Field Trip to Lancashire - weekend 5-7 May 2000

On Friday, 5th May this 'novice' explorer braved the enigmas of modern day travel and transportation and set off from Vienna, Austria to Manchester for two full days of geology field-tripping around the wilds of Lancashire (S260 - Saturday) and Alderley Edge, Cheshire (OUGS, Northwest branch - Sunday - but that's another story!). After an uneventful flight I arrived at Manchester airport to discover that my small suitcase containing walking shoes, warm clothing, insect repellent spray, etc. had not been as lucky as me and had remained in Vienna! Ah, well, I thought, at least I have my brand new hard hat, purchased specially for the trip, in my back-pack. So I pressed on.

On Saturday morning our small party of Chris and Dave Arkwright, Alan Diggles (the OUGS Northwest Branch organiser) and myself headed for Trowbarrow Quarry, near Silverdale on the Lancashire coast. There we met about 20 people from all over the northwest, most of whom are studying S260 this year. After a short walk up to the quarry Chris gave us a brief outline of what we were about to see. The main type of rocks found are limestones, there are also some thin clays and mudstones, although these are not so obvious. The rocks form part of the Carboniferous Limestone Formation, belonging to the Lower Carboniferous period (about 340 million years old) of which the most abundant is that of the Urswick Limestone. The rocks have been dramatically tilted from the horizontal to a vertical position.

We began our investigation of the area at a cliff face to one end of the elliptical-shaped quarry which is composed of fragmented skeletal remains and shells of shellfish, sea-lilies, corals, algae and plankton. The most numerous of the easily identifiable fossils are the long cylindrical stick-shaped trace fossils formed by creatures burrowing below the surface, eating, digesting, and passing the inedible waste to back-fill the tunnel - this is known as 'bioturbation'. The other fossils found at this site are colonial corals (Giantoproductus of the Brachiopod family) which are radial-spoke fossils about 30cm across. Growing conditions for these marine creatures suggest that they thrived in clear, warm, shallow, tropical seas that would have been Britain which is believed to have been sited south of the equator at this time.

Walking across the quarry it is easy to see distinct bedding planes in the vertical, and these form the limb of a monoclinal fold (my interpretation is of a one-limbed feature where one side of the plane is found at a lower angle to the other in a kind of dog-legged attittude - sort of like a capital L with a horizontal extension from the top of the L out to the left). This fold is thought to have been made at the end of the Carboniferous period.

About 60m to the left is a high face of massive rock covered in stick fossils with several large cracks running up it which have broken the beds into large rectangular blocks, with fluting at the top. There is a large pile of scree at the base of the face. Further to the left is an area showing three distinct bedding planes filled with red clay which is called 'paleokarst' (paleo-ancient; karst-landform resulting from the action of carbonation*) and has an appearance similar to limestone pavements. The paleokarst surface is evidence for a land surface, with the red clays possibly being remnants of the soil cover. The colouring of the rock is thought to be as a result of solution by water, possibly mineralising solutions flowing within the fractures.

Walking south we saw a rock face showing signs of tree root-ball activity. One theory suggests dissolved carbon dioxide exuded from the trees produced an acidic fluid which corroded the rocks to produce these surface depressions. Further on were two clear faults, which have moved, with different limestones to either side and showing the beds lying at different angles. One bed, the Urswick Limestone, is formed from calcite, a dense, hard, weather-resistant rock - and is almost vertical. The other, Park Limestone, shows less well-defined, irregular, horizontal beds. (Additional geological information from Countryside Management service,
Arnside, Lancs.)

After a picnic lunch we made our way to Heysham via Morecombe. There we joined up again for our walk along Heysham Head. The rocks here are part of the Millstone Grit Group of Namurian age, consisting of deltaic sands, silts and muds deposited 315 million years ago and approximately 2.5km thick. They are from the lower sequence of sandstone deposition infilling the basin with 'turbidite-generated' sediments. The most abundant rock is Ward's Stone Sandstone showing thick, coarse grains of gritstone with mainly angular quartz clasts. The finer mudstones show turbidite activity possibly due to an undersea avalanche, and there are red blotches of mudflake.

Three types of white minerals may be seen calcite, quartz and barytes (barium sulphate and associated with the Carboniferous rocks in the area). Much of the faulting is reverse thrust fault resulting in the Heysham Head Anticline. On our way to the beach area we practised using a clinometer and found that the angle of dip in one particular area was approximately 20 degrees to the west.

The red colouring of the sandstone rocks is thought to be due to weathering in a highly oxidized and arid environment or haematite leaching down from the Permian rocks that were subsequently laid down and which have now eroded away. Alan gave a great show of digging out the soft deposits of haematite with his fingernail to demonstrate their intense colouration. This was unforgettable as he got his hands covered in red dye and he tried fairly unsuccessfully to wash it off in the rock pools left by the tide!

Further along the cliff section is broken by complex faulting. Slickensides (lines on a fault or bedding plane where one side frictionally rubs against another) are very distinct showing the direction of movement, and they are mineralized with barytes. Other rock types are in evidence, such as interbedded sandstones, siltstones and shales of the Roeburndale Formation deposited before the Ward's Stone Sandstone. These show normal and reverse faulting as well as competent (able to withstand pressure of folding) and incompetent beds, and cross-bedding and ripple marks are abundant. Here another trace fossil, Diplocraterian (a U-shaped dwelling burrow created by an animal living in the sediment*) bioturbation can be found. Multiple faults are obvious a little further along the beach where the lower older beds of the Roeburndale have been compressed and pushed up over the younger Ward's Stone Sandstone beds. They lie in the approximate direction of the Heysham Head Anticline.

We adjourned the meeting at the local pub and then made our way 'home' and to dinner, which for me was the much dreamed of 'fish and chips'. I was further rewarded with the arrival at 6pm of my 'long-lost' suitcase!

After another wonderful day on the Sunday I returned to Vienna, tired, stiff, but most definitely exhilerated after what was the best weekend I've had in a long time. I would like to take this opportunity to thank all those OUGS members, tutors, and friends for making me feel SO welcome on both trips. Ummmm, memorable .........

May 2000 Ann Wiltner, Vienna, Austria

(With grateful thanks to Chris Arkwright who provided copious notes and a
worksheet for this trip)
* Oxford Dictionary of Earth Sciences, Oxford University Press, 1999