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MINES OF SPAIN Horseshoe Bluff
Cliff of Horseshoe Bluff Quarry
The Galena Group dolomites that outcrop at the Mines of Spain provide a record of marine life from a time when the mid-continent was submerged 400 million years ago. Some features of these rocks are large calcite crystals, chert nodules, marine fossils, indications of burrowing marine organisms, evidence of karst processes and vertical fracture systems known as joints. The quarry at Horseshoe Bluff provides an area to observe these features. The rock in this quarry has long been mined due to its strength and resistance to weathering. The quarry, however, was closed in 1980 when the land was purchased by the state and designated as a park.
Many of the fossils in the Galena Group have been diagenetically altered due to its dolomitization but it still displays a number of different groups. The fossils are commonly poorly preserved, but a number of invertebrate species have been recognized. These include crinoids, brachiopods, rare trilobites, bryozoans, and mollusks. The Wise Lake Formation includes areas in which large gastropods can be found. Large networks of dwelling tunnels can also be found in the Galena Group.
The chert nodules that occur in the Galena Group are especially abundant in the Dunleith Formation. The development of these nodules is thought to precede the dolomitization of the sequence. Chertification of the carbonate sediments is regionally extensive in this area in both the limestone and dolomite facies. This likely relates to the mobilization and reprecipitation of biogenic silica in the sediment. The source of silica is thought to be provided from siliceous sponges. The Wise Lake Formation that overlies the Dunleith is primarily chert free. These formations also contain large crystals of calcite. Calcite is the primary mineral component of limestone. It has more than 600 different crystal forms and varies in color from white to pink, yellow, green, and blue depending upon its impurities. The calcite in this limestone is clear with a rhombohedral form. The Mines of Spain is a great place to explore the occurrences of ore deposits associated with the Upper Mississippi Valley Lead-Zinc District. The ore present in this area was galena. It was found as mineralized vein-fillings along crevices and cave openings. These features are associated with the regional north-south and east-west joint systems that extend vertically through the bedrock. These joints are stress related and caused by the warping of Paleozoic strata. They are especially noticeable where they shape long linear bluff lines and cause abrupt, sharply-angled stream direction changes. The precipitation of the lead ores was caused by the upward migration of mineral-rich hydrothermal brines through these fractures. Mining patterns exhibit different zones of activity in which the sulfide ores of lead, zinc, and iron were concentrated along the vertical fractures that intersected with the horizontal bedding planes of the sedimentary rocks.
Example of cavern development in an oxidized lead ore deposit The vegetation of this area consists mostly of juniper groves and scattered trees and shrubs. Because of the harsh environment the junipers dominate the rock face area. These trees can grow in the most adverse of conditions. As visitors can see they are growing on the quarry walls.
Mines of Spain Recreation Area
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