Chapter 8 -- Texas Landforms and Geology

Chapter 8 -- Texas Landforms and Geology - GEO 1013...

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GEO 1013 E.R. SWANSON – spring 2009 LANDFORMS AND GEOLOGY OF TEXAS Landforms Texas on television and in the movies is commonly portrayed as a rugged, bleak, rural landscape dotted with clapboard towns through which dust blows and cattle wonder aimlessly. Change the channel and it is oil barons or cigar chomping politicos vying for wealth and power in urban Dallas or Houston. Flip again and its moss-draped oaks at the old plantation or shrimpers in the Gulf. Ranchland or rice, mountains or marsh or metroplex, Texas is a very large state with tremendous diversity in terms of landforms and climate. East to west and on a continental scale, Texas contains parts of the Gulf Coastal Plain, the nation’s great Interior Lowlands, the Great Plains, the Rocky Mountains, and the Basin and Range Province. On the local level, more than fifteen distinctive provinces exist in Texas, but a more manageable number of seven will do for our purposes. We will cover those seven provinces beginning in the east. Figure 1. Physiographic Provinces of Texas 1
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GEO 1013 E.R. SWANSON – spring 2009 Gulf Coastal Plain From the Sabine River border with Louisiana to the Rio Grande border with Mexico, the Texas part of the Gulf Coastal Plain forms a 180 mile wide arc around the western margin of the Gulf of Mexico (Fig. 1). As the word plain plainly implies, this is a relatively flat surface. Gulf Coastal Plain elevations do, however, increase from sea level at the coast to an inland elevation of about 500 feet in the northern Gulf Coastal Plain, increasing southwestward to about 1,000 feet above sea level at Del Rio. While the elevation may increase in the Del Rio direction, the amount of rainfall and therefore the thickness of the soil and the general height of the vegetation definitely do not. The Gulf Coastal Plain is crossed by numerous rivers like the Sabine, Trinity, Brazos, Colorado, Rio Grande, etc. carrying enormous loads of sand and mud and discharging it into the Gulf of Mexico. “The present is the key to the past” and this situation has persisted throughout Cenozoic time. The result is that, from east of a line extending from Del Rio through San Antonio, Austin, and near Texarkana, there exists a thick wedge of sediment dumped onto the subsiding crust of the Gulf of Mexico during the past 65 million years, all during Cenozoic time. While there is still a long way to go toward filling in the Gulf of Mexico, the Gulf Coastal plain represents about 180 miles worth of progress adding about a three mile-wide strip of sedimentary rock to wonderful Texas every million years. Naturally, the oldest Cenozoic sedimentary rocks are found inland and the youngest are just now forming at the coast. Sedimentary layers of sandstone, mudstone and clay dip gently toward the Gulf like
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Chapter 8 -- Texas Landforms and Geology - GEO 1013...

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