Category Archives: GeoPic of the Week

Geo-pic of the week: Aerial Photography

Aerial Photo-Central Arkansas

Sometimes the best way to see what’s on the ground is to get as far away from it as possible.  Geologists use a variety of tools to do just that.  One of those tools is aerial photography.

The picture above shows the Arkansas River where it leaves the mountainous western uplands and enters the bottomlands of the Mississippi Alluvial Plain, east of Little Rock (upper left).  Driving east across that boundary, it’s easy to get the impression that the Mississippi Alluvial Plain is a broad flat expanse of land with little to no distinguishing features.  That’s far from the case as this photo reveals.  

From high-altitude imagery, subtle relic features created by the Arkansas River can be easily recognized.  Note the swirling landforms that characterize the lowlands on the right side of the picture.  Over time the Arkansas River has meandered through the valley carving new channel courses and abandoning old reaches of channel.  The continually changing river has left a mosaic of oxbow lakes (water-filled abandoned channels) and arc-shaped river deposits known as point bars. 

Geo-pic of the week: Cleavage

clip_image002Calcite at atomic resolution

The mineral in the above pictures is calcite, a common mineral in earth’s crust that is the main component of the sedimentary rock limestone.  The stack of samples (top) exhibit a physical characteristic known as cleavage.  The cleavage of calcite causes it to break into a rhombus-shape (see picture).

Cleavage is the tendency of a crystalline substance, such as a mineral, to break along parallel planes that reflect the internal arrangement of the atoms in the crystal.  All crystals, by definition, have a uniform atomic arrangement.  To illustrate this property, I’ve included a second picture (bottom), borrowed from Dr. Cathy Sutton, that shows an extremely magnified calcite crystal.  The repeating rhombus-shapes in the picture are individual calcite molecules.  Basically, cleavage is the outward expression of the internal structure of a mineral.

The samples on the left were collected from Midwest Lime Quarry, Batesville, Arkansas.

Geo-pic of the week: Trilobite Romance

trilobite meeting in pennsylvanian

The photo above shows trace fossils that record the travels of two trilobites.  Trilobites are an extinct group of marine invertebrate animals, resembling horse-shoe crabs, that flourished for 100s of millions of years in the Paleozoic Era (540-250 mya).   The tracks the animal left are known as the trace fossil, Cruziana.  It appears that one traveled from the right side of the photo, the other from the left, until they met in the middle where they rested for a while.  At the center of the photo are resting traces known as Rusophycus.  Perhaps they became friends or maybe they were even more than friends?  It is Valentine’s Day.  Their traces are preserved in the Atoka Formation of west-central Arkansas.  

Geo-pic of the week: Fracture-Fill at Shine-Eye

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The photo above shows a vertical dark rock in the center of flat-lying white rock. The dark rock is a sandstone deposit, probably Mississippian-aged, and the white rock is Silurian-aged limestone. If one were to follow the sandstone dike upward, it would lead to a sandstone bed sitting on top of the limestone. Since the limestone was deposited first, we can infer that it was exposed to weathering. The limestone was solutioned and deep fractures or cracks formed. Afterwards, sand was deposited in the area, filled the fractures in the limestone, and eventually lithified into sandstone. There are several of these sandstone-filled fractures present along the Buffalo National River in Silurian-aged limestone. The one pictured above is located at Shine-Eye.

Geo-pic of the week: Sand and clay of the Wilcox Group

wilcox sand canyon pit enhanced

This rather handsome outcrop of the Wilcox group consists of alternating layers of sand and clay of the Eocene Epoch which lasted from about 56-34 million years ago.   The Wilcox Group is a non-marine unit mostly composed of sand with lesser clay, silt, gravel, and lignite (low-grade coal).  

This geologic unit is part of a larger sequence of loosely-consolidated sedimentary rocks exposed in south central Arkansas, south of Pulaski county.  These rocks are the northern extent of the West Gulf Coastal Plain, a physiographic province that stretches from central Arkansas, south, to the Gulf of Mexico.

Geo-pic of the week: Pegmatite from Magnet Cove

Agerine from Magnet Cove, AR

In the picture above, large black rectangular aegerine crystals are prominent in a rock type known as a pegmatite.  Pegmatites are igneous rocks characterized by extremely large crystals.  Sometimes they also contain unusual mineral species.   This sample was collected from Magnet Cove, Arkansas.  Magnet Cove, which is approximately 10 miles east of Hot Springs, is one of the few places in Arkansas where igneous rock is exposed at the surface. 

Between 84 and 100 million years ago, magma was injected into the earth’s crust under central Arkansas where it slowly cooled and crystallized into igneous rock.  Millions of years of erosion eventually unearthed that rock.  Despite only being exposed over approximately 5 square miles, the rocks of Magnet Cove have yielded more than 100 different minerals.  Rare minerals have been discovered there including a new variety of zirconium-rich garnet called Kimzeyite.

Geo-pic of the week: Phantom Quartz

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Phantom quartz refers to quartz crystals that show the outline of smaller quartz crystals inside of them.  They are thought to form when there are at least two periods of growth.  The phantom crystal is visible either because it differs slightly in composition or because it was coated with some material prior to the second round of crystal growth. 

In the crystals above, all of which were collected near Mt. Ida, Arkansas, partial dissolution of the phantom crystals prior to the second round of growth left the phantoms with a ragged appearance.