Introduction
This EarthCache takes you to Bridge 63, a sandstone bridge over the Leeds & Liverpool canal where it passes through Standish. The coordinates given are for the south side of the bridge.
The banner image above shows an object fashioned from metal rather than stone which, whilst not geological in nature and hence not the focus of this EarthCache is nonetheless important because it's key to answering the questions correctly - so get a good look at it to make sure you can locate it once you're there 😉.
I did wonder what it was and why it was there (there's a matching one on the north side of the bridge) and after a little research I now believe that it's a mounting point for a rope roller - a long, narrow metal cylinder affixed to the side of the bridge which rotated freely with the rope rather than rubbing against it.
Before the days of powered canal barges they were towed on a long rope by a horse. You may have walked past a canal bridge with grooves worn into the stones at the arch edges by these ropes repeatedly rubbing over them. Rope rollers were a means of preventing this from happening, while extending the life of the rope and making the horse's life a little bit easier at the same time.
Logging Tasks
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The given coordinates take you to the south side of the bridge. Follow the towpath under the bridge to access the north side. Based on your observations at these locations and the information on this page you should be able to tell me:
- Locate the iron rope roller mounting bracket anchored into the north side of the bridge (it looks like the banner photograph at the top of the page). See how the grey lead that's been packed into the hole around the iron to give a tight fit is flush with the surface of the stone. Now look at the bracket on the south side and see that weathering and erosion here has created a gap between the lead and the surface of the stone. Measure this gap and tell me how wide it is. This tells us how much stone has eroded away at this point. Assuming a constant rate of erosion over the lifespan of the bridge, what is that rate in millimetres per year?
- Describe any differences in appearance between the stone in the the north side of the bridge and that on the south side, including the degree of weathering to each side. What about the environment and the location of these stones influences these differences?
- On the south side of the bridge please describe any features arising from differing concentrations of mineral cement. Which type of mineral cement is most obvious?
- Optional task: feel free to add any photographs of your visit that do not show the specific features from the logging tasks - no spoilers please. In the interests of allowing everyone to experience the EarthCache fully for themselves obvious spoiler photographs will be deleted.
Background
Figure 1 - a sandstone rich in iron oxide cement
Bridge 63 is a bridge over the Leeds & Liverpool canal, built of sandstone - but possibly not the most hard-wearing sandstone available.
This section of the canal was completed in around 1800, which makes this bridge around 220 years old - and time and the elements have not been particularly kind to the structure. Today it looks somewhat the worse for wear.
Over the 220 years the bridge has stood here a significant amount of the rock has been eroded away, but can we tell how much?
Fortunately there is a fixed reference point on the bridge that allows us to measure how much rock has been removed by the efficient processes of weathering and erosion.
Clastic Sedimentary Rocks
Clastic sedimentary rocks are formed mainly from fragments of other rocks and minerals. Geologists call these individual fragments clasts, a word derived from the greek word klastos meaning broken.
Individual clasts, or grains if you prefer, vary in size from clay at a few thousandths of a millimetre found in claystones and shales through silt (a few hundredths of a millimetre) in siltstones, sand (0.0625mm to 2mm) in sandstones through gravel, cobble and even boulder sized fragments found in conglomerates and breccias.
These individual clasts are transported by some flowing medium, such as wind or moving water, before being deposited elsewhere. Geologists call accumulations of these deposited clasts sediments and thus rocks made from these accumulations are known as sedimentary rocks.
How sediment becomes rock
Sediments laid down under force of gravity will typically be laid down loosely and include lots of space (pore space). Pore spaces may also be filled with fluid - if the sediments have been laid down in a body of water such as a sea or lake for example, or where groundwater percolates down from above.
As more and more sediments accumulate, the weight of the higher layers pushes down on the lower layers, forcing out the fluids and squeezing those lower sediments closer together, reducing the amount of pore space in the process (compaction).
Mineral rich fluids percolating through the sediments leave behind traces of those minerals which cement the individual sediments together to form solid rock once more (lithification).
Weathering and erosion
The terms weathering and erosion are often used interchangeably but they are actually different processes.
Weathering is a collection of physical and chemical processes that pulverize rocks over time. Weathering makes rocks softer and weaker and more easily eroded. It can involve the physical disintegration and chemical decomposition of rocks into soil, loose clasts (rock fragments), dissolved chemical components (ions), and solid chemical residues. In other words, weathering is so powerful that it can break rocks down into their fundamental molecular structures.
Rocks are most stable in the conditions in which they were formed. When rocks are moved to a new location where conditions differ, weathering will break down the rocks into substances which are stable in those new conditions.
Weathering is an in-place process, breaking down rocks into substances which are more easily transported but not actually transporting those substances anywhere. When those products of weathering are transported to a different location then erosion has taken place.
Transport refers to the processes by which the products of weathering are moved along – for example, pebbles rolled along a river-bed or sea shore, sand grains whipped up by the wind, salts carried in solution or just loose material sliding downhill under the force of gravity.
Most weathering and erosion processes typically involve repeating cycles over long periods of time - wetting and drying of the rock dissolve cementing minerals and wash them away along with loosened bits of rock, heating and cooling causing the rock to gradually break down as a result of expansion and contraction, and freezing and thawing - the expansion of water within remaining pore spaces and any cracks in the rock, as it freezes to become ice, acting as a lever to gradually weaken and break the rock apart.
In the simplest of terms then, the nature and severity of the weather conditions rocks are exposed to will be a major factor in the nature and rate of any weathering and erosion taking place, and there can be as much to learn from looking at the placement of the rocks in the surrounding environment as from looking at the rocks themselves 😉.
Strong and stable?
The sandstone used to build the bridge formed at considerable depth within the Earth, under conditions of higher temperatures and pressures and lower humidity than are found at surface. Now at the surface, exposed to the elements, weathering has acted on the stone to convert its constituent parts into substances which are stable in these new conditions. Some of the weathered stone has been carried away by the process of erosion.
Some sandstones resist weathering well. Some sandstones don't. Two features which influence this are the amount of compaction and the type and degree of cementation of the sediments.
In well compacted sediments the amount of contact between individual sedimentary clasts is greater, reducing porosity and increasing the chances that mineral cements will bind the clasts together strongly.
Figure 2 - concretions in sandstone
In sandstones, the two most common cements are silica and calcite, which are pale in colour. Iron minerals such as red hematite, yellow limonite and brown goethite may form cements on their own, giving red/orange, yellow and brown colouration respectively to the rock, but are more often found alongside silica or calcite cements.
Mineral cements may be evenly distributed throughout the rock or may be concentrated in particular areas. High concentrations of mineral cement tend to make those areas more resistant to weathering than areas with lower concentrations.
These differences in the rock and its resistance to weathering can lead to differential weathering, where the harder parts end up raised, sometimes quite significantly, above the softer parts - as has happened in figure 1 above and in figure 2 to the right.
If you've carefully read and digested the information from this cache page your tasks at the cache location should prove relatively straight forward, although you may wish to take a printed copy of the page with you so that you can check your answers while there 
.
Please submit your logging task responses before posting your log.