Ohio Collector's Series       v.2   art.5
Septarian Minerals of the Huron River near Lamereaux Road Bridge

Monroeville, Huron Co., Ohio

by Joseph W. Vasichko

    Some of the more interesting, non-quarry localities in the state of Ohio are outcrops of the Huron member of the Devonian shale that produce large, mineral-bearing, limestone septaria.  Mineral specimens from these septaria are unique and distinctly different from those available at most limestone and dolostone quarries of Western Ohio, although the assemblage is very similar.  While large septaria have been found at numerous outcrops, roadcuts, stream exposures and occasional quarries throughout the Devonian shale in Ohio, the most familiar of these localities to collectors are Copperas Mountain on Paint Creek in Ross County, the Olentangy River in Delaware and Franklin Counties and the Huron River in Huron and Erie Counties.  This is most likely due to the fact that these localities are located along larger streams and are therefore in a constant state of change.  These localities tend to expose more septaria each year, while roadcuts and outcrops along smaller streams very rarely produce any new septaria.  Also, outcrops of the Huron shale above the limestone and dolostone in the few active quarries where it is exposed are typically off limits to collectors.  As there are several known localities along the Huron River, this article will discuss only the mineral habits noted in septaria from a series of shale exposures along the West branch within a short walking distance of the Lamereaux road bridge, North of the town of Monroeville in Huron County, Ohio. 

smoky quartz crystals on ferroan dolomite
(Specimen John D. Vasichko)
largest crystal is 1.9 cm
West branch of the Huron River at Lamereaux road bridge
Monroeville, Huron Co., Ohio
    In the late 1700’s a portion of land in Northern Ohio, appropriately named “The Firelands”,  was granted to the civilians of Connecticut whose lands were spoiled and properties burned by the British during the Revolutionary War.  However, because the Indians who actually owned the land were not completely paid until 1808, most who suffered loss were no longer alive to settle in this area. Many others had sold their claims to land speculators.   When the first settlers to the area did arrive in 1809, this region became Huron County, as organized by the State of Ohio.  The name Huron is believed to be a derogatory term meaning “rough”, from the French word hure, describing the Wyandotte inhabitants who hunted, fished and lived in this same area.  In 1840, this region was reorganized into Erie and Huron counties and has remained unchanged since that time.  Because of this, descriptions of unnamed localities in Huron County from the early 1800’s may describe sites in present day Erie County.   The Huron River was of great importance to the canal system as it fed the Milan Canal, established in 1839.   This canal, which ran from Milan to lake Erie, allowed the town of Milan to become a major port for the export of wheat.   Along the banks of the scenic, winding Huron River are numerous high shale banks which produce large limestone septaria.  Although the first thorough geological description of the occurrence would not be made until 1874, it is unlikely that these septaria would have gone unnoticed by the earliest settlers and travelers to the area, especially one as large as the Huron River Boulder, as photographed by Jesse Earl Hyde. 

huge septaria known as the "Huron River Boulder".  C.1910 
The Jesse Earl Hyde Collection, Case Western Reserve University 
(CWRU) Department of Geological Sciences


Cliff exposing the late Devonian Huron Shale
East side of the West branch of the Huron River
South side of Lamereaux road bridge
Photo 2004 
    The North-flowing Huron River drains Northern Richland County, Northeastern Crawford and Seneca Counties, and a large percentage of both Huron and Erie Counties.   The town of Monroeville in Northern Huron County is located approximately 2/3 down stream from the headwaters of the West Branch of the Huron River to its mouth at the town of Huron on Lake Erie in Erie County.   At the Lamereaux road bridge locality the river is generally shallow with occasional deep pools.    A series of high steep cliffs expose portions of the middle and upper units of the late Devonian age Huron Shale.  The upper unit, which is exposed in the top 6-8 meters of these cliffs, consists of thinly bedded black shale that weathers light tan to orange red after exposure.  The middle unit, consisting of alternating layers of black and dark gray shale which weathers light gray, extends from the base of the creek to approximately 9-10 meters up the face of these cliffs.  The contact between the two units is clearly visible in the photo to the left.   There are two zones which produce large septaria at the Lamereaux road locality.  The upper zone exists at the contact between the middle and upper units.  Septaria from this bed range between 1 and 3 meters across and have large mineralized veins and pockets.   In the center of the photo to the left, one of the large septaria is still in place, partially obscured by a group of bushes.  The second zone of septaria is found in the floor of the river.  Septaria in this zone are more abundant than in the upper zone, but rounder and smaller in size.  Also, septaria from this lower zone do not generally have well developed veins and pockets and therefore rarely produce the quality of mineral specimens found in septaria from the upper zone.
    The large septaria throughout the Huron Shale are theorized to have formed around arthrodire remains or fragments of petrified wood, as both have been found in the centers of concretions at some localities throughout the state.  However, this has not been reported nor observed at the Lamereaux road bridge locality.  Expanding through the shale, a network of shrinkage cracks developed throughout the septaria, allowing for later mineralization.  The septaria are chiefly composed of fine-grained limestone, dolomite, clay and quartz.  The vein minerals which typically fill the shrinkage cracks of these septaria include crystalline aggregates of apatite, aragonite, barite, calcite, dolomite, ferroan dolomite, galena, marcasite, pyrite, quartz, sphalerite and whewellite.   Isolated pockets occur in the veins and yield fantastic specimens of  complex rhombohedral ferroan dolomite crystals, occasionally in combination with 

