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Earth Science Conservation Review
| Orlock Bridge - Structural | Down |
| Site number: | 652 | ||
| Locality Type: | Foreshore, Inland exposure | Status: | ASSI |
| Grid Reference: | J565829 Centroid | ||
| Google maps: | 54.67032,-5.57413 | ||
| County: | Down | Council area: | Ards & North Down Borough Council |
| Period: | Silurian, Ordovician | ||
| Site Description | |
|---|---|
| Introduction: | |
Orlock Bridge is located on the main County Down coast road (A2) 6.4km east of Bangor. The principal areas of geological interest lie upstream from Orlock Bridge and on the adjacent seashore (Fig. 21). Geological interest centres on the Orlock Bridge Fault which runs NE-SW through the site and can be traced for a distance of 2km offshore on the Copeland Islands. The Orlock Bridge Fault is the major bounding structure, which separates the Gilnahirk Group (Ordovician), and the Strangford Group (Silurian). | |
| Description: | |
The strata exposed in the vicinity of Orlock Bridge are Lower Palaeozoic turbidite facies rocks that range in age from Ordovician to Silurian. The topographic expression of the Orlock Bridge Fault lies upstream from Orlock Bridge in a small wooded valley containing a reservoir. Downstream, on the seashore about 200m south of Orlock Bridge there is a 100m wide exposure of a brecciated zone related to the fault. Along the shore, south of the Orlock Bridge Fault, strata are characteristically sandy turbidites and shales (080 35S) with sole markings and cross-laminations. These sedimentary structures indicate that the succession is inverted. Rocks south of the Orlock Bridge Fault have only a weakly developed cleavage. This contrasts with turbidites and slates north of the fault which are noticeably more tectonized. An intense and penetrative cleavage has formed in the more argillaceous lithologies of these strata and a steeply plunging stretching lineation with two later crenulation cleavages has been recorded on the steeply southwards dipping slaty cleavage. The principle folded structures at Orlock Bridge relate to the primary phase of deformation (D1). They formed as a result of a NW-directed subduction of the Iapetus Oceanic crust which led to accretion and deformation of marine sediment against the Laurentian continental margin. Subsequent phases of deformation were probably related to the final closure of the Iapetus Ocean and uplift of the accreted rocks. | |
| Notes: | |
For general information on the Structural Geology of Northern Ireland, directions to other Structural Geology sites and an extensive reference list see Key Site 641 - Introduction to the Structural Geology of N. Ireland. The full range of ESCR site relating to the Lower Palaeozoic are as follows: Outer Ards Stratigraphic, Palaeontological and Structural sites Coalpit Bay – Lower Palaeozoic (record no. 1350) Helen’s Bay – Grey Point – Lower Palaeozoic (record no. 1351) Kearney Point – Knockinelder Bay – Lower Palaeozoic (record no. 1352) Orlock Bridge – Lower Palaeozoic (record no. 1353) Tieveshilly – Carrstown – Lower Palaeozoic (record no. 1354) Orlock Bridge - Structural (record no. 652) Whiskin Rocks - Structural (record no. 653) White House Port, Cloghy - Structural (record no. 654) Knockinelder Bay and Kearney Point - Structural (record no. 655) Millin Bay - Structural (record no. 656) Pomeroy area Craigbardahessiagh and Bardahessiagh River – Palaeontology (record no. 29) Lime Hill Farm – Palaeontology (record no. 33) Little River and Slate Quarry Bridge – Palaeontology (record no. 31) | |
| Keywords | |
|---|---|
| Rocks: | Shale, Turbidite, Slate |
| Structures: | cross-bedding, fault, cleavage, sole marks |
| Relations: | Marine sediments |
| Record type: | ESCR report | Recorded by: | T.P. Johnston |
| Entered by: | E M Porter | ||
| Last updated: | 23/09/03 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Orlock Bridge - Structural, County Down, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=652. Accessed on 2026-11-08. |
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