 | | Mercia Mudstone Group, Collin Glen. |
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| Site Type: | Stream section | | Site Status: | ESCR | | Council area: | Belfast City Council | | Grid Reference: | J270720 | | Bing maps: | 54.58076,-6.03579 | | Google maps: | 54.58076,-6.03579 | | Rocks |
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| Rock Age: | Cretaceous, Jurassic, Triassic | | Rock Name: | Collin Glen Formation, Hibernian Greensands Formation, Mercia Mudstone Group, Penarth Group, Sherwood Sandstone Group, Ulster White Limestone Formation, Waterloo Mudstone Formation | | Rock Type: | Glauconitic sandstone, Greensand, Limestone, Marl, Mudstone, Sand, Sandstone | | Interest |
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| Fossil Groups: | Brachiopod, Crinoid, Crustacea, Echinoderm, Fish | | Other interest: | fault, Marine sediments |
| Summary of site: |
| Collin Glen has a very special place in the development of geology in Northern Ireland. Its proximity to Belfast meant that, in addition to the early professional geologists (in the 19th century Portlock described what is now the Penarth Group; Tate the Jurassic and Cretaceous; Hume the late Triassic and Jurassic), the first generation of amateurs, initially in the Belfast Natural History and Philosophical Society and later (under Tate’s tutelage) the Belfast Naturalists’ Field Club, cut their geological teeth here. Even in the twentieth century its allure was undimmed; the Belfast Geologists’ Society held its inaugural field excursion there in 1954 and celebrated its fiftieth anniversary year by a return visit in 2004. The growing significance of the scientific and amenity importance of the site was recognized by a concerned group of local people and it was largely their enthusiasm and influence that led to the creation of the Collin Glen Forest Park, now firmly established with its own visitor centre. | | The geological importance of the glen lies in the enormous time span represented in the long succession of rocks exposed in its walls. Over 170 million years of Earth history can be read here, commencing 240 million years ago in the Triassic deserts and ending 65 million years ago in the remains of the fuming lava fields of the Tertiary basalts at the top of the Upper Glen. | | There is less rock exposed now than at any time in the nineteenth and twentieth centuries but enough survives for a cohesive picture to be presented. The section commences at the base with red-brown, lime-rich mudstones and silts of the Mercian Mudstone Group, formed in the second half of the Triassic period from about 240 million years ago. It is seen intermittently from Suffolk into the glen where the topmost 75m appear to contain no gypsum which elsewhere is common. At the top of this section near Glen Bridge, 3-4m of the last formation of the Mercia Mudstone, the Collin Glen Formation can sometimes be seen. They are strongly jointed, green mudstones with thin, red laminations. Their total thickness is around 9m and the formation is named and defined from this outcrop which is, therefore, the stratotype. | | The succession continues with the Triassic Penarth Group which was clearly exposed in the nineteenth century but is now largely obscured by overgrowth. Tiny glimpses of shales and shaley mudstones can be seen around Glen Bridge, with somewhat better exposure a further 90m upstream where there are also limestones. These rocks are the Westbury Formation, the lower of two divisions in the Penarth Group, and they contain rich fossil horizons yielding typical bivalve molluscs and fish teeth of a division in time called the Early Rhaetian. About 25m further upstream, silty lime-rich mudstones, possibly part of the Lilstock Formation of Late Rhaetian age, can be seen. | | The Collin Glen Fault interrupts the succession here and on its north-east side the succeeding Waterloo Mudstone Formation (of earliest Jurassic age) can be seen in three outcrops amounting to no more than 6-7m of dark grey, shaley mudstone with shelly bands. A fossil fauna of stone lilies (crinoids), sea urchins, a variety of bivalve molluscs, ammonites and microscopic ostracods have been collected here and they firmly establish the age as Hettangian (the first stage in the Jurassic period), formed around 205 million years ago. Although rocks of the next two stages are found elsewhere in Northern Ireland, neither stage appears to be present here. | | The best exposures in Collin Glen are of the Hibernian Greensand Formation, the earliest beds here of the Cretaceous period. The time interval between the last of the Jurassic rocks and these is over 100 million years, a huge gap in the geological record when large parts of the area were almost certainly dry land at the zenith of the age of the dinosaurs. One can only speculate on events in this interval. | | The nomenclature of the Hibernian Greensand Formation has been revised in recent years and somewhat confusing rock names, such as Glauconite Sands, Glauconitic Sandstone, Yellow Sandstone and Grey Marl, have given way to three new members. The Belfast Marls Member forms the base, around 2.5m of dark green to black, pebbly, loosely consolidated sands (the green colour due to grains of the mineral glauconite); the Island Magee Siltstone Member follows, around 6m of sandstone, grey to brown at first with pale yellowish grey above with many oyster shells; and finally the Collinwell Sands Member, about 1m of pale grey, medium grained sandstone with a large proportion of glauconite grains. These three members are richly fossiliferous in part and confirm their Cenomanian and Turonian age. This means that the first Cretaceous rocks here were deposited from an encroaching ocean about midway through the period, about 95 million years ago. | | The prominent waterfall in the Upper Glen cascades over a cliff of 11m of Island Magee Siltstone. 65m north of the waterfall, the junction with the first chalk of the Ulster White Limestone Formation can be seen. Initially the rock is medium grained, chalky sandstone grading into glauconitic chalk - the two together forming the Glauconitic Chalk Member. The Glauconite grains become less frequent towards the top. The succeeding bed is the Creggan Chalk Member, better seen on the north coast. There is a time break in the rocks of over 5 million years between the Glauconitic Chalk and the Creggan Chalk followed immediately by another of roughly 5 million years where five chalk members are absent. The Portrush, Ballymagarry and Tanderagee Chalk Formations follow, and are capped by, the Tertiary Lower Basalts. | | Collin Glen has one of the most complete sequences through the Mesozoic (Triassic, Jurassic and Cretaceous) rocks in Northern Ireland, despite its present poor exposure. The story it tells starts in the Triassic deserts and records their invasion by erratic fluctuations of the Jurassic and Cretaceous seas as first the massive continent of Laurasia split from Pangea and later the fragmentation of Laurasia spawned the continent of Europe. As the drifting plates moved beyond 30ºN the climate became a little cooler, perhaps similar to the Mediterranean now. | | Colin Glen is an undoubted Area of Special Scientific Interest awaiting only the formalisation of status. Almost all the geological interest is within the bounds of the Collin Glen Forest Park and is well protected with excellent access. A strong case can be made for the restoration of the degree of rock exposure in the Glen to that previously enjoyed in the 19th century and a sensitive plan for such work and its future maintenance ought to be achievable without significant change to the existing habitats. A classic geological site deserves to have its interest fully displayed and interpreted. |
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