Introduction
There is nothing geologically remarkable about this location. In fact it's precisely the sort of place you'd walk by without a moment's thought.
But if you do pause for a moment and take a look you'll see that at this point two buildings butt up against each other and, while they are both constructed of sandstone that might look similar at first glance, the stone in each building has distinct differences which reveal clues about the environment the rock formed in.
Specifically those clues provide insight into the amount of energy involved in the laying down of the sediment grains that were later compressed and cemented together to form the solid rock we see today.
Logging Tasks
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Based on your study of the stone used in the buildings either side of the given coordinates and using the information on the cache page please tell me:
- Are there any bedding planes visible in the stone used for either building?
- Is there any variation in grain size or degree of sorting in the left-hand building? If there are - please describe them.
- Is there any variation in the grain size or degree of sorting in the right-hand building? If there are - please describe them.
- Which rock would you say results from a lower energy environment and why?
- Which rock would you say results from a higher energy environment and why?
- 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
Sedimentary rocks are formed from fragments of other rocks, fragments which vary in size, shape and weight.
Those fragments are typically transported by some flowing medium - water, wind or ice, before being deposited, compacted and cemented together by some form of mineral cement - oxides of iron for example - to form solid rock. It's tempting to describe these rocks as new rocks but we have to remember that they are built of fragments of rocks that already existed and which may have been recycled many times before.
Variations in the speed of the flowing medium govern how the various sediment grains are laid down. Faster, higher energy flows can carry along larger, heavier sediments. Slower, lower energy flows can carry only smaller, lighter sediments. If a flow starts to slow, the heavier sediments are dropped first, while lighter sediments continue to be carried along. If the flow continues to get slower and slower the size and weight of sediment it can continue to carry gets less and less and so progressively smaller sediments are deposited.
If the energy of the flow increases again the size and weight of sediments that the flowing medium can carry along increases again and only larger, heavier sediments will be deposited - or even none at all until the energy falls again and the flow speed drops.
We can see the results of these sediment grading processes in the resultant rocks.
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Bedding - sedimentary rocks are made of fragments of other rocks called sediments, stuck together by mineral cement.
Because the sediments are laid down under the force of gravity they tend to settle in horizontal layers called beds. Beds are made up of sediments of roughly the same size and type. If the size and/or type of sediments being deposited changes distinctly a new bed is formed. The interface between beds is called a bedding plane.
The image to the right shows a section of sandstone in which the sediments (grains), beds (layers) and bedding planes (darker bands between the layers) are clearly visible.
- Sorting - geologists use the term sorting to describe the grain size distribution of sediments, either in unconsolidated deposits (such as boulder clay) or in sedimentary rocks. The following diagram shows examples of what the various levels of sorting might look like:
Rocks consisting of very poorly sorted or poorly sorted sediments typically arise from high energy environments such as a flash flood. In such an event the powerful flow of water picks up a load of sediments of a range of sizes and jumbles them all up during transportation. When the flood subsides the flow energy drops rapidly and the jumbled up sediments are all dropped together.
Well sorted sediments are a good indicator of a low energy environment. Very well sorted sediments arise from very low energy environments - a stream where the current is barely moving at all for example and may even cease altogether for periods of time. Such a cessation would typically result in a bedding plane in the rock.
A sandstone consisting of sediments laid down during an extended period of moderate flow, where energy levels remained fairly constant, would tend to have moderately sorted to well sorted grains. Where energy levels have been very low for an extended period of time the sediments deposited would tend to be small enough to produce siltstone / shale (or is it called clay rock these days?).
On the other hand, sandstones which were laid down quickly during bursts of high energy would tend to have poorly sorted sediments, even very poorly sorted sediments in extreme cases and we would see grains ranging from sand-sized (1mm - 2mm) inter-mingled with much larger grains / small pebbles.
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.