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QuestionAnatomy and Physiology

Observations and Predictions A. Sedimentary Grain Sizes in Lake McClure Based on the erosion of Yosemite Valley, we can expect a range of sedimentary grain sizes in Lake McClure. Yosemite Valley is known for its waterfalls, steep gradients, and glacial erosion, which can produce large sediment loads with varying grain sizes. Given the distance and energy of the Merced River, it is likely that the river will transport and deposit sediment with a range of grain sizes in Lake McClure. This may include clay, silt, sand, gravel, and even boulders, depending on the specific locations within the lake and the amount of energy available for sediment transport. B. Composition of Sedimentary Grains in Lake McClure The composition of most of the sedimentary grains deposited in Lake McClure from Yosemite Valley will primarily consist of silicate minerals, quartz, and feldspar, which are the most common minerals in the Earth's crust. Additionally, sedimentary grains may include metamorphic and igneous rock fragments, as well as mineral particles derived from weathering and erosion of pre-existing rocks in Yosemite Valley. II. Changes in Clastic Sediment as it Travels Downstream As clastic sediment travels downstream from Yosemite Valley to Lake McClure, it undergoes various physical, chemical, and biological changes. These changes can include: A. Sorting and Size Reduction As the sediment is transported by the river, it will be sorted by size, shape, and density. Coarser sediment, such as gravel and sand, will be deposited closer to the source (Yosemite Valley), while finer sediment, like silt and clay, will remain in suspension longer and be transported further downstream. Additionally, sediment particles will experience abrasion and collision during transport, leading to size reduction and rounding of the particles. This process is known as comminution. B. Chemical and Mineralogical Changes Sediment particles can undergo chemical changes as they travel downstream due to interactions with water, dissolved ions, and atmospheric gases. These changes may result in the formation of new minerals, the dissolution of existing minerals, or the precipitation of minerals from solution. For example, as silicate minerals interact with water, they can undergo weathering and dissolution, releasing cations such as calcium, magnesium, and sodium into the water. These cations can then precipitate as carbonate or sulfate minerals, forming authigenic minerals within the sediment. C. Organic Matter and Biological Activity Sediment can also accumulate organic matter as it travels downstream, derived from the decay of plants, animals, and microorganisms. This organic matter can serve as a nutrient source for aquatic life, supporting primary productivity and contributing to the formation of organic-rich sedimentary rocks like coal and shale. III. Interpreting Clastic Rocks in General By studying the characteristics of clastic sediment in modern river systems, such as the Merced River, we can gain insights into the formation and interpretation of clastic rocks in the geologic record. For example, the composition, grain size, and sorting of sedimentary rocks can provide information about the depositional environment, tectonic setting, and climatic conditions under which the rocks were formed. Furthermore, the presence of specific minerals, such as quartz and feldspar, can indicate the provenance of the sediment, helping to reconstruct the tectonic history of a region. By understanding these processes and relationships, geologists can use clastic rocks to unravel the complex history of Earth's sedimentary basins and the evolution of the planet's surface.