Niagara Gorge

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					                                    Niagara Gorge
                                   Geologic History
              Summary of Book “Nature’s Niagara” by Paul Gromoside
             With additions from “Niagara Glen Nature Reserve” trail guide
               And “Geology of Niagara Falls” by Professor N.S. Shaler

Niagara Falls is of the type “where horizontal stratified rocks arranged in alternating beds
of hard and soft character are flowed over by a considerable stream. The bed of the river
lies on one of the hard resistant layers.” (Shaler) Below the resistant layer lies a soft
shale (mudstone) that is easily eroded. When the churning water erodes the underlying
shale the resistant layer (dolostone) is left without its support and fractures and then falls
below (Figure 1).

Figure 1: (Cover of Roadside Geology of NY). A rendition of the Niagara River flowing
over the resistant Lockport Dolostone, the churning waters easily erode the soft
underlying shale.




There a couple of stories here. One is the massive erosive power of water that produced
the gorge and the movement of the falls through time. The second is the geologic history
that the river exposed through cutting the gorge. Rocks in the gorge range from the
Ordovician Shale (500-435 mya) to Silurian shales, sandstones, limestones, and
dolostones (435-395 mya) (Figure 2).

The rocks laid down during this time were in an environment dramatically different from
today. North America at this time was south of the equator and the Earth was warmer.
Shallow seas inundated the continent and then regressed, and repeated this process
throughout as the rocks in the gorge were deposited. Throughout the rocks of the gorge
one can find numerous ocean fossils.
Figure 2: (Gromoside, p. 37) Rock Layers in the Niagara Gorge, The Queenston Shale is
the oldest unit to the youngest resistant Lockport Dolostone.




The following description of rock units comes from Gromoside:
(see attached for pictures of fossils)
Queenston Shale: Deposited on a coastal plain, iron-rich muds and silts exposed to the
atmosphere give this unit its characteristic red color.

Whirlpool Sandstone: Shallow sea, ripple and waves marks, ostracodes, trilobites, and
lingulid brachiopods.

Power Glen Shale: Gray-Greenish in color, few fossils poorly preserved (deposited on the
shores of an inland sea).

Grimsby Sandstone: Mottled appearance, pale green/yellow surrounded by red. Delta
and mudflats, brachiopods, pelcypods, worm burrows.

Thorold Sandstone: White to buff in color. Brachiopods, beach of ancient sea.

Neahga Shale: Green to olive-green (few fossils), deposits from a shallow stagnant sea.

Reynales Limestone: Dark grey to grey-blue in color. Contains pyrite (“fool’s gold). A
lot of fossils.

Irondequoit Limestone: Light grey – dark grey – white and tan (weathered). Invertebrate
fossils (brachiopods, crinoids, bryozoans, and corals). Shallow sea.
Rochester Shale: dark brownish grey (lower) and dark bluish grey (upper). Many fossils.
(somewhat deeper sea environment).

Lockport Dolostone: Calcium-magnesium Carbonate, resistant rock that forms the
Niagara escarpment.

Part II.

The last ice-age carved out the great lakes from former river valleys. After the retreat of
the ice the Niagara connected Lake Erie to Lake Ontario (whose sizes have varied). The
falls retreated back creating the gorge in only the last 12,000 years. A number of stages
describe its development (Figure 3).

Figure 3. Gorge Formation and Retreat of the Falls. (Gromoside)
Figure 4: Map of the Niagara Region
Pictures of Fossils

Ostrocod




Trilobite




Crinoid
Brachiopod




Bryozoan




Pelecypod

				
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posted:12/5/2011
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