wells creek impact crater

Shatter Cone from Wells Creek Impact Crater

Sometime between 100 and 300 million years ago, a violent event occurred in what is now Wells Creek Basin in Stewart and Houston Counties of Central Tennessee. A bright light appeared and streaked southward across the sky. A meteorite, traveling 10 to 25 miles per second (36,000 to 90,000 mph), struck the earth with a shuddering impact, accompanied by a supersonic air blast, and penetrated to a depth of 2,000 feet before the tremendous downward force together with the earth’s forces of resistance resulted in a gigantic explosion. The earth shook and earthquake waves raced in all directions. A mighty fiery mushroom of masses of rock and clouds of pulverized rock dust rose high into the air, and fell back to earth. The rock fragments landed quickly, but some of the dust stayed in the air. A great crater was formed, about four miles in diameter and half a mile deep, rimmed by a surrounding pile of shattered rock debris. The crater was mostly filled in by the central uplift. The deafening noise died away and all returned to the peace and quiet that prevailed before the catastrophe.

The earth’s surface appeared to be damaged forever, but millions of years passed and erosion and vegetation softened the ugly scar. The rim of scattered rock disappeared, and the level of the region was lowered many hundreds of feet by the work of rain and running water, mass wasting, gullying, and downward and lateral cutting by streams. Because of the shattered character of the rock in the impact area, the circular scar was eroded faster and therefore deeper than the surrounding region. Thus, circular Wells Creek Basin, as it is today, was born.

Perhaps 10,000 years ago, man first saw Wells Creek Basin, Indian tribes found haven in this pleasant basin that had so much to offer. Game was plentiful, and the streams yielded a variety of fish and mussels. Springs and clear streams were present. The low hill in the middle of the basin afforded an excellent place to camp and to live safely above the highest flood water. Outposts of watchers could guard the trail that entered the basin from the south along Wells Creek as well as paths that crossed the protective rim of hills surrounding the basin. The low central hill could be defended easily. Dense flint found in great abundance in the hills about five miles to the west could be patiently worked into arrow and spear points for war and hunting, large ceremonial flint objects, and many other useful items.

When the first white settlers came from North Carolina and Virginia, they were impressed by the relatively flat, well-drained basin. The soil was better than that of the surrounding hill country. Not only was the soil fertile and essentially free of chert blocks, but it also presented within a small area a variety of soil types formed by the weathering of many types of rock exposed there. Here in the basin, a highly agricultural society developed. The owners, their neighbors and visitors, knew that the soil was different and that the rocks were unlike others in the region. Undoubtedly many wondered why.

The center of the Wells Creek crater contains some of the finest shatter cones in the world. A shatter cone is a conical fragment of rock that is formed from the high pressure of a meteorite impact and has striations radiating from the apex of the cone. The Wells Creek shatter cones were actually formed by the shock waves that arrived before the limestone beds were tilted by the meteorite. The cones formed pointing toward the place from which the shock waves came. The Wells Creek shatter cones were formed by shock waves coming from a position which was (at the time of impact) more than 2,000 feet underground.

On Saturday, February 25, 2006 about 20 members of the Memphis Archaeological and Geological Society were given a grand tour of the Wells Creek Crater by Tennessee state geologist, Marvin Berwind. Mr. Berwind gave us an overview of the formations and fault patterns that make up the Wells Creek structure. We were given an opportunity to view the structure from several locations along the rim before venturing into the crater for an afternoon of exploration and shatter cone collecting at ground zero. Shatter cones were very abundant, although most required a little bit of work to extract from the limestone formations. The Wells Creek shatter cones averaged from about a quarter of an inch to two or two and a half inches in diameter, although shatter cones outside that range were also available.

[01] Charles W. Wilson, Jr. and Richard G. Stearns. Circumferential Faulting Around Wells Creek Basin, Houston and Stewart Counties, Tennessee. Journal of the Tennessee Academy of Science. Vanderbilt University, Nashville, Tennessee. 1966.

[02] Charles W. Wilson, Jr. and Richard G. Stearns. Bulletin 68: Geology of the Wells Creek Structure, Tennessee. State of Tennessee. Department of Environment and Conservation. Division of Geology. Nashville, Tennessee. 1968. Reprinted 1993.

[03] Wells Creek Crater. Wikipedia. Answer.com. http://www.answers.com/topic/wells-creek-crater. 27 February 06.


upper cretaceous fossils of frankstown, mississippi

Concretions in Twenty Mile Creek, Frankstown, Mississippi.

Once or twice a year MAGS members travel to Frankstown, Mississippi to search for sharks' teeth and other vertebrate fossils in the creekbed sand of Twenty Mile Creek.

