Unconformities
In addition to creating joints and faults, the stresses of tectonic plate movement can also result in other types of strain. This can make it initially more difficult to obtain clear information from a geological record, but ultimately reveals a great deal more about geological history than more easily discernible features. Two primary concepts upon which the science of geology are founded are ideas that layers of rock initially occur in "flat" layers, parallel to the Earth's surface, and that younger layers of rock remain closer to the surface unless otherwise disturbed (Alden 2010). This would produce a highly conformed geological record; the unconformities that exist in reality define the disturbances of geological history.
The most obvious and earliest discovered and described type of unconformity is the angular unconformity, in which the rocks below a certain level have hall been tilted in a similar direction and sheared off at a specific height (Alden 2010). This shows a period in the geological development of the bedrock in which an upheaval or other tilting force occurred, forcing what were parallel rocks into their new angular positions. The top layer of these angled rocks were then eroded down to a level surface by natural forces -- generally wind and/or water -- and subsequent layers of level and parallel rock were laid down in successive geological epochs (Alden 2010). The high degree of persistent regularity in angular unconformities despite the stress to which these areas are subjected is a large part of the reason behind their easily recognized features and the clarity of the geological history they relate.
Disconformities or paraconformities, known as nonsequences and not true unconformities to many British geologists, occur when successive layers of sediment deposit and rock strata buildup occur with an obvious interruption or hiatus in such sedimentation, but no other disturbance exists (Alden 2010). What this essentially represents is a gap in the geological record; a time when sediments did not accumulate to create more layers of rock, and then the start of more deposits that, looked at now, can represent gaps of millions of years between layers. Sudden transitions from fossils of one...
plate tectonics is responsible for changing continental landmasses through geological occurrences. Thousands of years ago the earth's surface has been hypothesized as one big landmass. The Earth's surface has been constant motion. "Fragmented into giant sheets of solid rock that glide atop a layer of hotter, more pliable material, the globe's appearance is forever changing." [Cowen, 1999]. These plates are semi-rigid, floated on flow of mantle. The plates measured around
The valley floor itself can experience a variety of topographical changes. As was discussed, the Napa River and other streams deposit clays, shales, and a variety of other river sediments in the valley lowlands, while a historical narrowing of the river has resulted in large concentrations of sedimentary rocks. The Napa River also can flood from late fall to early spring, depositing river sediments far past its typical banks (Larson). The
Rising From the Plains John Mcphee, a writer of creative nonfiction books, started writing about the earth in 1985. He described the structure and movement of the earth's crust and mantle based on geology. He focused on the theory called plate tectonics which describes the earth's crust as several plates that bump with each other while gliding over the mantle. That theory was accepted only in the latter part of 1960's
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