King George III Obelisk

The sandstone obelisk in the square was erected to commemorate the jubilee of King George III in 1810. There is a set of stocks beside it and a couple of slate 'fish slabs'

In order to log a find against this EarthCache, you will need to visit GZ and make some observations and then answer some questions and send answers to me via my Geocaching profile or through the Messenger Centre.

You won't need a magnifying glass or anything like that to complete the earthcache but if you have one, or ideally a hand lens or even a magnifying app on your smartphone, you'll be able to see the makeup of the rock and any fossils it includes in much greater detail.

1. Described the stone of the Obelisk and steps. Paying particular attention to the colour and grain size you can see.

2. Using the information bellow and your answer to Q1, how would you classify type of sedimentary stone.

3. On the rear of the Obelisk you will find a ring, what colour is it and what mineral do you think has caused it.

4. Take a photograph of you, your GPS or another identifiable item near GZ, taking care not to include anything in the photo that may give away your answers.

Sandstone, simply put, is sand cemented together into rock — this is easy to tell just by looking closely at a specimen. But beyond that simple definition lies an interesting makeup of sediment, matrix, and cement that can (with investigation) reveal a great deal of valuable geologic information.

Sandstone Basics 

Sandstone is a type of rock made from sediment, sediment particles are clasts, or pieces, of minerals and fragments of rock, thus sandstone is a clastic sedimentary rock. Sandstone may include finer and coarser material and still be called sandstone, but if it includes more than 30 percent grains of gravel, cobble or boulder size it's classified instead as conglomerate or breccia (together these are called rudites). Sandstone with a lot of matrix is called poorly sorted. If matrix amounts to more than 10 percent of the rock, it is called a wacke ("wacky"). A well-sorted sandstone (little matrix) with little cement is called an arenite. Another way to look at it is that wacke is dirty and arenite is clean. You may notice that none of this discussion mentions any particular minerals, just a certain particle size. But in fact, minerals make up an important part of sandstone's geologic story.

Minerals of Sandstone 

Sandstone is formally defined strictly by particle size, but rocks made of carbonate minerals don't qualify as sandstone. Carbonate rocks are called limestone and given a whole separate classification, so sandstone really signifies a silicate-rich rock. (A medium-grained clastic carbonate rock, or "limestone sandstone," is called calcarenite.) This division makes sense because limestone is made in clean ocean water, whereas silicate rocks are made from sediment eroded off the continents. Mature continental sediment consists of a handful of surface minerals, and sandstone, therefore, is usually almost all quartz. Other minerals—clays, hematite, ilmenite, feldspar, amphibole, and mica— and small rock fragments (lithics) as well as organic carbon (bitumen) add color and character to the clastic fraction or the matrix. A sandstone with at least 25 percent feldspar is called arkose. A sandstone made of volcanic particles is called tuff.​

The cement in sandstone is usually one of three materials: silica (chemically the same as quartz), calcium carbonate or iron oxide. These may infiltrate the matrix and bind it together, or they may fill the spaces where there is no matrix.

Depending on the mix of matrix and cement, sandstone may have a wide range of color from nearly white to nearly black, with gray, brown, red, pink and buff in between.

Types of Sandstone.

Breccia. Breccias are a type of clastic sedimentary rock which are made of angular to subangular, randomly oriented clasts of other sedimentary rocks. They are formed by either submarine debris flows, avalanches, mud flow or mass flow in an aqueous medium. Technically, turbidites are a form of debris flow deposit and are a fine-grained peripheral deposit to a sedimentary breccia flow. The other derivation of sedimentary breccia is as angular, poorly sorted, immature fragments of rocks in a finer grained groundmass which are produced by mass wasting. These are, in essence, lithified colluvium. Thick sequences of sedimentary (colluvial) breccias are generally formed next to fault scarps in grabens.


Conglomerates. A conglomerate, by contrast, is a sedimentary rock composed of rounded fragments or clasts of pre-existing rocks. Both breccias and conglomerates are composed of fragments averaging greater than 2 millimetres (0.079 in) in size. The angular shape of the fragments indicates that the material has not been transported far from its source. Breccias indicate accumulation in a juvenile stream channel or accumulations because of gravity erosion. Talus slopes might become buried and the talus cemented in a similar manner.

Sandstone. It is composed mostly of sand particles, which are of medium size; therefore, sandstone is a medium grained clastic sedimentary rock. More precisely, sand is between 1/16 millimeter and 2 mm in size (silt is finer and gravel is coarser). The sand grains that makeup sandstone are aptly referred to as framework grains.

Each of the following classification represents a different type of sedimentary rock based on the size, shape, and composition of the clasts, as well as the depositional processes that led to their formation. Understanding the characteristics of different types of sandstone can provide insights into the environmental conditions under which they were deposited and geological history..

# Arkose. Typically grey to reddish in colour, a feldspar-rich sandstone. The sand grains making up an arkose may range from fine to very coarse, but tend toward the coarser end of the scale. Fossils are rare in arkose, due to the depositional processes that form it.

# Lithic. Lithic sandstones can have a speckled (salt and pepper) or gray color, and are usually associated with abundant type of lithic fragment grains in addition to quartz and feldspar, indicating a mixed sediment source.

# Fossiliferois. Typically contains a mix of shell fossils and casts of gastropods (snails), oyster shell, clam shell, mussel shell, and wood fragment.

# Quartz. This is a pure sandstone composed of well sorted rounded grains of quartz, with a high degree of purity. Cemented by further quartz between the grains. It is often white, gray, or light tan colored due to the dominance of quartz.