During the summer of 1990, construction on Highway 45, about seven miles south of Booneville, Prentiss County, in northeastern Mississippi revealed two concentrated fossil beds. One contained an abundance of sharks' teeth and the other an abundance of oyster shells.

The fossils in the Frankstown creekbed are about 75 million years old. These fossils come from the base of the Demopolis Formation, just above the Coffee Sand. The Coffee Sand can be seen in the exposure along Twenty Mile Creek near where Highway 45 crosses it. One interesting aspect of the Coffee Sand is the large boulder-sized concretions that are scattered along the creekbed. I took the picture above on a recent visit to Frankstown. After many visits to this location, I am still fascinated by these structures. Concretions are rounded masses of rock that formed by the cementation of sediments where lime or silica is concentrated in certain layers of ground water. Though the Coffee Sand concretions of Twenty Mile Creek are boulder-like in size and shape, they were not rounded by being rolled along the creek bottom. Rather, they acquired their rounded shape by cementation in place within the formation, and being sculpted by the once-swift currents of Twenty Mile Creek.

Many fossils from the Frankstown site are from groups of animals that died out in a great extinction at the end of the Mesozoic, indicating that the site is no younger than the Cretaceous Period. These animals include the oyster Exogyra, ammonites, some of the sharks (Hybodus, Squalicorax), some of the rays (the sclerorhynchid sawfish), some of the bony fishes, the mosasaurs and the dinosaurs.

The cause of the great extinction at the end of the Cretaceous is believed to have been an asteroid impact which spread a layer of soot across the earth, filling the atmosphere, blocking sunlight and causing plants (and the animals that depended on them) to die. One interesting detail of the great extinction is that not everything died. Gars, turtles, crocodiles, opossums and many others lived on as if nothing had ever happened.

A partial list of fossils found at the Frankstown site include: formaminifera, sponge, arthropod (ostracod, crustaceans), mollusks (oysters, jingel shells, scallop, gastropod, scaphopod), annelid worms, brachiopod, bryozoan, more than 12 varieties of shark, rays, bony fish, turtles, mosasaur, crocodiles, and dinosaurs (theropod, hadrosaurid); microfossils and trace fossils.

SOURCE: Earl Manning and David Dockery. A Guide to the Frankstown Vertebrate Fossil Locality (Upper Cretaceoous), Prentiss County, Mississippi. Circular 4; Mississippi Department of Environmental Quality. Office of Geology. Jackson, Mississippi. 1992.


meteorites: bits of the solar system that have fallen to earth

Barringer Crater, Arizona

As you stare up into the star-filled sky on almost any moonless night, you will probably see a few meteors every hour. During meteor showers (such as the Leonid shower which occurs each fall) you may see as many as one hundred meteors every hour. Meteor showers can be very impressive. A meteor is a bright streak of light in the sky created when a small meteoroid enters the Earth's atmosphere. Meteors are sometimes called shooting stars or falling stars. Very bright meteors are called fireballs.

Meteorites are bits of the solar system that have fallen to Earth. Most meteorites come from asteroids which have strayed from the asteroid belt located between Earth and Mars. Some have been found that are believed to have come specifically from the 4 Vesta asteroid. Some probably came from comets. A few have been found which came from our Moon and a few are believed to have come from Mars. One famous meteorite, called ALH84001, is believed to show evidence of early life on Mars. Meteorites are very important in scientific research, because they are our only material link to the universe beyond Earth.

Meteorite Types:
[01] Iron: primarily iron and nickel; similar to type M asteroids
[02] Stony iron: mixtures of iron and stony material like type S asteroids
[03] Chondrite: by far the largest number of meteorites fall into this class; similar in composition to the mantles and crusts of the terrestrial planets
[04] Carbonaceous Chondrite: very similar in composition to the Sun less volatiles; similar to type C asteroids
[05] Achondrite: similar to terrestrial basalts; the meteorites believed to have originated on the Moon and Mars are achondrites

A fall means the meteorite was witnessed by someone as it fell from the sky. A find means the meteorite was not witnessed and the meteorite was found after the fact. About 33% of all meteorites are witnessed falls. More than one hundred tons of meteoroid material enters the Earth's atmosphere every day. Most of this material is very small, weighing only a few milligrams each. They are called micro-meteorites. Only the largest bits of meteoroid material ever reach the surface of the Earth to become meteorites. Weighing in at 60 tons, the Hoba Meteorite, found in Namibia, is the largest on record.

