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Earth Science Conservation Review
| Bloody Bridge River Fan | Down |
| Site number: | 454 | ||
| ESCC: | IS | ||
| Locality Type: | Inland exposure | Status: | TBC |
| Grid Reference: | J383270,J384270,J387269 Centroid, Not Entered | ||
| Google maps: | 54.17382,-5.88206 | ||
| County: | Down | Council area: | Newry, Mourne & Down District Council |
| Period: | Quaternary | ||
| Stages: | Late Midlandian | ||
| Site Description | |
|---|---|
| Description: | |
Downstream of the most easterly ridge, a subhorizontal fan widens downvalley. The fan cuts across ridges located on the coastal tract lowlands which trend subparallel to the coast. The fan has a maximum width of 260m at the modern coastline and the Bloody Bridge river has incised into the fan producing a trench up to 100m wide. Downcutting has produced three exposures along the river trench. An exposure on the south bank 100m east of the limiting readvance moraine consists of diamict and crudely stratified gravel (J383270). A north bank exposure is situated 80m downstream and mainly consists of stratified gravel (J384270). An exposure 300m downstream at 25m a.s.l. consists of two lithologically distinct diamicts overlain by variable stratified gravel (J387269). Description of the sediments is confined to the sand and gravel components, and proceeds from west to east. Facies description Crudely stratified, poorly sorted, cobble/boulder gravel (<1m thick) has a planar erosional contact with underlying diamict. Beds of pebble to cobble gravel (<30cm thick), are clast to matrix dominated and have amalgamated contacts. Matrix is generally of coarse sand. Beds dip eastwards at 7-8°. Boulders (<1.4m) and cobbles frequently form chaotic clusters. The site is 150m downstream of the greywacke/granite junction. Clasts are almost entirely of granite. The contact of underlying bedrock with the north bank exposure, located 80m downstream is within 0.5m of the base of the exposure. The base of the exposure consists of a 1m thick bed of massive greywacke boulders and cobbles. Clast a-axes are frequently imbricate, with upstream dips. Pebbly and sandy inclusions fill the voids between the larger clasts. This bed is overlain by 2m of tabular, mostly horizontal, but occasionally cross-bedded, granule to pebble, clast to matrix supported gravel. Maximum bed thickness is 40cm, but most are <10cm. Bed contacts are conformable, non-erosional, and frequently welded. Clasts are well-sorted, with normal grading. Imbrication is frequent, fabric is mostly a(p), and clasts dip upstream. Greywacke (<20%) is more frequent than in the upstream exposure. Well-sorted gravels pinch out downstream, and are overlain by more crudely and variably stratified gravel in a channel (2m deep). Beds (<30cm thick) are laterally discontinuous, with erosional and non-erosional contacts, and exhibit both fining up and coarsening up. Most beds consist of pebble to cobble clasts, although a few beds consist of granule to pebble clasts. Beds are matrix to clast supported and clasts are <1.2m. Most boulders (80%) are of granite. The largest boulders occur mainly at the base of the channel, but some form a cluster with cobbles, 1.5m across by 1m deep near the top of the exposure. The most distal exposure occurs 420m east of the lowest cross-valley ridge and is up to 1m deep. Contact with the underlying diamict is gently undulating and erosional. The western end of the exposure consists of low angle (ca. 20°) cross-stratified coarse sand with occasional pebble beds (<10cm). There are occasional boulders along the contact with the underlying diamict. The sand and pebble beds wedge out in a downslope (easterly) direction, and are overlain at a non-erosional contact by poorly stratified cobble and boulder beds with amalgamated junctions. Log jam type cobble clusters are frequent, and are occasionally openwork. Clasts are often imbricate clasts, with both a(p) and a(t) fabrics, dip upstream. | |
| Interpretation: | |
The topographic location of the fan at the exit of the Bloody Bridge river valley indicates that it post-dates the deposits in this area of the valley and those on the coastal tract. The sediment supply is interpreted to originate from the morainic ridges which resulted from the main phase of ice retreat westwards along the valley. Textural variation in the fan sequences record variable flow conditions (Harvey, 1984). Sedimentation resulted from the sharp decrease in transport efficiency downstream as meltwater breached the outer moraine and deposition occurred on the eroded surface of massive diamict. Two general phases of sedimentation can be recognised. Firstly, the bouldery gravels represent deposition at the most ice-proximal location. These are succeeded by fine gravel in the middle section, and cross-bedded sand and gravel in the most distal exposure. This trend is the result of declining flow competence and sediment supply with distance from the efflux and in interchannel areas. Bouldery gravels are interpreted as debris flows, with little internal structure or preferred fabric (Harvey, 1984). Well-sorted, mainly clast-supported imbricate gravels are typical of within-channel deposition (Harvey, 1984). Welded contacts indicate rapidly recurring flows. The increase in cross-bedding between the middle and distal exposures is typical of increasingly distal fan deposits (Rust & Koster, 1984). Secondly, there is a phase of increased flow competence, recorded in the more variably stratified gravels of the middle exposure, which are interpreted as the result of deposition in a major channel. The coarse gravels of the most distal exposure are included in this depositional phase. The massive boulder bed at the base of the north bank exposure is interpreted as a lag deposit, due to the reworking of the subglacial till matrix. Pebble and sand inclusions are the result of sieving of sediment from overlying deposits. Cobble-rich beds with erosional contacts record more energetic flows. The boulders at the base of the channel record a highly erosional flow pulse which cut into the underlying stratified gravels. The cobble/clast cluster is interpreted as a debris flow buoyed up by dispersive pressures (Postma, 1986). Log jams in the distal exposure are also interpreted as debris flows. The non-erosional contact with underlying cross-bedded sandy gravels is characteristic of high concentration cohesionless flow (Postma, 1986). | |
| Notes: | |
For site specific information on other sand and gravel sites within the Bloody Bridge River Valley Complex see the following sites: Key Site 452 - Bloody Bridge River Outwash Key Site 453 - Bloody Bridge River Valley Ridges For overview information and references on the Bloody Bridge River Valley Complex see the following site: Key Site 448 - Bloody Bridge River Valley Complex - Overview For overview and site specific information on other sand and gravel sites within the Mourne Mountains area see the following sites: Key Site 440 - Mourne Plain Complex - Overview Key Site 447 - Attical Complex, Mourne Mountains - Overview Key Site 449 - Northeast Mournes Complex - Overview Key Site 450 - Western Mournes - Overview Key Site 451 - Annalong Valley For general information and references on the sand and gravel landscapes of N. Ireland, and for directions to sand and gravel sites other than those within the Mourne Mountains area, see the following site: Key Site 439 - Introduction to the Sand and Gravel Landscapes of N. Ireland | |
| Keywords | |
|---|---|
| Geomorph: | glaciofluvial fan |
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Last updated: | 7 Jan 2005 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Bloody Bridge River Fan, County Down, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=454. Accessed on 2026-11-05. |
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