Geological strata and their fossils are mapped by paleontologists as past living layers, composed of sediments anciently laid down by rivers, lakes, oceans, and wind, and filled with associated organisms, and now turned into layers of sedimentary rock with fossils. Reading the layers as changing forms of past and present geologic environments, and periodic deposition, is based first on the study of present changes in rivers, lakes, and other geologic features. This branch of geology is called geomorphology, or the "study of earth forms". It can also be applied to ancient changes in the environment. The geomorphologists studying ancient river courses have borrowed a few terms from 19th century riverboat captains, such as documented by Mark Twain. While Murchison and Sedgwick were mapping the Paleozoic strata of Wales, Mark Twain was mapping the 19th century Mississippi River.
on the Mississippi by Mark Twain
was first published as a serial in
Magazine in 1875. The book is a
unique study of
about the river, as Twain discovers during
apprenticeship as a “cub” pilot, is a meticulous
which takes up a good
portion of the story. Every sandbar, shoal, chute, and channel in the
Before they are deeply covered by sediments, the point bars and levees of abandoned river channels are easy to see in aereal photos. Beginning in the 1940s, using aerial photos combined with numerous core samples, the geologist Harold Fisk mapped the Holocene courses of the Mississippi for about 10,000 years back to the last ice age (fig.1; Fisk 1944). Fisks’s initial study has been continued and refined to the present day by other geologists, with the same techniques applied to many rivers around the world. Many of the abandoned channels contain archaeological sites which were once on active rivers (Saucier refs).
Importantly for this discussion, the point bars, levees, and other features of river channels can also be reliably mapped in cross section, going far back in geological time. Thus, if you took the entire lower Mississippi Valley mapped by Fisk, Saucier, and other geologists for the last 10,000-12,000 years and turned it on its side, it would represent a vertical section about 1000 miles long from north to south and 100 miles wide east to west, but often not amounting to more than a few dozen meters in depth.
Fig.1: Map of current and abandoned channels of the Mississippi River (Fisk 1944).
million years from now, when the
The channels and banks known to Mark Twain and his riverboat mentors as point bars will then be visible in cross section as thin layers of shale and sandstone, called crossbeds by geologists, which run at angles to the prevailing beds or layers. Such crossbedding and other indicators of river currents provides essential clues to the locations of buried ancient river courses. In these strata, future paleontologists would find fossils of fish and plants contained in the shale and sandstone layers, along with the fossilized bones of various land animals (perhaps including a few careless 19th century rivermen who may have fallen from their rafts while brawling).
Extending far beneath the level of Mark Twain’s river are successively older layers, normally subject to interpretation by the common-sense principle of superposition (older layers are covered by younger layers), but inevitably, with some folding or faulting due to plate tectonic shifts in the Earth's crust (i.e., orogeny) or earthquakes. As "Strata Smith" and many others have demonstrated, typical fossils are to be found in each layer which reflect past environments and evolutionary stages. Below the glacial era of the Pleistocene (showing meandering vs braided channels, representing warm vs dry layers in the Mississippi valley) are layers from the Tertiary periods (Pliocene down to Paleocene, 2-65 million years ago), with fossils of ungulates and crocodilians and perhaps a few small Eocene primates. Below this are the Mesozoic periods (Cretaceous, Jurassic, and Triassic periods; 210-65 million years ago), the age of reptiles including dinosaurs and sub-tropical vegetation. Still further down, pehaps now a few hundred meters below the surface, begin the Paleozoic strata dating back 245-390 million years ago, when the first land animals (tetrapods) developed from fish into amphibians, reptiles, and synapsids (the ancestors of mammals), during the Permian, Pennsylvanian, Mississippian, and Devonian periods.
Pangaea, vertebrates were diversifying,
particularly reptiles whose amniotic (egg) reproduction allowed them
of movement through the land than amphibians. A typical Permian-age
the Russian Urals or northern
Clemens, Samuel 1875. Life on the Mississippi. Harpers Magazine, Harpers, New York.Fisk, Harold 1944. Geological Investigations of the Alluvial Valley of the Lower Mississippi River.
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