The average meteoroid enters the atmosphere at between 10 and 70 km/sec. But all but the very largest are quickly decelerated to a few hundred km/hour by atmospheric friction and hit the Earth's surface with very little fanfare. However meteoroids larger than a few hundred tons are slowed very little; only these large (and fortunately rare) ones make craters.

A good example of what happens when a small asteroid hits the Earth is Barringer Crater (a.k.a. Meteor Crater) near Winslow, Arizona. It was formed about 50,000 years ago by an iron meteor about 30-50 meters in diameter. The crater is 1200 meters in diameter and 200 meters deep. About 120 impact craters have been identified on Earth, so far.

A more recent impact occurred in 1908 in a remote uninhabited region of western Siberia known as Tunguska. The impactor was about 60 meters in diameter and probably consisting of many loosely bound pieces. In contrast to the Barringer Crater event, the Tunguska object completely disintegrated before hitting the ground and so no crater was formed. Nevertheless, all the trees were flattened in an area 50 kilometers across. The sound of the explosion was heard half-way around the world in London.

There are probably at least 1000 asteroids larger than 1 km in diameter that cross the orbit of Earth. One of these hits the Earth about once in a million years or so on the average. Larger ones are less numerous and impacts are less frequent, but they do sometimes happen and with disastrous consequences.

The impact of a comet or asteroid hitting the Earth was probably responsible for the extinction of the dinosaurs 65 million years ago. It left a 180 km crater now buried below the jungle near Chicxulub in the Yucatan Peninsula.

Freshly fallen meteorites and what to do if you find one:
Calculations based on the observed number of asteroids suggest that we should expect about 3 craters 10 km or more across to be formed on the Earth every million years. This is in good agreement with the geologic record. It is more difficult to compute the frequency of larger impacts like Chicxulub but once per 100 million years seems like a reasonable guess.

The most important primitive meteorites are generally those that were observed falling and collected soon after hitting the ground. These samples are particularly useful because they have not been altered by chemical and physical processes (such as rain) that are typical of the Earth's surface.

Freshly-fallen meteorites are usually the easiest to identify, because they are surrounded by a fusion crust produced when the surface of the meteoroid was melted by friction with the Earth's atmosphere. (You may recall that when space capsules or shuttles return to Earth, they are protected by heat shields to prevent the spacecraft from being consumed.). Fusion crusts are usually black or brown. Sometimes they contain flow lines radiating from one end of the meteorite towards the other, indicating the thin skin of melt was being blown backwards. These samples are called oriented meteorites, because we can tell how they were oriented when they fell through the atmosphere.

If a sample is unbroken and has a complete fusion crust, handle the sample gently to preserve the fusion crust. To see the interior of the object, only chip away or grind off a very small portion of the sample. If the interior resembles one of the meteorites described above, bring the sample to a credible laboratory where a complete examination can be properly documented and precautions can be taken to preserve the integrity of the sample.

If you believe you have found a freshly-fallen meteorite, try to photograph the area before removing any specimens. If a meteorite has produced a crater, measure its diameter and depth (some may only be a few centimeters deep). You should also look for multiple meteorite fragments scattered around the area. If you find more than one, try to estimate the distance between the samples.

Hunting for meteorites:
Scientists recently discovered that meteorites have been preserved and concentrated in certain regions of Antarctica. Consequently, within the last 20 years, Japanese and American teams of scientists have collected over 15,000 meteorite specimens from Antarctica, increasing the number of samples in our collections dramatically.

Until the work in Antarctica began, the total number of known meteorites was about 2,600. Of these, only about one-third were observed to fall. Most meteorites are classified as finds and were discovered by farmers, shepherds, ranchers, hikers, and so on. In some cases, however, they were found by diligent meteorite hunters. A few meteorites have also been found in fossiliferous limestones and two were found in rocks collected from the Moon by Apollo astronauts.

Although 93% of the meteorites observed to fall are chondrites and achondrites, only 56% of the meteorites that are found accidentally are of these same types. Iron meteorites are much more common among the meteorite finds, because they are so unlike most terrestrial rocks and because stony meteorites are highly susceptible to weathering on the surface of the Earth. While hunting for meteorites (in the case of iron meteorites), look for dense rocks with brown fusion crusts which are also magnetic and have a metallic silver interior.

If you find a possible meteorite, try to avoid handling the stone. In contrast to some myths, you will not be harmed by touching a meteorite, but you may inadvertently contaminate the sample with salts and oils from your hands. If possible, place the stone in a clean (and dry) plastic bag. Bring the sample to a credible laboratory as soon as possible.

