Anyone wishing to make a serious study of this area should start with "The Geology
of the Craven Arms area" by B.A.Haines, published by the 'Institute of Geological
Sciences' (now British Geological Survey, BGS) as a second edition in 1980.
Haines lists and describes 22 sites but many of these are not accessible, being
located on the north bank of the river, which is outside the boundary of the Cheney
Longville estate
Another useful guide is the GEOLOGISTS' ASSOCIATION GUIDE No. 45 "Onny Valley,
Shropshire Geology Teaching Trail" by Peter Toghill, published in 1992.
Toghill describes a more modest eight sites that can be viewed from the south
bank of the river.
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The exposures range from Hoar Edge Grit, the oldest rock of the Caradoc series in
location No.1, through Chatwall Sandstone (locations 2, 3 and 4). Alternata Limestone
(location No.5) named for its high content of the Brachiopod Heterorthis alternata
, Cheney Longville Flags (location No.6), members of the Acton Scott Group including
two beds of Bentonite or volcanic ash (location No.7) to the famous Ordovician/Silurian
unconformity (location No.8).
It was this unconformity in which Onny Shales, the highest beds of the Ordovician,
are overlain by Silurian Hughley Shales that redefined Murchison's Lower Silurian
as the Ordovician.
Toghill reports that Harnage Shales can be detected in the highest beds of the
quarry at location No.1 and in the bed of the river Onny at low water.
Sadly in Sepember 2006, fourteen years after the GA guide was published, some
of the sites were overgrown and could not be found. Neverless the trail is a
delightful walk and the exposures are well worth a visit.
Two general pictures of the trail are shown above and four pictures of exposures
are shown below:-
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