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16 A SKETCH OF THE GEOLOGY AND SOILS OF THE BIG THICKET BY SAUL ARONOW The Big Thicket, which occupies a major part of Hardin County and portions of the adjoining Polk, Liberty, and Jefferson Counties (see map facing p. 12), is very young as the geologist measures time. The clays and sands that support the extensive forest of the Thicket are perhaps less than a million years old. At this writing the best estimate of the age of the earth-an estimate based on the slow decay of radioactive elements -is about 4,500 million years. The past million years of this history certainly have been most eventful in producing the familiar features of southeast Texas. All of the coastal features such as deltas, barrier islands, bays, marshes, and lagoons were formed during this time. Also made during this period were the extensive highstream terraces bordering the flood plains of the Sabine, Neches, Trinity, San Jacinto, and Brazos rivers, which supply most of the sand and gravel to southeast Texas. Outside the Gulf Coast region, during the past million years, vast continental glaciers have spread at least four times from central Canada into the midwestern and northeastern parts of the United States, while in the west large glaciers descended from the Rocky Mountains and the coast ranges into adjacent lowlands. Most other continents were similarly glaciated. Geologists have 11. Saul Aronow. given the name "Pleistocene" to this time of the glaciers. Sometimes , an older name, "Quaternary," is used instead. The clays and sands of the Big Thicket, then, were deposited during the time of the Pleistocene glaciers. To begin with, over a million years ago the waters of the Gulf of Mexico were rolling over the Thicket area. Gradually, however, deltaic plains, with fringing marine deposits, were laid down by the Pleistocene ancestor of the present Trinity River. (Similar deltaic plains are being deposited today by the nearby Brazos, Rio Grande, and Mississippi Rivers.) And gradually, as the deposition continued, the water retreated and the land of the Thicket appeared. The deltaic and associated marine sediments found in the Thicket were originally classified as two geologic formations, the Beaumont Clay and the Lissie Formation. Recently, some geologists, with purer motives than to confound the uninitiated, have subdivided the Lissie into an Upper and Lower Lissie, giving us three 17 18 12. Climax hardwoods in Boggy Creek bottom. geologic formations. They are in relative age from youngest to oldest , the Beaumont Clay, the.Upper Lissie and the Lower Lissie. Now, it might well be asked what relevance the advance and retreat of the glaciers have to the abandoned deltaic plains under the Thicket and their division into three separate formations. Geologists believe that during the times of the advances of the great THE GEOLOGY AND SOILS glaciers the sea level went down, perhaps as much as 450 feet below its present level. The best explanation for this lowering of sea level is that much water from the oceans was incorporated into the glaciers and thus transferred to the surface of the continents. The shoreline of the Gulf of Mexico was then many miles out in the present Gulf, with much of the continental shelf exposed. All of the major streams in the Gulf Coast, as well as in the rest of the world, cut deep channels. The floors of some of these are now partly or completely filled but they remain below present sea level. Sabine Lake, Trinity Bay, and Corpus Christi Bay are the partly obliterated remnants of such channels. Other streams, such as the Brazos and Rio Grande, carrying more sediments, have long since filled theirs. When the glaciers melted and sea level rose again, the streams began depositing the deltaic plains. The three formations in the Thicket record three of these rises of sea level between glaciations. The boundary between the Beaumont Clay and the two other formations is perhaps the easiest of the formation boundaries to discern, for farm-to-market road 787 between Saratoga and Fuqua more or less follows it. The boundary is marked by a scarp about twenty-five or thirty feet high. In a few places, this scarp has been eroded by streams and the road descends to lower levels-for example , where the road crosses Little Pine Island Bayou about three miles southeast of Thicket. South and west of the road, and below the scarp, lies the Beaumont Clay; north and east, the Upper and Lower Lissie. About halfway between Votaw...


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