So you wonder, why has Hillgorilla put an earthcache in Bury, I hear the moans, oh yet another earthcache, there are nicer places etc. But what many may not realise, is that there is plenty of geology all around us, and I don't mean the hills or rivers. Look out your window now, and you will most probably see a bit of geology. Now many an earthcache, are not all that accesible, but why should those who are restricted or disabled in some way not get to encounter the wonders of geology?
The co-ordinates take you into urban Bury, forget the fancy bits of the Rock or the Town Hall, this is industrial revolution and post that era Bury. So you need to get onto Wellington Street, most probably named after the great duke, who beat Boney at Waterloo. Crossing Wellington Street is the Daisyfield Viaduct. It was originally constructed in the nineteenth century to carry the Bolton–Bury–Rochdale railway line across the Manchester, Bolton and Bury Canal and River Irwell taking trains towards Knowsley Street station in Bury. It no longer carries trains, as the line and viaduct were abandoned in the 1970s. It was restored in 1999 for use as a recreational feature for the general public, forming part of the Manchester to Preston leg of the National Cycle Network and the Bury Bolton Greenway Project. It is now hidden out of site from many, but it is a wonder of architecture and geology.
The geology of the area is Carboniferous sandstone, overlain by superficial river terrace deposits and alluvium. These superficial deposits comprise of sandy gravely silt. What I want you to look at it is the Viaduct itself, in particular the 4th stone up, just along from where the main wall joins the buttress.
So what type of rock have we got here?
It is sandstone.
Sandstone, which is sometimes known as arenite, is a clastic sedimentary rock composed mainly of sand sized minerals or rock grains. Most sandstone is composed of quartz, and / or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any color, but the most common colors are: tan, brown, yellow, red, grey, pink, white, and black. Sandstones are formed from cemented grains that may either be fragments of a pre-existing rock or be mono-minerallic crystals. The cements binding these grains together are typically calcite, clays, and silica.
The formation of sandstone involves two principal stages:
(1) First, a layer or layers of sand accumulates as the result of sedimentation, either from water (as in a stream, lake, or sea) or from air (as in a desert). Typically, sedimentation occurs by the sand settling out from suspension; i.e., ceasing to be rolled or bounced along the bottom of a body of water or ground surface (e.g., in a desert).
(2) Finally, once it has accumulated, the sand becomes sandstone when it is compacted by pressure of overlying deposits and cemented by the precipitation of minerals within the pore spaces between sand grains.
The most common cementing materials are silica and calcium carbonate, which are often derived either from dissolution or from alteration of the sand after it was buried. Colours will usually be tan or yellow (from a blend of the clear quartz with the dark amber feldspar content of the sand). A predominant additional colourant is iron oxide, which imparts reddish tints ranging from pink to dark red, with additional manganese imparting a purplish hue.
Sedimentary rocks are types of rock that are formed by the deposition and subsequent cementation of that material at the earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle in place. The particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, the sediment was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or galciers. Sedimentation may also occur as minerals precipitate from water solution or shells of aquatic creatures settle out of suspension.
Stratification is the way sediment layers are stacked over each other, and can occur on the scale of hundreds of meters, and down to submillimeter scale. It is a fundamental feature of sedimentary rocks.Over time the different sediment types will be stacked on top of each other and the migration of the shoreline will produce superimposed layers (stratification) of different types of sedimentary rock. If we assume that strata are layers we would think they would be straight, but that is not always the case You would think that strata would be laid down in a flat horizontal manner, as in the picture, with level layers, but it is not always that simple, at times the strata can be tilted, folded or cross bedded.Look at the stones that make up the viaduct, namely the stone I have asked you to identify. It is an example cross bedding
What is cross bedding?
Look at the stone, yes it is rectangular in shape, that bit was done by man. But look at the stone itself, you will notice that it has diagonal lines below horizontal lines. This is cross bedding, which is a feature that occurs at various scales, and is observed in conglomerates and sandstones. It reflects the transport of gravel and sand by currents that flow over the sediment surface (e.g. in a river channel), sand in river channels or coastal environments. When cross beddding forms, sand is transported as sand-dune like bodies (sandwave), in which sediment is moved up and eroded along a gentle upcurrent slope, and redeposited (avalanching) on the downcurrent slope (see below picture). After several of these bedforms have migrated over an area, and if there is more sediment deposited than eroded, there will be a buildup of cross-bedded sandstone layers. The inclination of the cross-beds indicates the transport direction and the current flow (from left to right in our diagram). The style and size of cross bedding can be used to estimate current velocity, and orientation of cross-beds allows determination direction of paleoflow.
Looking at the viaduct, there are two types of rock blocks, some are cut with a rough face, whilst others are dressed. This does not mean that they are wearing trousers, pants or knickers, it means that the stone has been worked by a mason. This is known as Ashlar.
Ashlar is finely dressed (cut, worked) masonary, either an individual stone that has been worked until squared or the masonary built of such stone. It is the finest stone masonry unit, Precisely cut “on all faces adjacent to those of other stones”, ashlar is capable of very thin joints between blocks, and the visible face of the stone may be as quarry-faced or feature a variety of treatments: tooled, smoothly polished or rendered with another material for decorative effect.
One such decorative treatment consists of small grooves achieved by the application of a metal comb. Generally used only on softer stone ashlar, this decoration is known as mason's drag.[3]
Ashlar is in contrast to rubble masonary, which employs irregularly shaped stones, although sometimes minimally worked or selected for similar size, or both.
This geocache is intended to be accesbile to all, it should be possible to get a wheelchair right up against the side of the viaduct at ground level, though some surfaces are slightly rough, and there is a small kerb to be encountered.
This being an earthcache, in order to log it, I ask that you answer some questions. Please send them to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile.
1. Run your hand over the sandstone blocks, what do they feel like, are they rough or smooth, are they all the same?
2. Look at the stone which is cross bedded, what direction do you think the paleoflow went, when the rock was deposited in its original location?
3. Can you identify some stones which are ashlar? Where are they?
4. Now this earthcache site is easily to be seen using google earth, so to prevent an armchair logger, please look up, what type of animal is looking down on you?
5. Feel free to take a picture of yourself or GPS at the site, though this is not a logging requirement