The sandstone, shale, and limestone rock layers exposed across much of southeast Minnesota Figs. Although many people are not aware of the geologic history of the Paleozoic bedrock in Minnesota, the rocks are familiar to anyone who has visited southeastern Minnesota.
The bluffs along the St. Croix, Minnesota, and Mississippi rivers, and their tributaries, are composed of layers of Paleozoic-aged rock such as the St. Peter Sandstone Fig. Paleozoic rocks lie beneath glacial deposits across much of southeast Minnesota, from as far north as Taylors Falls, southwest to Mankato.
They extend south into Iowa and east into Wisconsin. Paleozoic rock layers are sedimentary in origin. They are composed of particles of pre-existing rocks or minerals, or are precipitated by biological or chemical processes.
Sedimentary rocks are deposited by the accumulation of these particles into layers, or beds. Small grains are dropped by wind or settle in water to form sandstone and shale. Elements, such as calcium, magnesium, carbon, and oxygen precipitate from seawater or are left as biological remains, such as shells, to form what we call calcareous, or carbonate sediments and later rocks—either calcium-rich limestone largely consisting of the mineral calcite, or more magnesium-rich dolostone largely consisting of the mineral dolomite—with much of the magnesium commonly added later by percolating water.
Different rocks reflect the environmental conditions present at the time the original sediments were deposited. For example, where sand was scarce, carbonate minerals, chemically precipitated from seawater, and carbonate shells of marine organisms, accumulated to form limestone. If you visit the river bluffs in southeastern Minnesota, take note of the varied rock formations.
Remember that these rocks record what the world was like hundreds of millions of years ago when a shallow tropical sea existed right here in Minnesota and across much of central North America.
Rocks of Mesozoic age in Minnesota tell the story of two major marine trangressions or sea level rises : one during the Jurassic Period roughly million years ago and one during the Cretaceous Period roughly million years ago. The poorly understood Jurassic rocks are known only from drill core samples in the far northwestern corner of the state Fig.
They include shale, limestone, dolostone, sandstone and siltstone possibly deposited within streams, lakes, and other marginal-marine environments as a sea over much of North Dakota and other western states slowly advanced eastward.
The Cretaceous rocks form a nearly continuous blanket covering the Paleozoic and Precambrian rocks throughout the western half of Minnesota and numerous, small outliers in the eastern half of the state Fig.
These rocks consist primarily of weathered residuum and overlying sedimentary rocks of conglomerate, sandstone, mudstone, shale, marl, and minor limestone.
The kind of rock that was deposited, or how it has been weathered tells geologists a lot about what processes happened to create this area. They can see how the bedrock was tilted through plate tectonics, or the chemical constituents of the lava that formed the original rock, or what kinds of process occurred in the area since the rock originally formed. The depth of the bedrock changes from place to place on Earth. In some regions, the bedrock is right at the surface, exposed to air.
In other places, it might be hundreds of meters deep, beneath loose sediments and broken rock. A large chunk of rock at the surface, detached from the bedrock is known as float. Bedrock can be made of all the 3 types of rocks: sedimentary, igneous, and metamorphic. If no button appears, you cannot download or save the media.
Text on this page is printable and can be used according to our Terms of Service. Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. The rock cycle is a web of processes that outlines how each of the three major rock types—igneous, metamorphic, and sedimentary—form and break down based on the different applications of heat and pressure over time.
For example, sedimentary rock shale becomes slate when heat and pressure are added. The more heat and pressure you add, the further the rock metamorphoses until it becomes gneiss.
If it is heated further, the rock will melt completely and reform as an igneous rock. Empower your students to learn about the rock cycle with this collection of resources. Erosion is the process where rocks are broken down by natural forces such as wind or water.
There are two main types of erosion: chemical and physical. In physical erosion, the rock breaks down but its chemical composition remains the same, such as during a landslide or bioerosion, when plants take root and crack rocks.
Explore the process of erosion with this collection of resources. An abiotic factor is a non-living part of an ecosystem that shapes its environment. In a terrestrial ecosystem, examples might include temperature, light, and water. In a marine ecosystem, abiotic factors would include salinity and ocean currents. Abiotic and biotic factors work together to create a unique ecosystem. Learn more about abiotic factors with this curated resource collection.
Weathering is the process of the weakening and breakdown of rocks, metals, and manmade objects. There are two main types of weathering: chemical and physical. An example of chemical weathering is acid rain. Caused mostly by the burning of fossil fuels, acid rain is a form of precipitation with high levels of sulfuric acid, which can cause erosion in the materials in which it comes in contact. An example of physical weathering is wind blowing across the desert playas.
This process causes rocks to form a specific pyramid-like shape and they are called ventifacts. Select from these resources to teach about the process of weathering in your classroom.
Sedimentary rocks are one of three main types of rocks, along with igneous and metamorphic. Metamorphic rocks start as one type of rock and—with pressure, heat, and time—gradually change into a new type of rock. Weathering describes the breaking down or dissolving of rocks and minerals on the surface of the Earth.
Water, ice, acids, salts, plants, animals, and changes in temperature are all agents of weathering. Join our community of educators and receive the latest information on National Geographic's resources for you and your students.
Skip to content. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. Bedrock is the hard, solid rock beneath surface materials such as soil and gravel. Bedrock also underlies sand and other sediment s on the ocean floor. Bedrock is consolidated rock, meaning it is solid and tightly bound. Overlying material is often unconsolidated rock , which is made up of loose particle s.
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