| Site Type: | Stream section, Valley |
| Site Status: | local interest |
| Council area: | Belfast City Council |
| Grid Reference: | J314787,J314792 |
| Google maps: | 54.63978,-5.96478 |
| Rocks |
|---|
| Rock Age: | Tertiary, Cretaceous, Jurassic, Triassic (Jurassic Undifferentiated, Palaeogene, Upper Cretaceous, Triassic Undifferentiated) |
| Rock Name: | Collin Glen Formation, Creggan Chalk Member, Hibernian Greensands Formation, Island Magee Silts Member, Mercia Mudstone Group, Penarth Group, Ulster White Limestone Formation, Waterloo Mudstone Formation |
| Rock Type: | Basalt, Dolerite, Limestone, Marl, Mudstone, Sandstone, Shale, Siltstone |
| Interest |
|---|
| Fossil Groups: | Ammonite, Brachiopod |
The site is located 800m north-northeast of the Horseshoe Bend on the Upper Crumlin Road, The exposure in the steep valley of Carrs Glen, between Cave Hill and Squire's Hill, is sufficiently good for a section to be constructed through the Mesozoic rocks of the area. Horizontally bedded red marls with green bands up to 10cm thick and abundant green reduction spots and streaks, pass upwards into uniform marls of the Collin Glen Formation of the Mercia Mudstone Group (Triassic). These marls, overlain by shales of the Penarth Group (Upper Triassic), are baked by an overlying dolerite sill to a hard blocky mudstone. The sill is overlain by a small exposure of the Waterloo Mudstone Formation (Lower Jurassic) [J3140 7919]. Here, a hard, shelly limestone 15cm thick is overlain by medium grey mudstones which contain some brachiopod and ribbed ammonite remains, possibly of Caloceras type. The beds have a dip of 29° to the west and they are clearly affected by landslip movements.
Carrs Glen provides one of the best sections through the Hibernian Greensand Formation (Upper Cretaceous) in south County Antrim. Here, calcareous buff coloured siltstones alternate with soft greyish silty marls and are part of the Island Magee Siltstones Member. Intra-Cretaceous erosion has removed the succeeding Collinwell Sands Member so that the overlying Ulster White Limestone Formation rests directly on the Island Magee Siltstones Member. Formerly, Carrs Glen provided one of the best natural sections through the Creggan Chalk Member though this is now largely obscured. The entire Mesozoic section at Carrs Glen is cut by three Palaeogene dykes one of which causes a waterfall high in the glen 1.1km north-northeast of the Horseshoe Bend. Finally the distinctive red boulder clay that was developed on top of the Triassic marls and mudstones has been cut into by the stream at Carrs Glen.
