Eurypterids - Specimen of the Month
Samuel J. Ciurca, Jr., Rochester, New York           by Eurypterids.net
APRIL 2010
Honeoye Falls Dolostone
Honeoye Falls, New York - a slight diversion
Honeoye Falls is a small village in upstate New York, south of Rochester. I
have studied the rocks in the area since the 1960s when I discovered the
"Honeoye Falls Dolostone." The area was previously mapped as mostly Bertie
waterlime, a Late Silurian deposit, roughly 400 million years old. However, I
discovered the eurypterid,
Erieopterus, in the banks of the creek and this led
me to assume that the age of the strata was Early Devonian and that the rocks
were stratigraphically above the Bertie Group of the region.
Honeoye Falls Dolostone is unique to the region. While it was
quarried locally to construct several buildings within the village, I
find it amazing that the rock has lasted nearly 200 years within
buildings like the Mendon Town Hall. West of the Genesee River,
the Honeoye Falls Dolostone does not occur due to unconformity  
below the Onondaga Limestone (the very resistant rock that forms
the upper falls in the village). It is wonderful that these structures
are still with us (the lower mill structure, another beautiful building
within the village, is just to the north near where Honeoye Creek
crosses NY 65.
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ABOVE: The Mendon Town
Hall in the village of Honeoye
Falls (NY 65). The building is
made of Honeoye Falls Dolo-
stone, essentially a magnesian
limestone, a very fine-grained
rock often displaying  conchoi-
dal fracture. Lines of deposition
are readily seen in these phot-
ographs taken on April 15, 2010.
Originally, the structure was a
mill (see below).
Prehistoric Honeoye Falls
Prehistoric
Mendon
LEFT: Historical marker (plaque) near
entrance to the Mendon Town Hall,
photographed April of 2010. Note the
laminated rock, Honeoye Falls Dolostone,
surrounding the plaque.

BELOW: Closer view of a single block of
the dolostone showing cryptalgal structure
formed from cyanobacterial activity in
very shallow waters that occupied the
region almost 400 million years ago.
A Prehistoric Creature ...
LEFT: The aquatic eurypterid, Erieopterus microphthalmus, collected from
the Olney Limestone near Syracuse, New York. It is surrounded by small
brachiopods (
Howellella) characteristic of fossils found in the Manlius lime-
stones of central New York.
Erieopterus is the only known fossil occurring in the Honeoye Falls Dolo-
stone - the brachiopods have never been seen. Evidently, eastward, lime-
stone deposition occurred with an abundant marine fauna and westward,
shallow water deposition occurred with sediments being dolomitized within
a regimen of algal mat/stromatolitic sedimentation.
Erieopterus grew to at least one foot in length, but may have reached even
double this length. The best collection of the fossil remains of the animal is
located in the Peabody Museum of Natural History at Yale University, includ-
ing specimens I retrieved from Honeoye Creek and nearby localities in
years past.
The thick beds that overly the Honeoye Falls Formation are the cherty beds
of the Onondaga (limestone) Formation formed in warm tropical seas with
an abundance of marine life including small reefs in western New York.
ABOVE & RIGHT: Nearly complete section of the Honeoye Falls Forma-
tion exposed along Ulrich Lane. Note massive beds of dolostone. At the
top is cherty Onondaga Limestone.