[01] David A. Kring. Meteorites and their properties. Lunar and
Planetary Laboratory. Department of Planetary Sciences. The University
of Arizona. http://meteorites.lpl.arizona.edu/index.html. 27 Feb 2007.
[02] Bill Arnett. Meteors, Meteorites and Impacts. The Nine (8)
Planets. http://seds.lpl.arizona.edu/nineplanets/nineplanets/meteorites.html.
27 Feb 2007.


great smoky mountains geology

This is a view of Charlie’s Bunion as my Venture Crew and I trekked our way through the Great Smoky Mountains National Park on the Appalachian Trail several years ago. Thunderhead Mountain, Charlie’s Bunion and Sawteeth are my favorite parts of the trail.

Although the Rocky Mountains are taller and the peaks are more jagged, the Appalachian Mountains are the oldest mountains in the United States. Age . . . that’s actually why the Appalachians are smoother and rounder . . . a billion years of erosion. The Great Smoky Mountains are the highest peaks in the Appalachian mountain range. I grew up in the Smokies, and I get back to them as often as I can. When I was a boy growing up in Western North Carolina, I never wondered how the Smokies were formed, I just enjoyed hiking them, sitting on the balds, peering out over the vast expanse of blue and purple mountain tops. Now that I am older I find it fascinating to study the geologic history of the areas in which I hike.

Journey Back
So sit back and enjoy at journey back through time when an ancient sea flooded what is now the eastern United States, submerging the remnants of an old mountain range. The sea slowly deposited layers and layers of sediment onto the ocean floor. The intense pressure of thousands of feet of sediment compressed these layers into metamorphic rock. Almost 300 million years ago, the sea added yet another layer of limestone sediment that was composed of fossilized marine animals and shells. The stage was set for the formation of the Appalachian Mountains. [01]

As a result of the eons-old shifting of the earth's tectonic plates (large sections of the earth's crust), Africa and North America collided about 250 million years ago. This caused the older, underlying layer of metamorphic rock to tilt upward and slide over the younger limestone rock, slowly creating a towering mountain range, the Appalachians. The older rocks, known as the Ocoee Series, now compose most of the Great Smoky Mountains. Charlie’s Bunion (pictured above), Sawteeth and Chimney Tops are dramatic examples of how the rock layers tilted and buckled to form steep cliffs and pinnacles. In Cades Cove, erosion of the overlying metamorphic rock reveals the limestone layer beneath. [01]

During the ice ages, massive boulders were created by alternating freezing and thawing of the rock. You can see boulder fields on the Cove Hardwood, Noah "Bud" Ogle and Roaring Fork Motor Nature Trails. The Smokies originally looked more like the Himalayas than the rounded mountains we see today. The relentless erosive force of water has sculpted their present-day appearance. Water run-off has also helped to carve the alternating pattern of V-shaped valleys and steep ridges. Landslides caused by a torrential downpour in 1951 created the large V-slash on Mount LeConte, and rock slides in 1984 briefly closed Newfound Gap Road. As you explore the park, look for how water continues to sculpt the land. [01]

A look at the rocks and minerals in the park
Although the nature of the rocks of the Great Smoky Mountains is puzzling, they are geolocially interesting because they contrast the Paleozoic sedimentary rocks of the Appalachian Valley on the northwest with the metamorphic rocks and granite of the Blue Ridge on the southeast. Like the rocks of the Appalachian Valley, most of those in the Great Smoky Mountains are sedimentary. The rocks of the Appalachian Valley are made up of a variety of fossil-bearing limestone, sandstone, and shale. Those of the mountains are a great mass of pebbly, sandy, and muddy sedimentary rocks, devoid of fossil remains. [02]

Close examination of the rocks of the Great Smoky Mountains indicate that they were deposited later than most of the rocks of the Blue Ridge, which are of earlier Precambrian age (formed more than a billion years ago), but before the rocks of the Appalachian Valley, which are of early to middle Paleozoic age (formed 600 million to 300 million years ago). Most of the rocks of the Great Smoky Mountains were formed during some part of later Precambrian time (a billion to 600 million years ago). [02]

Basement Complex
Basement Complex, the rocks of the Blue Ridge Mountains, have a crystalline foundation. They extend along the southeastern side of the Great Smoky Mountains and reappear at several places within the mountains where tectonic forces have pushed them up or have thrust them into contact with younger rocks. [02]

The basement complex consists of a wide variety of gneiss and schist, including layered gneiss from sedimentary or volcanic rocks and non-layered granite-based gneiss. The layered gneiss contains various amounts of biotite, muscovite, quartz and feldspar. They also contain small amounts of mica schist and larger amounts of hornblende. The non-layered gneiss is mostly quartz monzonite and granodiorite whose chief minerals are biotite, epidote and magnetite. [02]

