THE CASTLE IS CURRENTLY FENCED OFF FOR SAFETY REASONS MAKING IT DIFFICULT TO VIEW THE FLINT COLOURED RED IN THE PHOTO. I HAVE ADDED QUESTION Q1A AS AN ATERNATIVE.
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Odiham Castle is a 13th Century royal hunting lodge known locally as "King John’s Castle" It is a scheduled ancient monument and is managed by the Basingstoke Canal Authority on behalf of Hampshire County Council. For reasons of safety and security Hampshire County Council discourage geocaching on their land during the hours of darkness. The site is accessed from the towpath running beside the Basingstoke Canal between North Warnborough and Greywell. Parking for about 6 cars is available at Tunnel Lane.
Since this cache is an EarthCache there is no physical container to search for. Instead, you will need to look at flints that have been used in the construction of the castle and also look at two large sarsen stones to the front of the castle. You will then need to answer the few simple questions set out below.
SEDIMENTARY ROCKS
Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks.
Clastic sedimentary rocks such as breccia, conglomerate, sandstone, siltstone, and shale are formed from mechanical weathering debris.
Chemical sedimentary rocks, such as rock salt, iron ore, chert, flint, some dolomites, and some limestones, form when dissolved materials precipitate from solution.
Organic sedimentary rocks such as coal, some dolomites, and some limestones, form from the accumulation of plant or animal debris.
LOCAL BEDROCK GEOLOGY
The bedrock geology of the general area comprises a mix of Palaeogene clays, silt and sands to the north around North Warnborough and Odiham Castle and Cretaceous chalk to the south between Odiham and Greywell. These are all sedimentary rocks.
The sands and clays around Odiham Castle contain a few flints. These mainly comprise relatively small rounded stained flint pebbles which generally lack fossils. In comparison, the chalk around Greywell contains abundant seams of large nodular flints. The flints are black or bluish black, sometimes mottled grey, with a thin white cortex. These flints commonly enclose bi-valve shell fragments.
THE FLINTS
The co-ordinates are for the arched entrance to the inner part of the castle. The castle was originally faced with dressed stone but all that remains now is the inner core formed by nodules of flints in a pale coloured cement.
Flint is a hard silica-rich form of the mineral quartz and it normally occurs as banded nodules of various sizes and shapes within chalk. In geology, a nodule is small, irregularly rounded knot, mass, or lump of a mineral or mineral aggregate that typically has a contrasting composition from the enclosing sediment or sedimentary rock. Inside each nodule, the flint can be dark grey, black, green or brown in colour due to varying trace elements. For example, the colour can be caused by inclusions of organic compounds (black), metal sulfides (black), and various metal oxides and hydroxides (yellow, orange, brown, reddish, etc.). Flint is slightly translucent to almost opaque, sometimes only thin chips are translucent at the edges. On freshly broken surfaces the luster is dull or waxy but because it is very hard, flint can take on a glassy luster over time. A thin layer on the outside of the nodules is usually white and has a rougher texture. Some flints may enclose fossils or shell fragments such as bi-valves, sponges and sea urchins.
THE SARSENS N51 15.678 W000 57.704
Sarsens are a relatively unknown quantity. Little is known about their age, formation, or even where they formed.
Sarsen stones are a very hard resistant form of sandstone. This consists of grains of quartz, a white clear mineral with a glassy appearance, together with grains from various other rocks. The grains may have come from deserts or been washed into the sea by rivers. They were later cemented together by silica that slowly trickled into the porous sediment and infilled the pore spaces to make a uniformly very hard and resistant rock. If the sun is shining you may be able to see the light glinting off some of the grains within the sarsens. Sometimes veins can form in sandstone when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation to form a distinct sheet like body of crystallized minerals within the rock.
Sarsens can be white, grey, yellow or orange depending on the mixture of minerals. Sarsen stones in Wiltshire are usually made up of quartz sand in a siliceous cement and are locally known as greywethers. In Hertfordshire the stones are known as puddingstones and contain much larger pebbles and in Essex the stones have a greater iron content.
QUESTIONS
PLEASE NOTE THAT ODIHAM CASTLE IS A SCHEDULED ANCIENT MONUMENT AND YOU SHOULD NOT ACTUALLY TOUCH ANY FLINT ON THE CASTLE WALLS.
Please walk through the entrance arch to the castle and look at the left hand side. You are looking for the specific flint nodule shown coloured red in the photo located approximately 30 cm/12 inches above the top of the start of the green metal fence. BUT SEE QUESTION Q1A BELOW IF THE FENCING IS STILL IN PLACE.
THE FLINTS
Q1 Please describe the colours that are present in the nodule and describe the size and shape of any visible minerals or fossils present.
ALTERNATIVE QUESTION Q1A WHILE THE CASTLE IS FENCED OFF
Q1A Please describe the colours that are present in the nodules to the right of the entrance arch and describe the size and shape of any visible minerals or fossils present.
Q2 Using the information that you have read on the cache page and the evidence you have found at on site, please explain, giving reasons, where you think the flints possibly came from.
THE SARSEN STONES
Q3 Please describe the colour of the stones and describe the size, shape and location of any visible minerals or fossils present.
Q4 Do you think these stones are examples of “puddingstones”? Please explain your answer.
BEFORE LOGGING THIS EARTHCACHE, please use the message centre or email me with your answers to the questions above. Please include the GC number and cache title. Please do not put the answers in your log.