broken septaria revealing a mineralized pocket
West branch of the Huron River
at Lamereaux road bridge
Monroeville, Huron Co., Ohio
crystals of other vein minerals especially barite, calcite, dolomite and quartz.  Because the carbonate minerals quickly decay when exposed to the elements of the river, the best mineral specimens are found in fresh unbroken pockets inside the septaria.  In the photo to the left a portion of a split septaria has been spread apart showing the formation of ferroan dolomite pocket with quartz crystals.  Aside from the septarian minerals, crystalline concretions of pyrite, similar to those from the more famed Ross County localities, occur throughout the shale.  Efflorescences of halotrichite-pickeringite, melanterite and gypsum are also commonly observed on pyrite and shale.

calcite and multiple generations of ferroan dolomite
(specimen Joseph W. Vasichko)
specimen measures 8.2 cm x 5.4 cm
West branch of the Huron River at Lamereaux road bridge
Monroeville, Huron Co., Ohio
Dolomite var. Ferroan Dolomite Fe, CaMg(CO3)2
      One of the most under appreciated species found in the state is dolomite and its companion ferroan dolomite.  At many localities, collectors will walk away from outstanding specimens of the species, believing them to be of no value.  However, collectors need to realize that the acquisition of Ohio minerals is not about an investment, but an appreciation for the unique habits of a species.  Ferroan dolomite is not a separate species but a varietal name for iron rich dolomite.  Up to 1/3 of the magnesium atoms in the mineral can be replaced with iron and still be considered dolomite.  Since this variety is more common than iron free dolomite at this locality, it has received precedence in this article.  The Lamereaux road bridge locality is one of the best places in the state to study ferroan dolomite crystal forms and habits.  Simple to complex rhombohedral crystals of ferroan dolomite are abundant in septaria at the Lamereaux road bridge locality and numerous habits of the species can be observed within a single septaria.  Furthermore, ferroan dolomite habits in various septaria are often quite dissimilar.  Crystals and masses of ferroan dolomite can be translucent, internally zoned, or opaque and vary in color from black to dark brown, brown, tan, gray, yellow, orange and red.  Crystals may be vitreous or dull and occasionally iridescent as in the photo to the right.   The most common crystals are typically tan, brown or dark brown and lusterless.  Individual crystals range in size, but are rarely larger than 2 cm.   Most of the crystals found at the locality are crude and have been corroded or stained by exposure to the elements of the river. 

iridescent ferroan dolomite crystals
(specimen Joseph W. Vasichko)
largest crystal measures 9 mm
West branch of the Huron River at Lamereaux road bridge
Monroeville, Huron Co., Ohio
Dolomite var. Ferroan Dolomite Development
      Six generations of dolomite and ferroan dolomite are known to occur at this locality.  However, specimens generally do not display all six of these generations.   For instance, a specimen may have been mineralized by only the 2nd, 3rd and 5th generations.   And of those, perhaps only the 5th generation has remaining crystals.  While not common, the most unique and interesting specimens are those that show crystals of multiple generations and forms.  The following general statements can be made about the various generations of ferroan dolomite in septaria at the locality.  The 1st generation is often massive, light tan and rarely has any pockets with crystals.  The 2nd generation is medium brownish tan and often forms a boxwork or honeycomb structure of very small pockets with the occasional larger openings for better crystals.  The 3rd generation is usually thin and very dark brown to black in color.  The 4th generation is comprised of crude crystals and varies in color from tan to white, yellow or even red.  The 5th generation is typically yellow, orange, red or brown and is often made up of larger curved crystals.  The infrequently seen 6th generation is thinner and generally made up of minute yellow to light brown crystals.   It should be noted that the color and form of ferroan dolomite can be different in any generation, and atypical dolomite and ferroan dolomite development does regularly occur.   In general, the number and record of generations is only visible on the broken edges of specimens as in the photo to the right.   In this photo the gray area at the bottom of the specimen is the limestone matrix.  Layered above are the first five generations of ferroan dolomite. 

5 sequential generations of ferroan dolomite growth
West branch of the Huron River at Lamereaux road bridge
Monroeville, Huron Co., Ohio

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