The hornblende gneiss of this area may have been part of a volcanic flow, and the granitic rocks which dominate the northwestern part of the Blue Ridge Mountains may have originated partly as magma that invaded the rocks. Rocks of the basement complex date as far back as one billion years with a scattering of rocks as young as 350 million years. [02]

Ocoee Series

The later Precambrian sedimentary rocks, which form most of the Great Smoky Mountains and large parts of the adjacent foothills, are known as the Ocoee Series. This series extends far beyond the Great Smoky Mountains to the northeast and southwest, along the trend of the ranges–from northeast of Asheville, NC, at least as far as Cartersville, GA, a distance of more than 175 miles. Near the Great Smoky Mountains this series extends across the ranges about 30 miles (wider in some places). [02]

Toward the northwest of the Ocoee Series the clay minerals in the sedimentary rocks have been altered to chlorite; southwestward these minerals have been transformed to biotite and garnet; to the southeast these minerals are represented by staurolite and kyanite. The southeastern rocks with dominant clay minerals have changed from shale to slate, phyllite and schist. [02]

The Ocoee Series can be divided into three groups: Great Smoky Group which forms the main mass of the Great Smoky Mountains; the Snowbird Group occurs in the middle, in the foothills just north of the mountains; and the Walden Creek Group which occurs in the northwest, in the part of the foothills nearest the Appalachian Valley. Characteristic Snowbird outcroppings of Pigeon Siltstone may be seen north of Gatlinburg and characteristics of Roaring Fork Sandstone may be seen southeast of the Great Smoky Mountains park headquarters. [02]

The Great Smoky Group is a thick mass of sedimentary rocks, pebble conglomerate, coarse to find sandstone, and silty rocks, which can be divided into three formations: the fine-grained Elkmont Sandstone below, coarse-grained Thunderhead Sandstone in the middle, and cark sily rocks of the Anakeesta Formation above. Both the Elkmont and Thunderhead Sandstones are gray and composed principally of quartz and potassic feldspar, with a small amount of plagioclase feldspar and light-colored granite and quartizite. The Thunderhead Sandstone may contain blue-tinted quartz grains. The Anakeesta Formation consists mainly of dark silty rocks altered to slate, phyllite or schist. [02]

Sedimentary rocks of the Walden Creek Group form the northern and northwestern parts of the foothills. This group is mostly chale and siltstone, but it includes masses of conglomerate and sandstone, as well as nimor layers of quartzite, limestone and dolomite. [02]


[01] Sculpted by Water. Geology. Great Smoky Mountains. American Park Network. http://www.americanparknetwork.com. accessed 19 April 2007.

[02] Bedrock Geology. Geology of the Great Smoky Mountains National Park, Tennessee and North Carolina. Geological Survey Bulletin 587. USGS. http://www.cr.nps.gov/history/online_books/geology/publications/pp/587/sec1.htm. accessed 19 April 2007.


fluorescent minerals

I had an opportunity to travel to Philadelphia at the end of last week on business, and I found it an opportune time to travel north a bit on Saturday to Franklin, New Jersey to one of my favorite places . . . the Franklin Mineral Museum and the fluorescent mineral collecting site directly behind the museum. This was my fourth trip to Franklin and I enjoy it more each time. I was looking for flourescent minerals and I sure found a load of them on Saturday. I just happened to be there on the first day of the collecting season. A week earlier and I would have found the site under snow.

The first rock that I spotted looked different to me. I didn't recall having seen one like it there before, so I held on to it and kept moving it to the top of my bucket each time I placed another rock in the pile to be sorted. My first rock turned out to be a combination of willemite, zincite and franklinite. When I placed it under the short-wave UV light, it fluoresced a bright blue. It is really stunning. I also found several red willemite and several green willemite specimens. I found a piece or two of barite, there were several specimens with just a tinge of yellow fluorescence and one or two with specs of beige. And of course, there was "tons" of calcite brilliantly glowing orange. I have always found the combination of calcite and franklinite to be an attractive-looking specimen. The pictures above are red willemite from my Franklin excursion. The top one in under fluorescent light. The bottom one is under short-wave UV light.

Now just a few words about fluorescence.
Fluorescence is a property not found in all minerals. Minerals that do fluoresce, glow when exposed to either short-wave or long-wave ultraviolet light. Examples of fluorescent minerals are autinite, calcite, diamond, eucryptite, fluorite, hyalite, scheelite and willemite. Minerals from the Franklin and Sterling Hill area of New Jersey are known for their fluorescence. There are over 80 fluorescent minerals found in that area. Franklin/Sterling Hill fluorescent specimens usually contain 2-4 minerals in a typical specimen though some have up to 7 fluorescent minerals found together.

