The Edge of the Edwards Plateau

The most striking geological feature near Abilene is the Callahan Divide. Often called mountains or hills, the correct terms are escarpment or bluffs.  The divide is over 100 miles long stretching across Taylor County.  It turns north in Callahan County to encompass Clyde and the steep hill on I20 at Baird is part of it.  The shear, crumbly cliffs make most of it very difficult to climb.  The pioneers called a passable valley a gap, thus the names Cedar Gap, Lemon Gap, and Buffalo Gap.

The hard caprock on top has hindered the erosion of the hills.  It is a gray limestone, an Early Cretaceous shallow ocean sediment called the Edwards Formation.  The cap rock is actually made of many layers.  At the top are 40 feet of blocky hard stone separated by layers of crumbly limestone.  Exposed, the blocks break down quickly into smaller and smaller blocks.  Small chunks of stone litter surrounding soils many miles from the escarpments.  These layers are made up of fragments of shells and other marine life.  Occasionally you can find a complete specimen but the hardness of this matrix (stone around fossils) tends to outlive the softer fossil material. (abundant fossils – Exogyra texana, foraminifers)

The lower layers of the Edwards Formation are composed of a crumbly limestone.  It erodes easily into rubble.  Nodular chert beds formed within this limestone, which was used by early man to make arrowheads.  (Box of chert flakes, arrowheads?)  Many kinds of invertebrate fossils erode easily from the matrix.  (abundant fossils – foraminifers, gastropods, rudistids as molds and silicified remnants, snails and urchins)  Vertebrates (fish) have been found but they are rare.

White Mines gravel tops the dirt roads and alleys around Abilene.  It is from the lower limestone layer and has been misnamed as caliche.  Caliche crystals are formed as calcite evaporates out of a soil in an arid climate.  It covers the surface and can become quite thick.  There are caliche beds around Abilene.  They are not very thick or used for road material locally.  Our caliche beds are Pleistocene to recent in age (the age of the mammoth to now).  (Block of caliche)

At the base of the Callahan Divide is a thick layer (almost 200 feet) of Early Cretaceous sands, called Antlers Sand.  These layers were deposited at the same time as the dinosaur footprints at Glen Rose. (Need a geologist for more info.  Fossils,?  Why sand: desert, beach?  Why didn’t it turn into stone?).  Pink, brown, and gray sandstones interbed with brown to purpleish mudstones, sand, and conglomerates.  (Sandstone ball and organic forms, mudstone, conglomerate)  Sandstones are composed of sand that has concreted together.  The sand can come from several sources: beaches, sandbars (river or ocean), and deserts.  Ours is most often freshwater deposits.  Layered in thin horizontal bands a geologist can determine the environment that produced the sedimentation.  (Mudstone slab showing interrupted layers)  Animals often dig into soft mud.  These tunnels can be found filled with sediment.  (Sandstone slab of burrows)

Beneath the Early Cretaceous layers are Early Permian layers.  About 150 million years of sedimentation is missing.  That is almost as long as the entire existence of the dinosaurs!  It is not known what removed all those layers.  Non-deposition and erosion are prime suspects. 

The red clay hills surrounding the Big Country and reaching all the way through Oklahoma represent only the first third of the Permian.  Still, this is about 4500 feet of the most complete and uninterrupted Early Permian layers on Earth!

The geologic name for the redbeds near Abilene is the Undivided Clear Fork Formation.  It is undivided because determining the boundaries within it is often impossible.  Layers are made by changes in the environment.  Formations are drastic changes.  Different kinds of stone and the fossils they preserve allow geologists to recognize layers.  Near Abilene, only the occasional Permian pond, river, and mudflat contain fossils.  Earlier Permian formations of vast swamp lands, near Seymour Texas, are abundant with fossils.

Most of our redbeds represents arid continental flood plains crossed and dotted by meandering rivers, streams, and lakes.  Flooding brought fine grained sediments inland.  The mud became thin layers of shale, siltstone, and mudstone.  (Clay is considered shale.)  Sediments in fast flowing rivers have larger grains.  Occasionally one finds blocks of conglomerates, pebbles of all kinds cemented together with a natural limy mortar. (Samples of red shales)  Ribbons of light blue-gray (green if wet) layers were slow moving river courses within swamps.  Their color is due to fresh water leaching the iron (red) out of the soil. Occasionally sheets of red shales are found with green patches.  (Samples of green shales) 

Traveling I-20 east, the layers become older.  Between Putnam and Weatherford, they belong in the Pennsylvanian Period, which contains many formations.  This Period can also be found at Brownwood.  Both are rich in shallow marine invertebrates.  At Thurber, a coal layer can be seen from the highway as a dark band in the hillside.  Before the widespread use of electricity, this coal was mined for fuel.  It is evidence of a rich swamp area.

