"Tropical World" and the Geology of Roundhay Park :: Fotopic.Net

"Tropical World" and the Geology of Roundhay Park

26th January 2008

The fun thing about Roundhay Park in Leeds is that there is a small "zoo" there called "Tropical World" - and a little over 300 million years ago this really was a tropical world, lying in equatorial latitudes!

The Late Carboniferous rocks exposed in Roundhay Park range from about 317 to 315 million years in age. They tell the story of a tropical low-lying coastal area which was from time to time submerged by rising sea-level. Rivers draining from highlands in the North and West built swampy deltaic coastal plains which repeatedly advanced towards the South and East. The central area (now Lancashire and Yorkshire) towards which the rivers drained is known as the Pennine Basin.

Two main kinds of rock can be seen in the park, being a) mudstones and b) sandstones. The mudstones were deposited in lakes and seas. The sandstones were deposited in river channels and in coastal deltas where river mouths built out into standing bodies of water. The mudstones can be seen in The Gorge at the northern end of the park whereas the sandstones are exposed in the higher ground of the golf course, the falls near the Upper Lake and the old quarries to the east of Waterloo Lake.

The geology of the park is separated into two areas by an East-West fault, the Roundhay Park Fault. The rocks on the southern side of this fault have moved downwards, by a vertical distance of 200 m or more relative to the rocks on the northern side. These down-thrown rocks are the Elland Flags, consisting of fine-grained sandstones, rich in mica, with some thin mudstone and siltstone intercalations. They commonly exhibit cross-lamination, recording the passage of ancient ripples. The Elland Flags have been widely used as building stone in the area, as for example in the Mansion.

The older succession on the northern side of the fault begins with mudstones at the base (seen in The Gorge) which are overlain by a unit of coarse pebbly sandstones called the Rough Rock. The thickness of these sandstones ranges from about 8 to 25 m across the Leeds area. The top of the Rough Rock runs along the southern margin of the Upper Lake and represents an important geological horizon. This is the boundary between two major subdivisions of the Late Carboniferous, with the Namurian Series below and the Westphalian above. Above this boundary are dark mudstones with bands containing marine fossils and thin coals. Coals are not seen at the surface in Roundhay Park, but the Soft Bed Coal (a metre or less in thickness), the first coal in the Coal Measures, is present just beneath the surface between Tropical World and the Mansion. Several other coal seams have been worked in the Leeds area. Roundhay Park is situated at the northern limit of the Yorkshire Coalfield.

Google Earth image of the northern part of Roundhay Park.	Geological map of Roundhay Park. The thick red line at the northern end of Waterloo Lake represents the trace of a geological fault called the Roundhay Park Fault. Rocks to the South of this fault are younger than those to the North. The red-orange colour to the North of the fault represents the Rough Rock and the yellow colour to the South represents a younger body of sandstones known as the Elland Flags.	Geological map of Roundhay Park showing a suggested clockwise walk around the gorge and Waterloo Lake.	Vertical succession of rock types and fossil content in Roundhay Park. (sketch).
Non-marine bivalve fossils from The Gorge, Roundhay Park.	Fossil goniatites from the gorge. a bedding plane in “paper shales” crowded with nautilus-like goniatites of the species Cancelloceras (Gastrioceras) cumbriense.	An imagined marine scene in Roundhay Park 315 million years ago.	Marine fossils from The Gorge, Roundhay Park. several examples of the thin-shelled scallop-like bivalve Dunbarella.
An anticlinal fold in mudstones, perhaps related to movement on a nearby fault.	Soft, easily eroded mudstones overlain by a unit of bedded sandstones, belonging to the Millstone Grit Group in The Gorge. Hammer for scale.	A spring in the Rough Rock. To the north of the Roundhay Park Fault. The sandstone here is very rich in mica and small plant fragments.	A link between Leeds and the forests of equatorial Africa.