Fluorescent minerals contain particles in their structure which respond to ultraviolet light by giving off a visible glow. Ultraviolet light is a form of electromagnetic radiation invisible to the human eye. It is given off by the sun and by common fluorescent lamps used for lighting, but they also give off considerable white light (visible light), preventing the fluorescence from being seen. The ultraviolet reaction is visible with a special fluorescent lamp with a filter that blocks white light but allows ultraviolet light to pass through. This lamp is known as an ultraviolet fluorescent lamp, or UV lamp. The reaction will only be visible in a dark area, where the presence of white light is weak.

There are two ultraviolet wavelengths: long-wave and short-wave. Some minerals fluoresce the same color in both wavelengths, others fluoresce in only one wavelength, and yet others fluoresce different colors in different wavelengths. Some UV lamps have two separate filters: one for long-wave and the other for short-wave. There are more minerals which fluoresce in short-wave than there are in long-wave.

Color and intensity of the fluorescence varies among specimens of a particular mineral. However, specimens from the same locality almost always fluoresce the same color. For example, calcite may fluoresce red, orange, yellow, white or green.

When a fluorescent lamp is lit, never look at the light source, as it can damage the eyes permanently. In addition, skin should not be exposed to the light source for extended periods, as it can cause sunburns and long term skin problems.

If you have a question about fluorescent minerals, if you have additional information to add to this post about fluorescent minerals, or if you have a question or comment about another subject, please click the comment link below.


tennessee geology

Here is a look at Tennessee physical geography beginning with the eastmost region and moving westward to the Mississippi River.

Unaka Mountains: The bedrock here consists of a variety of igneous and metamorphic rocks, and is quite resistant to erosion. Due to the resistance of these rocks to erosion, and uplift associated with the mountain-making processes of the past, and the isostasy (equilibrium in the earth's crust such that the forces tending to elevate landmasses balance the forces tending to depress landmasses) of this area, the elevation throughout this area is generally 1000's of feet above sea level.

Valley and Ridge: This area consists of a large number of thrust-faulted layers or thrust sheets of rock dipping to the east at low angles. Imagine a deck of cards lying in a neat pile on a table. Now imagine a dealer spreading those cards out so that the stack is now splayed out over a much larger surface area of the table top. That should give you some idea of the nature of these thrust sheets -- except that each is several hundred to thousands of feet thick. Because the sheets dip shallowly to the east, their edges are exposed at the surface as a series of linear outcrops that are roughly oriented north to south. These thrust sheets were created as a result of continental convergence (mountain building).

Outcrops that contain mostly resistant rocks (such as sandstone or siltstone) form ridges. Outcrops that consist primarily of less resistant rocks (such as limestone or soft shale) form valleys. The result of this arrangement on a large scale is a series of north-south oriented ridges and valleys. Superimposed on these thrust sheets are smaller scale anticlines (folds with strata sloping downward on both sides from a common crest) and synclines (folds in rock in which the rock layers dip inward from both sides toward the axis) that complicate the geology somewhat. Elevations are highly variable, but generally 100's to a couple thousand feet lower than those in the Unaka's.

Cumberland Plateau: Structural geology played an important role in the development of this area. Continental convergence triggered mountain making in the Unakas and thrust faulting in the Valley and Ridge during the development of Pangaea. Much of the bedrock of this area is weather resistant, flat-lying, hardened sandstone. The resulting landscape is a tableland, or plateau, with typical elevations of 1200 to 2000 feet above sea level. These elevations are equal to, or higher than, those of the Valley and Ridge. This plateau is capped by a thick, nearly continuous sheet of resistant sandstone.

Eastern Highland Rim, Central Basin, and Western Highland Rim: Uplift of the Nashville Dome accompanied each mountain building episode in Tennessee. As a result, the regions of the Eastern and Western Highland Rims and the Central Basin all experienced periodic increases in surface elevation during the Paleozoic and early Mesozoic. At one time, the sandstones of the Cumberland Plateau probably extended westward over these areas as well. Fractures, resulting from uplift along the crest of the Nashville Dome, however, made the sandstones and the underlying limestones more susceptible to erosion. Consequently, the only remnants of these sandstones in Middle Tennessee are preserved in features such as Short Mountain. Isolated, resistant bedrock features like Short Mountain are termed erosional remnants.

