Earth Science Conservation Review
| Ballyharry - site of local interest | Down |
| Site Type: | Railway cutting |
| Site Status: | local interest |
| Council area: | Ards Borough Council |
| Grid Reference: | J501753,J501754 |
| Google maps: | 54.60405,-5.67714 |
| Rocks | |
|---|---|
| Rock Age: | Silurian, Ordovician (Ashgill, Caradoc, Lower Silurian) |
| Rock Name: | Strangford Group, Drumhirk Formation, Lessans Formation |
| Rock Type: | Mudstone, Sandstone, Turbidite |
| Interest | |
| Fossil Groups: | Graptolite |
Description:
The rocks exposed at the disused railway cutting at Ballyharry in northeast Newtownards form part of the Upper Ordovician Lessans Formation and the succeeding Lower Silurian Drumhirk Formation of the Strangford Group. Exposure of the Lessans Formation is restricted to isolated inliers which are usually fault-bounded and contain black graptolitic mudstones, grey barren mudstones and siltstones. The formation ranges in age from Llandeilo to Caradoc with the Ballyharry section exposing rocks towards the younger part of this range.
At Ballyharry the Lessans Formation consists of blue-black mudstones, some finely laminated and slaty, overlain by grey, blocky, silty mudstones. The dark mudstones are fossiliferous and during the Geological Survey's resurvey of the area during the 1970's the following graptolite assemblage was collected:
?Arachniograptus sp.
Climacograptid {with basal spines}
Climactograptus cf. tubuliferus Lapworth
Glyptograptus sp.
?Lasiograptus sp.
Leptograptus aff. macer Elles and Wood
Orthograptus amplexicaulis (Hall) cf. abbreviatus Elles and Wood
This fauna indicates an assemblage in the Pleurograptus linearis Zone that spans the Caradoc-Ashgill boundary.
The dark mudstones also contain a thin discontinuous buff coloured claystone probably representing a volcanic ash fall.
These argillaceous beds are succeeded, apparently conformably, by turbidite sandstones which contain mostly acid and intermediate volcanic and plutonic rock fragments together with subordinate basic volcanic and metasedimentary clasts. These turbidite sandstones are included in the Lessans Formation and appear to pass conformably up into the Drumhirk Formation, which is taken here as the base of the Strangford Group.
At Ballyharry the Lessans Formation consists of blue-black mudstones, some finely laminated and slaty, overlain by grey, blocky, silty mudstones. The dark mudstones are fossiliferous and during the Geological Survey's resurvey of the area during the 1970's the following graptolite assemblage was collected:
?Arachniograptus sp.
Climacograptid {with basal spines}
Climactograptus cf. tubuliferus Lapworth
Glyptograptus sp.
?Lasiograptus sp.
Leptograptus aff. macer Elles and Wood
Orthograptus amplexicaulis (Hall) cf. abbreviatus Elles and Wood
This fauna indicates an assemblage in the Pleurograptus linearis Zone that spans the Caradoc-Ashgill boundary.
The dark mudstones also contain a thin discontinuous buff coloured claystone probably representing a volcanic ash fall.
These argillaceous beds are succeeded, apparently conformably, by turbidite sandstones which contain mostly acid and intermediate volcanic and plutonic rock fragments together with subordinate basic volcanic and metasedimentary clasts. These turbidite sandstones are included in the Lessans Formation and appear to pass conformably up into the Drumhirk Formation, which is taken here as the base of the Strangford Group.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Ballyharry - site of local interest, County Down, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=702. Accessed on 2024-12-26 |
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