment layers and eventually embedded in sedimentary rock. Limestone commonly contains fossils of prehistoric organisms and can harbor nodules (clumps of rock or mineral debris). In Madison County, limestone walls, cliffs, and boulders are reminders of this geological past. Limestone in Madison County can take a gray-to-beige color. 

What is Chert?

Chert is a sedimentary rock that is found in nodules of limestone. Chert can have large fractures, allowing North American indigenous groups to use flint (a chert) to make arrowheads. Chert is biochemically formed from plankton. When plankton skeletons accumulated and deposited at the bottom of the Carboniferous Sea, they became lithified in sedimentary rock as it formed. Over time, silica is the main mineral that replaces marine organic debris, eventually forming chert. Chert comes in various colors that depend on the amount of impurities present. Brown and black chert tend to have a high amount of organic matter in the rock. Red shades of chert are often attributed to the presence of iron oxide. Tan/beige chert is usually associated with traces of magnesium. Below are some examples of what chert looks like.

Logging Tasks:

Four pronounced nodules of chert are located to the right of the steps and several feet to the right of the bottom entrance of the tower. Three of the four specimens are located 5 to 6 feet up and embedded in limestone bricks. You must find at least one nodule in the rocks to complete the logging tasks.

1. Locate a nodule of chert in the limestone. Describe the coloration and texture of the specimen. 
2. What is the shape of the nodules? Are they circular, elliptical, or another shape?
3. Compare the chert nodules to the parent rock (limestone). What differences and similarities do you observe?
4. As of June 2019, earthcaches now contain required photo logging tasks.  Please provide a photo of yourself, your GPSr, or a personal item that proves that you have visited this site.  Please post this in your log.

Sources:

https://www.iihr.uiowa.edu/igs/publications/uploads/2017-04-27_15-04-11_em44.pdf 

https://www.exploreiowageology.org/assets/text/GeologyOfIowa_UNI/IowaGeologyUNIstudentprojects/MBeelerMadisonCo.pdf 

https://geology.com/rocks/chert.shtml

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