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Onny Valley

The Onny Valley is an important site because of its historical significance in the development of the Silurian and Ordovician systems. This is highlighted by a teaching trail that runs along the valley. It is also the reason this is an SSSI (click here to see the reasons for notification).

Even for those not so interested in geology, this trail along permissive paths makes a nice walk.  It runs along the line of the old Bishops Castle Railway that once ran west from Craven Arms. 

View of Onny Valley

The bridges provide an opportunity to see some of the characteristics of the local rock. Chatwall Sandstone is used in the bridges and the characteristic purple banding of this rock can be seen.

Bridge in the Onny Valley

The geological trail along the valley passes exposures of units within the Caradoc Series of the Ordovician system . These sites straddle the important Church Stretton fault and the effect of the fault on the rock can be seen.

The sites of the Onny Valley trail show each of the rock types in the Caradoc series of the Ordovician System. These start with the unconformity between the Precambrian and the Hoar Edge Grit member of the Ordovician.

Photo of the Precambrian – Ordovician unconformity in the Onny Valley.

Precambrian Silurian contact in the Onny Valley

The beds at this point (to the west of the Church Stretton fault are almost vertical. This can be seen well in the quarried faces in the Hoar Edge Grit to the left of the above unconformity.

The photo shows almost vertical beds of Hoar Edge Grit with cross-bedding picked out by the effects of weathering on the thin layers of slightly different resistance.

 

Precambrian Silurian contact in the Onny Valley

The trail passes sites which show the other rock types until it reaches the top of the Ordovician and the unconformity with the Silurian.

The very slight change in dip between the Ordovician and Silurian rocks make this unconformity very difficult to spot!

Precambrian Silurian contact in the Onny Valley

This is the view upstream from the Silurian – Ordovician unconformity.

Precambrian Silurian contact in the Onny Valley