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INTRODUCTION
The area around Dubuque is one of the most interesting and rich in histories, both natural and geologic, for the state of Iowa. The area now known as Dubuque was under water during much of its beginning in the Paleozoic Era (280-570 million years ago). During this time shallow inland seas covered the mid-continent of the United States. This environment was suitable for the deposition of various sedimentary rocks. The primary sedimentary rock that was deposited was limestone. Limestone is produced when calcium carbonate (CaCO3) is precipitated from seawater and from the accumulation of calcareous shells of marine plants and animals. Many times these seas advanced and retreated leaving behind various sequences of rocks.
In the Ordovician, an early part of the Paleozoic, what is stratigraphically known as the Galena Group was formed. This group, which includes the Dunleith and Wise Lake formations, will be the rocks focused on during this trip. These formations known as the Galena group dolomites were originally deposited as limestone, but have undergone a process called dolomitization. This process is a secondary diagenetic feature possibly related to the progression of mixed marine-freshwater groundwater systems through the sediments when seaways retreated from the region near the end of the Ordovician.
Due to the great expanse of the continental sea there was an abundance of marine animals. As these animals died, their bodies became fossilized and incorporated in the sedimentary rocks. Iowaís best known fossils are probably crinoids. Complete crinoids appear to be growing in slabs of rock found in Mississippian limestones and large numbers of articulated crinoid cups are known from Pennsylvanian, Devonian, and Silurian formations. The oldest crinoids include beautiful intact specimens found in Ordovician limestones in northeast Iowa. The final retreat of these seas occurred during the late Pennsylvanian-early Permian. At the same time major tectonic events, such as the African continent colliding with the North American continent, were taking place. These tectonic events caused a variety of different responses in the rocks located in this area. The carbonate sedimentary rocks began to warp due to stresses produced from these events, eventually forming fractures, and joints. After the Paleozoic era, this area remained exposed and subject to weathering and erosion for a very extensive length of time. Due to this exposure the rocks were susceptible to a form of chemical weathering called dissolution. When carbon dioxide from the atmosphere mixes with rainwater it creates a weak acid, known as carbonic acid. When this water flows into cracks of carbonate rocks it begins to dissolve it away. After time these cracks can grow larger forming large underground crevices and caves. These joints and crevices would eventually become the source of the mineral galena in which miners from near and far would come to extract the valuable lead ore for well over one hundred years. Iowa was generally flat and low for millions of years before the dinosaurs evolved. Like most of the Midwest, it was frequently covered with shallow seas during the Paleozoic Era and was a question mark during the Mesozoic Era (65-250 million years ago). Mesozoic rocks and fossils have simply been eroded away from this region. Cretaceous formations in western Iowa contain fossils of marine invertebrates, fishes, and long-necked marine reptiles called plesiosaurs. A few fragments of dinosaur bones have been found in this area, but not enough to identify species. During the Cenozoic Era (present-65 million years ago), land was generally low lying and subject to extensive erosion.
Just 2 million years ago giant continental glaciers progressed from the north and plowed their way through the bedrock creating new landscapes. The advance and retreat of these glaciers greatly influenced the present appearance and deposits within the Upper Mississippi Valley, including that of Catfish Creek. Indications of this landscape change can be viewed in the records of floodplain and terrace deposits, alluvial fans, and abandoned stream and river channels. From 14,000 to around 9,500 years ago the melt water form the retreating Wisconsinan ice sheet notably impacted erosion and deposition of the Mississippi River. During this time there were periods of river down cutting that was occasionally interrupted by massive melt water floods. At this time the Mississippi back flooded into its tributary valleys depositing laminated silts and clays which can be seen today as preserved stream terraces near the mouths of these side valleys.
It is estimated that the first Native American inhabitants occupied this area sometime around 4000 BC. The Mesquakie Indians moved into the area during the mid-1700ís and mined here until the 1830ís. Many archaeological sites are located here and include remnants of prehistoric campsites, villages, burial mounds, and mine pits.
Mines of Spain Recreation Area
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