This is an EarthCache that ‘I Want to Tell You’ about. Portland Stone is a high quality building stone made in the ‘Sea of Time’ and is a form of limestone. It is found and quarried on the Isle of Portland in Dorset. Many buildings and monuments in London and a few in Liverpool are made from this rock.
Briefly, there are three classifications of rock that encase the entire planet:
Firstly, sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form deposits that accumulate on the earth’s surface and often have distinctive layering or bedding. This rock formed by sediments of eroded rock particles that have settled and accumulated mostly in association with bodies of water or sometimes on land. Within these sediments there can be organic material which may become fossils. As the layers of sediment builds up over time the pressure increases under its own weight, sometimes displacing the water in a process called compaction. The particles starts to bond together by crystallisation as the minerals form. This mineral salt cements the particles together creating solid rock over the course of millions of years. Old sedimentary rock is found at the bottom of the layers and young rock at the top. When you study sedimentary rocks you can occasionally see objects such as fossilised shells, bones and plants.
Igneous Rocks. This rock was once extremely hot and molten called Magma. The molten rock changes when it reaches the earth’s crust and cools becoming either solid bodies over time or volcanic rock if it gets through the surface. The slower it cools the bigger crystals are formed in the rock like in granite for example. No fossils are found in this rock as it comes from within the Earth where there are no living organisms to be fossilised.
Metamorphic Rocks. This type of rock is made from either pre-existing sedimentary and/or igneous rocks; the minerals of these rock will have been chemically changed by the pressure and/or heat by the Earth’s movements. Marble is example of limestone chemically changed by the action of very high temperatures. When shale or mudstone is subjected to great pressure the grains are aligned and layered and becomes altered to crystals during metamorphosis and can become slate. Sometimes, metamorphic rock also go through another process of changes as before. Fossils are not found in metamorphic rock, but only in rare occasions in very low grade metamorphosed rocks.
Back to GZ on the Abbey Road. In front of you is a monument that is made from Portland Stone, a sedimentary rock. ‘Portland Stone’ is also found outside of the area of Portland as it refers to the type of rock rather than just the location of Portland on the south coast of England. So, how was it formed? It was formed during the Tithonian stage of the Jurassic period (152-145 millions years ago) when the seas were very warm and shallow, rather like the Great Barrier Reef as it is today. For many centuries sediments of Oolities, sea micro-organisms and sea shells accumulated on the sea bed. Warm sea is unable to hold dissolved gas so the calcium and carbonate ions in the water chemically interacts and form calcium carbonate. The water is squeezed out by the pressure of its own weight and the accumulation of calcium carbonate, called calcite, over time, started the process of cementation and the organic materials like shell and bones became fossilised. Portland Stone is about 95% calcium carbonate.
This type of Portland stone, today, is found in Dorset on the belt of hills known as the Jurassic Coast, a World Hertiage Site, forming the coastline to the English Channel. It has been quarried extensively, so why is this rock a desirable building material? Unlike most limestone rock, Portland Stone, have cemented extremely well and evenly, it has very few layers that is associated with many sedimentary rocks. Because of this it does not spilt along layered lines when carved. Stone masons have been working with this stone since the Middle Ages. It became very popular building material for London after the Great Fire of London in 1666 when the law was changed that all buildings must be made of stone or brick.
Stonemasons can carve this rock into all forms and make fine details on the surfaces, but it can affected by prolong weathering. In most cities acid rain from industrial pollution is troublesome and can dissolve the alkaline stone. Portland Stone is a porous rock and if it is not protected from constant moisture such as rain from above or water from the ground it will absorb this moisture. Freezing and thawing conditions can lift fine shavings of the stone off the surface or at worst split the rock. Builders protect this stone with roofing and flashing from the rain and damp proof membrane near ground level from the moisture in the ground.
For the tasks to claim this EarthCache, you will have to study the monument at the given coordinates and ‘Tell me what you see’ by answering the following questions:
1. On which face of this monument do you see the most seashells embedded in the Portland Stone; N, S, E or W?
2. Describe the shells and what is the approximate length of the largest?
3. Which part of this Portland Stone monument is weathering the most and what do you think is causing it to do so?
4. There have been some obvious repairs, estimate the percentage of new stone in this monument?
5. Optional fun question: Did the ‘Yellow Submarine’ dive in the warm seas during the Tithonian age?
Please send your answers either by email or through the messaging centre found at my profile, and thank you for taking time to learn about this Portland Stone. You may log this cache instantly. If your answers are obviously incorrect I will be in contact.