The Bertie Waterlime - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 62

The Bertie
Waterlime

B ACKGROUND
At the end of the Ordovician Period
there was a major extinction event
which coincided with a worldwide
glaciation and, in New York State, with a
major erosional episode that produced
the Cherokee Unconformity at the
Ordovician-Silurian boundary. The
Silurian Period in New York begins,
therefore, with a marine transgression, as
the sea invaded the land following the
melting of the ice sheets and the wearing
down of the exposed land surface. The
Silurian is a relatively shorter period than
the Ordovician and, although most of the
time sea levels were fairly high, there was
a considerable variety of environments
which are represented today by different
rock types. The sea spread across New
York from the west, laying down
sandstones and shales derived from
erosion of the land to the east. By mid-
Silurian times, open sea covered most of
the state and shales and sandstones gave
way to increasing amounts of limestone.
Increased tectonic activity rejuvenated
mountains to the east which resulted in
the sea floor becoming smothered
with sand (in the east), shales (mid-state)
and deep-water muds in the west. As the
mountains wore down, so quiet-water
conditions again prevailed and the
later half of the Silurian Period in New
York is represented with rock sequences
predominantly carbonate in nature.

These include limestones, dolomitic
limestones (dolostones), and also beds of
evaporite (salt, gypsum, and anhydrite).
This last sequence of rocks is known as the
Salina Group.
Dolostones and evaporites are char-
acteristic of shallow-water, restricted
circulation basins: a landlocked sea with an
arid climate (New York was about 30° south
of the equator at this time). The widespread
evaporation caused the formation of
primary dolostone (magnesium carbonate-
rich limestone) and the growth of crystals of
halite (rock salt, NaCl), anhydrite (CaSO 4 )
and its hydrated form, gypsum (CaSO 4 .
2[H 2 O]).
The period of sedimentation of the
Salina Group of dolostones and evaporites
was brought to a close by the deposition of
the Camillus Shale: a thick accumulation
of mottled shales with halite and gypsum
crystals, an important source of these
minerals in western New York. But the
rock sequence we are most interested in
overlies the Camillus Shale. The highest
beds in the Silurian of New York are the
Bertie and Rondout groups. The Bertie
consists of a muddy dolostone having the
peculiar and useful characteristic of being
able to be turned into cement which will
harden under water; hence it was given
the name Waterlime. Arid, evaporating
conditions continued through the Bertie
Waterlime times, and some horizons in
the rocks of this group contain the