Just before arriving in Weatherford, the Cretaceous once more overlays the old layers.

Pretty Polished Gravel

Above all the Mesozoic and Paleozoic layers near Abilene is a thin to 25 foot layer of Pleistocene (the age of the Mammoth).  It comes in the form of polished gravel and is concentrated near rivers.  (Container of Pleistocene stones) In the last severe ice age, the ice sheet covered half of the North American continent.  It picked up rubble from the mountains of Colorado and New Mexico, polishing them in the glacial rivers.  When the ice finally melted, debris covered most of our state. Thick deposits of Pleistocene have produced fossilized mammoth.

The materials are very hard and resilient; any soft material would have been destroyed in the polishing process. (Individual Pleistocene stones)

1. Quartz crystals, which grow within molten granites.

2. Quartzite, a metamorphosed quartz sandstone (translucent whites and yellows).

3. Agates and jaspers (opaque quartz in red, yellow, and brown).  Common are black pebbles with white stripes of quartz that grew in old cracks. 

4. Metamorphic gneiss, usually a light stone with bands of color.

Permian Basin

Geological forces have bowed the Permian layers of Texas upward into a basin shape.  The edges are exposed and can be seen in the stone outcroppings at Baird, Cisco, and Ranger.

What happened to the Jurassic layers of Texas? 

Although North America has some of the best Jurassic fossil bearing layers to be found, Texas is sadly lacking.  Only a couple of outcroppings have been identified, none of which bear fossils.  Why?  First of all, Texas was dry during the Jurassic, which does not lend itself to sedimentation or fossilization.  Another suspect is erosion.  The normal process of water and wind over soil and rock washes it away.

What Animals Lived in the Jurassic?

The Saurischian (Apatosaurus, Diplodocus, and Allosaurus) dinosaurs had become well established throughout the world during the Triassic times.  The beginning of the Jurassic shows a diversification of the Ornithischian (Stegosaurus, Ankylosaurus).

While the Dinosaurs grew larger, mammals and birds stayed small.  Plants and insects, then as now, filled every niche, but there were no flowering plants and no grass.  The shallow oceans were teaming with life where the great marine reptiles ruled.

Kinds of Rocks

1. Clastic Sedimentary – from the accumulation of weathered particles. 

a. Shale – fine grained and layered, splits along bedding planes (red mud is a shale)

b. Mudstone – fine grained and layered, does not split on bedding planes

c. Siltstone – slightly gritty and layered, does not split on bedding planes

d. Silty shale – slightly gritty and layered, splits along bedding planes

e. Slaty shale – shale subjected to heat and pressure, harder and more brittle, but not enough to change it into slate

f. Sandstone – gritty grained and often layered, lens-shaped deposits from ponds, floodplains, beach sand

g. Shale concretions – nodule formed within a shale deposit that has a different composition or hardness, bedding planes are present

h. Volcanic Ash

i. Coal – compressed accumulation of plant and animal material

2. Non-clastic Sedimentary – dissolved minerals that are precipitated chemically

a. Limestone and Chalk – cemented by calcium carbonate (at least 50%), often layered in large blocks

b. Dolomite -- cemented by magnesium carbonate (at least 50%), often layered in large blocks, fossils are often dissolved leaving casts

c. Siliceous Limestone – limestone with much more silicon, much harder, splits along bedding planes, fossilization of delicate specimens and soft tissue

d. Chalk – very fine grained limestone, often uncemented

e. Chert – masses or nodules of silica from algae or protozoan

3. Non-clastic Sedimentary Evaporates – dissolved minerals that are precipitated by evaporation

a. Gypsum – the evaporation of salty lakes or bays in an arid environment, fine grained with large crystals

b. Salt -- the evaporation of salty lakes or bays in an arid environment

c. Caliche

4. Igneous – solidified directly from magma

a. Granite – magma solidified underground, course grained, large crystals, no banding

b. Pegmatite – magma solidified underground, very course grained, large uneven crystals, often form in dike zones, cavities filled with crystal drusy

c. Gabbro – magma solidified underground, course grained, some banding, easily eroded

d. Basalt and Obsidian – magma solidified above ground (from lava), sponge-like or glassy

5. Metamorphic – Sedimentary or Igneous rocks that have been changed by high pressures and temperatures

a. Marble – evenly course grained, uneven banding, from sedimentary rocks

b. Slate – compressed and heated mudstone and shale

c. Gneiss – elongated mineral crystals form bands of uneven alternating colors

d. Schist – elongated mineral crystals form flaky layers

Other local rocks

Hematite; rose rocks; petrified wood