This EarthCache takes you to the entrance to the park from Glasgow Road. We are here to look at the low wall, which surrounds the entrance, namely the carving of AVONDALE, and the stone forming the gateway.
You will notice that it is formed from sandstone, and in places has some lines on it. So lets explore geologically this location.
The rock that we can see 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.Grain sizes in sands are defined (in geology) within the range of 0.0625 mm to 2 mm (0.002–0.079 inches). Clays and sediments with smaller grain sizes not visible with the naked eye, including siltstones and shales, are typically called argillaceous sediments; rocks with greater grain sizes, are termed rudaceous sediments.
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.
Beds are seperated by bedding planes, cm to m thick units of sedimentary rock that were deposited approximately horizontally (beds) and are separated by horizontal planes (bedding planes); the rocks typically weather more along these planes. Beds are usually fairly uniform or change gradationally in composition. Bedding planes usually represent breaks in sedimentation. When sediment is initially deposited, it is laid down in horizontal layers, with the oldest strata at the bottom. Horizontal bedding usually indicates that little or no structural deformation has occurredto a sedimentary succession. These situations are quite common in sedimentary basins and in regions flanking active mountain belts. Sediment derived from the erosion of the mountains is laid down in successively younger layers according to the Law of Superposition.
Now 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.
Here, at the EarthCache site we have an example of cross bedding.
So what is cross bedding?
Well look at the LE part of AVONDALE, you will notice that the strata is not level or horizontal.It 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-bedding 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 upper half of 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.
Cross beds can tell geologists much about what an area was like in ancient times. The direction the beds are dipping indicates paleocurrent, the rough direction of sediment transport.
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. Please measure the height of the band of cross bedding in the stone block which has LE on it.
2. At what angle does the cross bedding lie at in degrees , from the horizontal on this block?
3. Look at the cross bedding, and assume this block was here naturally, which direction was the paleae flow. Please explain the reason for your answer?. You can explain the direction in terms of the start of the lettering or the end of the lettering.
4. Look at the blocks forming the gatesposts, there are different bedding planes on some of the blocks, which ones have these, and what rationale do you think there is for different bedding planes?