Elsewhere in the Eastern Highland Rim, erosion has exposed carbonate bedrock of Late Paleozoic age. These carbonate rocks contain variable amounts of chert, and are often interbedded with fine grained, fragmented (clastic) rocks. As a result, these rocks are more resistant to erosion than the underlying, purer limestones of the Lower (Early) Paleozoic. Therefore, the Eastern Highland Rim stands above the Central Basin where Lower Paleozoic limestones crop out and erode rapidly. Structural fracturing would have been most intense over the top of the dome; therefore, the Central Basin is more deeply eroded than the adjacent Highland Rims. The geologic characteristics of the Western Highland Rim closely parallel those of the Eastern Highland Rim, resulting in very similar physical geography as well. Elevations in the Highlands Rims typically range from 600 to 1200 feet. Within the Central Basin, the elevation rarely exceeds 800 feet, with 500 to 600 foot elevations more typical.

Mississippi Embayment/Coastal Plain: The Coastal Plain is the western-most physical geographic area in Tennessee. This geographic region roughly corresponds with that of the Mississippi Embayment. In other words, the Coastal Plain was once covered by a shallow sea; when that sea regressed southward, this area became a low relief coastal plain for a while. This sea deposited numerous, flat-lying sequences of sand, silt, and mud, lying between beds of strata which together form a thick blanket of sediment. This blanket is draped over a much older carbonate bedrock surface consisting of Lower Paleozoic carbonate rock.

These relatively young marine fragmented sediments have never been deeply buried, and so are not very hard. As a result, they do not resist erosion very effectively. Instead they form a subdued, low elevation, low relief landscape, consisting of rolling hills, poorly drained lowlands, and shallow, wide stream valleys. Elevations are usually less than 500 feet and decrease rather steadily toward the Mississippi River. Recent (i.e. geologically very young) terrestrial deposits, which are simply reworked marine sediments, are slowly accumulating in many lakes and streams.

If you would like to contribute information on Tennessee geology, or if you have a question about Tennessee geology, please leave a comment to this post.


foraminifera: an amoeba with a shell

from guest contributor Howard Allen . . .

I LOVE forams! Their biggest appeal to me is their stunning diversity of forms. The smallest ones are dust-particle sized, while the largest can be nearly the size of a frisbee (see Guiness Book of Records under "largest protozoan"). A more typical size is like a grain of sand.

You can think of foraminifera (forams for short) as "an amoeba with a shell". They are classified on the basis of the composition and structure of their shells. One group has shells made up of particles of silt or sand that are glued together by the foram animal. Some of these little guys can even select particular mineral types from the sediment to make their shells: magnetite, quartz, mica, diatom shells, sponge spicules, etc. How 'bout that for a single-celled organism! Another group makes its shells of opaque white calcite that looks for all the world like fine bone china. Another group uses transparent (sometimes colored) calcite that looks like glass.

The diversity of shell shapes is mind-boggling. One critter, Lagena, has a shell that looks like an elegant, fluted glass bottle or vase. Some look like miniature ammonites. Some look like bunches of grapes. Some look like strings of beads. Some look like bananas with ribs. Some form straight tubes. Some have multiple chambers inserted at precise angles. Some look like stars. Some look like rice grains. Some look like lentils. Some are attached to the sea-bottom like little trees. The variety is endless.

The geological range of forams is very long. The oldest ones are (I think) Ordovician, and they are very common in today's seas. They are almost exclusively marine (sea water), but some are tolerant of or prefer higher or lower salinities. They are also sensitive to water temperature, depth, energy level (calm water, pounding surf).

The only real limiting factor in collecting them is that you need a microscope, preferably a binocular dissecting scope, with magnification in the range of 10x to 50x. Many of these are available for a few hundred dollars or even less for used 'scopes.

You can find forams in all sorts of sediment. The easiest way is to simply grab a vial of sand from an ocean beach and look at it under the microscope. Tropical beaches, especially those near coral reefs (Florida, Caribbean, Mexico, South Pacific islands), tend to have more and larger forams than cooler water beaches, and muddier sediment tends to have more forams than coarser sand sediment. Bermuda's famous pink sand beaches are colored by billions of red and pink forams. Planktonic (floating) forams can be collected by dragging a very fine net behind a boat. I have obtained wonderfully rich collections of forams from gobs of mud brought up on a ship's anchor in the Arctic Ocean. I ask my friends (especially those who snorkel or scuba dive) to bring me back pill bottles or film cans or small zip-lock bags with sediment from their holidays. One person brought me a bottle of sediment from the Egyptian pyramid of Cheops that was full of fossil forams (Eocene). Turns out that the pyramids were built of limestone that is chock-full of big, lentil-shaped forams. I have rice-grain sized Pennsylvanian-age forams (fusulinids) weathered loose from limestone in Texas. I have collected similar, Permian-aged forams in the mountains of central British Columbia. Chalk and marl beds are a great source of foram fossils. I have seen beautiful forams in rock cuttings from deep in oil and gas wells I've worked on in Canada and Costa Rica.

