URBAN EARTH - Rock Triple-Decker EarthCache

Introduction

At the given coordinates you will find yourself next to a stone bridge which spans the River Ribble. The river is 121km in length and runs through Yokshire and Lancashire into the Irish Sea.

The river is tidal at this point so varies in depth throughout the day . You will need to visit at low tide in order to complete this EarthCache.

The structure of this bridge includes three different sandstones:

• Sherwood Sandstone - piers and spandrel walls
• Fletcher Bank Grit - arches / superstructure (parapets)
• Haslingden Flags - road deck / roadway

Logging Tasks

IN ORDER TO COMPLETE THESE LOGGING TASKS PLEASE EMAIL YOUR ANSWERS TO US VIA OUR GEOCACHING PROFILE. PLEASE DO NOT INCLUDE ANSWERS IN YOUR ONLINE LOG. YOU CAN GO AHEAD AND LOG YOUR FIND AS SOON AS YOU HAVE SENT YOUR ANSWERS IN ACCORDANCE WITH GROUNDSPEAK GUIDELINES. LOGS WITHOUT ADEQUATE LOGGING TASK EVIDENCE MAY SUBSEQUENTLY BE DELETED.

1. At the given coordinates you will have a good view of the full span of the bridge. From here you should be able to clearly see the red Sherwood Sandstone in the spandrels and thus compare and contrast it with the darker Fletcher Bank Grit which makes up the arches and superstructure, including the parapets which the are the bits that stop you accidentally falling off the bridge into the water . Once you've done that, notice that underneath each arch, in the wall, there are a number of projecting stones in a particular arrangement - please describe the number of projecting stones and how they are arranged
   
   
2. Go up onto the road deck of the bridge and study the Haslingden Flag stone setts. Describe their colouration and texture. What makes this stone particularly suitable for this use?
   
   
3. You should now be able to visually identify the three types of stone which this bridge is built from - but the whole bridge is sitting on an outcrop of one of those three types - and this is why you needed to be here at low tide because if you look over the bridge you should find that the water is low enough that the river bed - and the stone it is made of - is clearly visible. So what stone is the river bed made of?
   
   TIP: There's quite a lot of algae growing on the surface of the stone which makes up the river bed which can make it difficult to see through to the colour of the stone so: Stand in the waypointed pedestrian refuge (labelled PREF1) facing downstream (Winter Hill should be behind you, in the distance) and look over the parapet, straight down toward the river bed. In your field of vision you should have the dark coloured parapet, the red spandrel wall and the rock of the river bed and with that view you should be able to identify the closest colour match between the river bed and the matching stone in the bridge. I found that the stone in the riverbed slightly to my right had the most visible colouration.
   
   
4. At waypoint PB2 you'll be able to see the red Sherwood Sandstone up close. Please describe it - what does it feel like? Are the grains large/small/all the same size? Is the stone all the same shade of red or are there variations? Why do you think the stone looks like this?
   
   
5. Still at waypoint PB2, compare the texture of the red Sherwood Sandstone with the Fletcher Bank Grit above - do they look the same? Do they feel the same? Which one would you say has the largest particles in it?
   
   
6. On some parts of the bridge the Fletcher Bank Grit reveals its internal structure more (waypoint PREF2 is a good example). Find one of these areas and study the particles - is there variation in the size / colour of the particles or do they all look the same?
   
   
7. OPTIONAL: Please feel free to add photographs to your log which do not give the answers away.
   
   

Background

Haslingden Flags are a member of The Millstone Grit Group which comprises over thirty individually named sandstones. The Millstone Grit Group dates from the Namurian Stage of the Carboniferous period, making these stones somewhere between 313 and 326 million years old.

Haslingden Flags is a hard-wearing sandstone with a hardness and silica content equivalent to that of granite and is also fissile, making it relatively easy to split into slabs and blocks which can then be used for paving. London's Trafalgar Square is paved with Haslingden Flags.

Fletcher Bank Grit is also a member of the Millstone Grit Group and provides some of the best and most durable building stones in the UK. This particular stone is a very course form, described in the British Geological Survey Lexicon as Sandstone and pebbly sandstone, coarse-grained with angular grains, with quartz and quartzite pebbles, massive and current-bedded, and subordinate beds of mudstone and coal

At the end of the Carboniferous Period the climactic conditions changed drastically. What would become Lancashire moved north of the equator resulting in a switch from vegetation rich tropical swampy forests into the more arid desert-like conditions of the Permian and Triassic periods (300 to 200 million years ago).

Red Sherwood Sandstone was formed during the Triassic period (250 to 200 million years ago) and gets its name from Sherwood Forest in Nottinghamshire which is underlain by rocks of this age.

The rocks of the Sherwood Sandstone Group are found in Northwest England as far north as Carlisle, although they are largely obscured by superficial deposits. The stone itself is described as monomineralic i.e. comprised of a single mineral, which in this case is grains of quartz (although negligible amounts of other minerals may be present). The quartz grains are cemented together with the ferric iron oxide haematite, (Fe₂O₃) which is what provides the red colouration. This colouration varies in intensity depending on where the stone was quarried from and a single stone may feature bands of varying colour intensity due to variations in the quantity of haematite present as each sedimentary layer was laid down.