GeoPic of the Week: Honeycomb Weathering In Sandstone

Honeycomb weathering in sandstone

This type of weathering produces pits of varying size on a rock surface.  The pits in the picture above are small approximately less than 1 inch up to 2 inches in diameter.  This type of weathering is also known as tafoni, a Sicilian word for window.  The dominant process that forms these features is probably chemical weathering.    Sandstones are made up of sand size grains of quartz and other minerals.  The grains are held together with a cement or glue such as quartz, calcite, or iron.  In most instances, there are patches where the cement is no longer present allowing the grains to fall apart.  Honeycomb weathering is abundant in the sandstones in the northern part of the state.  This picture was taken of the Wedington Sandstone Member of the Fayetteville Shale in northwestern Arkansas.

 

GeoPic Of The Week: 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.