Google "foraminifera" to find photos and more information on the web. The "bible" for foram enthusiasts is Part C (2 volumes) of the Treatise On Invertebrate Paleontology, which is available at bigger libraries (especially university libraries). Have fun!


geologist offers answers

The editor of the Alberta (Canada) Palaeontological Society's newsletter, and professional geologist in the Canadian oil and gas industry has graciously offered to field questions you might have in the areas of sedimentary rocks and palaeontology. Howard has spent much time hunting dinosaur fossils, but his primary interests are invertebrate fossils and microfossils (especially foraminifera).

Howard also offers MAGS greetings and the best of luck on the launching of this blog.

I have a few questions for you already Howard. I know that foraminifera are small, single-celled organisms, of which many look like little grains of rice. Can you tell us a bit more about foraminifera? How old are they--do most of them come from the eocene? Were they ocean dwellers or shallow sea dwellers? How and where do you hunt for this particular type of microfossil?

Blog users--if you have a question that you would like to ask about invertebrate fossils or microfossils, or if you have information in this area that you would like to share with us, please click the comment link below.


geodes: how do I open them?

Opening geodes is always a lot of fun. You never know what is inside until it is open, therefore, in order to get the most enjoyment out of your geodes, you have to crack them open. There are a few ways to do that:

[01] Professional geode crackers use a set of two hardened steel points fastened to a power press, which allows you to apply pressure on the geode from two points at the same time. This is a great way to open a geode, but it is a little bit expensive.

[02] The method I prefer to use is a plumber's pipe-cutting tool. These can be found for sale on the internet and through plumbing supply houses. To open a geode using this method: find the most logical fissure or wrinkle along the outside of the geode; lock the geode in the plumber's tool and apply slow, even pressure to the handle. The geode will almost always open with a nice, even crack (no jaggies).

[03] One method is to score the geode all the way around rather deeply with a trim saw. Do this by raising the splashguard and rotating the geode by hand until it is cut all the way around. Now you can use a screwdriver to pry the two sections of the geode apart.

[04] The most common method is to lay the geode in a soft (earth) depression. Look the geode over for any cracks, weak spots, or lines that look like they might be good places to crack it open. That' where you need to apply pressure. When you have found the ideal spot on the geode, use a medium weight hammer (or rubber mallet) and center punch (NOT A STEEL CHISEL). Put the punch in a spot most likely to keep if from sliding off, and hit it several times; easy at first, then harder. If three or four blows do not open the geode, repeat in other spots until a crack does appear. Then use a screwdriver to pry your geode apart.

No matter what method you use, always use gloves and safety glasses. After you have cracked your geode, put the halves back together and secure them together with tape or rubber bands until you get them home. Never hit geodes with a hammer! Although you have on safety glasses, chips from the geode will fly in all directions, placing everyone around you in danger, and leaving chips of the brokern geode strewn all over the ground. Don't open the geodes in the field . . . take them home or to your shop or to someone who can open them for you . . . and leave the property from which you collected the geodes in just as good or better condition than it was when you arrived.

If you have additional information about geodes, unique or interesting ways to open them, or information on geode collecting sites, please leave a comment to this post.


geodes: what are they?

Geodes are natural inorganic objects which are hollow. Geodes are usually roughly spherical in shape and can occur in igneous or sedimentary rock. The interior may be lined with crystals, usually quartz, pointing toward the center. Quartz consists esentially of the elements silicon and oxygen, but exhibits many varieties of color and form. Geodes are sometimes lined with chalcedony, a variety of quartz and, on occasion, closely related opal.

The hollow interior of the geode is it's most characteristic feature. If the interior of a formation is completely filled in, it is then classified as a nodule, not a geode. Nodules can be composed of a number of minerals, such as agate, quartz or calcite.

Geodes usually occupy the sites of former gas cavities in the volcanic rocks, basalt, rhyollite and tuff. Geodes can also occur in various other environments, such as sedimentary rock, which occurs as shale formed by ancient oceans. The interiors of geodes are very different from their exteriors and until a geode is opened, there is no easy way to determine what is inside.

If you have additional information about geodes, unique or interesting ways to open them, or information on geode collecting sites, please leave a comment to this post.