The middle Devonian Old Red Sandstone, a thick sequence of chiefly
nonmarine sediments in Great Britain and northwestern Europe (Fig. 104), is the
main expression of a mountain building episode called the Caledonian orogeny.
The formation comprises great masses of sand and mud that accumulated in the
basins between the ranges of the Caledonian Mountains from Great Britain to
Scandinavia.The sediments are poorly sorted (dissimilar in size) and consist of
red, green, and gray sandstones and gray shales that often contain fish fossils.
Erosion leveled the continents and shallow seas flowed inland, flooding
more than half the landmass. The inland seas and wide continental margins
along with a stable environment provided favorable conditions for marine life
to flourish and proliferate throughout the world. Seas flooding North Amer-
ica during the Devonian produced abundant coral reefs that lithified (became
rock) into widespread limestones (Fig. 105).
The rising Acadian Mountains on the east side of the inland sea eroded
away. Their sediments produced flat-lying, fossiliferous deposits of shale in
western New York State, possibly the best Devonian section in the world.The
vast Chattanooga Shale Formation, which covers virtually the entire conti-
nental interior, was laid down during the Devonian and Carboniferous. The
seas also blanketed much of Eurasia in the late Devonian. Terrestrial clastics
containing rock fragments eroded from the Caledonian Mountains overlay the
western part of the continent.Figure 103Location of
the ancient Acadian
Mountains in North
America.
Historical Geology
Grenville orogeny
Acadian Mountains