Tag Archives: Ordovician

Geopic of the week: Breccia

Dolostone breccia (1) corrected

A Breccia is a rock made up of angular rock fragments mixed with finer sediment.  The one above was deposited about 450 million years ago (Ordovician) in a shallow sea in what is now the Ozark highlands, north of Mountain View, Arkansas.

Breccias can form by a variety of processes such as landslides, volcanic eruptions, storm events, cave and sinkhole collapses and others.  This one likely formed during a storm.  Turbulent ocean waves and currents washed fragments of marine organisms (dark gray) into calmer water where marine mud (light gray) had recently accumulated.  The turbulence ripped some of the mud up and mixed it with the organic material.

Breccias are just one of many clues, recorded in rocks, that help geologists understand how the earth has changed through time: a story that itself constantly evolves as new rock is unearthed and studied.

For more views of breccias click here

Geopic of the week: “Quicksand Structures”


Sandstone pipe1

Pictured above is approximately 450 million year old (Ordovician) St. Peter Sandstone photographed near the White River, north of Mountain View, Arkansas.  At the center of the picture is a feature not uncommon in this formation: a “quicksand structure”.  Though not well understood, this kind of structure is believed to be a preserved conduit through which ground water once flowed to the surface in a boiling spring. 

A boiling spring is where ground water rises up through unconsolidated sediment such as sand, keeping the sediment in continuous motion so that it appears to be boiling.  Because ground water contains dissolved minerals, the tunnel through which the water is rising can eventually become hardened by precipitated minerals.  Today, as the sandstone that formed from that sediment is being eroded away, the conduit stands in relief  because it’s a little harder than the surrounding sandstone due to that mineralization. 

Clues like these “quicksand structures” offer us a glimpse into what the environment was like when the rock was being deposited.    

Geopic of the week: Recumbent folds

City Quarry Fox Pass6

Tight recumbent folds in a fresh exposure of Bigfork Chert, Ouachita Mountains Arkansas.  Thin chert beds divided by siliceous shale beds are the basic lithology of this Ordovician age (540 to 490 Ma) formation.  Complicated structures like those pictured below are characteristic of the rocks that form the Ouachita Mountains.  The complex folds and faults resulted from plate tectonic forces that compressed the rock and caused many of the structures to rotate and overturn.

For more views of this and similar folds click here

GeoPic of the Week: Large Quartz Vug


This large quartz vug, or cavity, (approximately 3 ft wide x 3 ft high) was extracted from one of the Coleman Quartz Mines and brought to the Arkansas Geological Survey Learning Center in the 1980s. This vug was found in the Crystal Mountain Sandstone (Early Ordovician, 485-470 mya), a massive, coarse-grained, well rounded, light gray sandstone from Montgomery and western Garland Counties, Ouachita Mountains. These quartz crystals formed secondarily from silica-rich fluids that resided within this large cavity.

GeoPic Of The Week: Fenestrate Bryozoan In Limestone

Fenestrate bryozoan in limestone

Fenestrate bryozoan in limestone

Bryozoans are colonies of tiny individuals, approximately a millimeter or less, living from tide-level to abyssal depths in today’s oceans.  They have been abundant and diverse throughout geologic time since the Ordovician Period.  Bryozoan colonies vary in shape and can be stick-like, massive, or lacy and net-like.  Lacy bryozoans are also called fenestrate bryozoans.  The lacy bryozoan in the picture is approximately 2 inches (5 cm) long and is present in Pennsylvanian age rocks in northwest Arkansas.

GeoPic of the Week: Sandstone Pipe In Sandstone

Sandstone Pipe In Sandstone

Sandstone Pipe In Sandstone

Sandstone pipes are vertical cylindrical features that are commonly preserved in the St. Peter Sandstone in northern Arkansas.  They are made up of the same sand as the surrounding rock. These features were observed in Ordovician-aged sandstone in Arkansas by geologists as early as 1916.  Research by  other scientists showed that these pipes formed in sand that was slightly deformed by a column of water rising through it from a lower horizon and feeding a spring at the surface.  This sand then lithified into the rock we see today which includes the sandstone pipe.  A modern-day example of sandstone columns forming in springs is present in the Dismal River, in the Nebraska Sand Hills.  At this location, boiling (motion from water pressure, not temperature) sand springs have developed, fed by groundwater moving upward along cylindrical conduits.  In the picture above, the sandstone surrounding the pipe has eroded away leaving the sandstone pipe standing in relief.

GeoPic Of The Week: Ripple Marks In Sandstone

Ripple Marks In Sandstone

Ripple Marks In Sandstone

Ripple marks are sedimentary structures preserved in sandstone and limestone. They may be asymmetrical in shape, with the steep side pointing downstream in the direction of current flow.   In this picture the steep side is toward the viewer and so is the current direction.  Ripples form naturally by the movement of water currents in rivers and streams, on beaches of tidal and long-shore currents, and in deep-ocean basins.  This picture was taken of Ordovician age sandstone in the Everton Formation along Beaver Lake.