From about 543 million to 290 million years ago, vast inland seas covered most of what is now the central United States. During this time, sediment was deposited on the seafloor and under millions of years of intense pressure and heat, these sediments slowly turned into sedimentary rocks, forming the incredible limestone bluffs along the Illinois and Mississippi Rivers. Repeatedly through time, natural processes have eroded the bluffs to form what they are today.
The bluffs you will see today have been shaped by a variety of geological processes, including flooding by ancient seas, sediment deposition, tectonic movement, weathering, erosion, and glaciation. The St. Louis limestone is fine-grained with interlaid beds of dolostone, shale, and chert. The bottom limestone, or Salem Limestone, is made of marine organisms, mudstone, and microcrystalline dolomite from the warm shallow ocean more than 350 million years ago. The layers present in the rock can give geologists insight into some of the ancient history of this land.
Throughout the bluff you can see fossils that contribute to the sedimentary rocks. This is how scientists can see evidence that an ocean once existed here. Marine fossils are exposed throughout the layers of the bluff. At this site, you can see both St. Louis limestone (the top part of the bluff) as well as Salem limestone (the bottom part).
Along with important natural history, humans have a rich history in association with the bluffs. This area had many quarry sites throughout Alton, dating back to 1815. Down the road, closer to Alton, one can find a cave that was created due to the quarrying process. Not only did this business shape the land, it helped Alton develop into a thriving city with a booming glass manufacturing business.
To log this EarthCache you must:
-
Take a photo of yourself standing by the bluffs (optional).
-
Answer at least two of the following questions. When finished please send your answers.
Questions
-
Estimate the height of this bluff and explain how you came up with that answer.
-
Looking at the bluff, can you see evidence of quarry activity in the rocks? If so, please describe what you see.
-
What are some of the differences you see between the St. Louis Limestone and the Salem Limestone?
-
What geological forces might still be contributing to the erosion of this bluff?