| Site Type: | Stream section |
| Site Status: | local interest |
| Council area: | Belfast City Council |
| Grid Reference: | J294768,J288769 |
| Google maps: | 54.62324,-5.99658 |
| Rocks |
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| Rock Age: | Tertiary, Cretaceous, Triassic (Middle Triassic, Palaeocene, Upper Cretaceous) |
| Rock Name: | Belfast Marls Member, Collin Glen Formation, Collinwell Sands Member, Hibernian Greensands Formation, Island Magee Silts Member, Lower Basalt Formation, Mercia Mudstone Group, Port Calliagh Chalk Member, Ulster White Limestone Formation |
| Rock Type: | Basalt, Glauconitic sandstone, Greensand, Limestone, Marl, Mudstone, Olivine Basalt, Sandstone |
| Interest |
|---|
| Minerals: | Glauconite |
| Fossil Groups: | Trace fossils |
| Other interest: | amygdales, vesicles |
The Collin Glen Formation of the Mercia Mudstone Group (Triassic) is exposed in the bed of the Forth River at Crows Glen [J294 768] where it is cut by eight fine-grained basaltic dykes (Palaeogene). The rocks comprise red and red/green variegated marls and mudstones. In the nearby abandoned brick pit [J291 760] up to 20m of these beds are exposed. At Crows Glen, a small isolated exposure of the Hibernian Greensand Formation (Upper Cretaceous), can be seen 41m upstream (west) of the highest exposure of the Mercia Mudstone Group. Here the Greensand is composed almost entirely of grains of glauconite. These beds belong to the lowest Belfast Marls Member. Some 27m upstream from this point, beneath a small waterfall, about 3m of pale brown and grey fine-grained sandstone and marls are visible. The rocks are nodular in places and carry fossil Serpula and Pectinid fragments. Belonging to the Island Magee Siltstones Member, these rocks are overlain by 1.2m of very coarse and gritty chalky glauconitic sandstones of the Collinwell Sands Member. These sandstones contain many reworked and phosphatised sandstone pebbles from the underlying Island Magee Siltstones Member indicating a period of erosion prior to the onset of Collinwell Sands Member sedimentation. Thallasinoides burrows up to 60cm deep are common in both members.
The glauconitic sandstones of the Collinwell Sands Member lie directly below the Port Calliagh Chalk Member of the Ulster White Limestone Formation. Above this, four flows of the Lower Basalt Formation (Palaeocene) are exposed [J288 769]. Flow 1 (at the base) is a 10m thick flow of amygdaloidal basalt. Flow 2 is over 15m thick and is composed of massive medium-grained basalt. Flow 3 consists of 3.6m of slaggy vesicular basalt followed by 7m of massive dense blue basalt and 2m of slaggy vesicular basalt. Flow 4 consists of over 3m of coarse olivine basalt. Finally a distinctive red boulder clay, which developed on the Triassic marls and mudstones during the last Ice Age, can be seen.
