Church Micro 12945...Knaresborough EarthCache

The Chapel of Our Lady of the Crag is a beautiful Grade I listed chapel located deep in the Nidd Gorge, Knaresborough. It was carved from the cliff face by John the Mason in 1408, permission being granted by King Henry IV. The Chapel was probably built as a wayside shrine for pilgrims walking between the town and the Priory, and possibly going further afield. It would also have been a place of quiet and prayer for the quarrymen working in the quarry nearby. 

Geologically the chapel is a beautiful thing, carved into the rock here for over 600 years. The rock at first glance may appear to be sandstone and a little online research would quickly confirm that fact as most articles, including a popular online encyclopedia and the St. Mary's Church website both openly state the chapel is carved out of sandstone. However, the British Geological Society disagree - they confirm the chapel is actually hewn from Lower Magnesian Limestone - Upper Subdivision. The limestone is composed of large-scale (5-18 m. sets) of cross-bedded oolite.

This EarthCache looks at the rock of the Chapel of Our Lady of the Crag with a view to understanding why so many people have incorrectly identified the rock. You don't need any previous geological knowledge, you are primarily making observations and all the information you need is contained within the listing. Don't be put off by the length of the questions - I have done much of the writing so you can just read a little and write less!

If you wish to go inside the chapel please be aware it is only open to visitors on Sundays during the summer from 2 p.m. until 4 p.m. (usually from about Easter or the beginning of April to the end of September). Access to the chapel outside these times may be possible depending on the availability of a volunteer - you can make contact via the chapel website to request access. You do not need to enter the chapel to complete the EarthCache, you only need to study the cliff at or near the chapel. The gate at the bottom via the garden may be locked but you can usually access the chapel via a path to the east beginning at the foot of the cliff. At the time of publication access to the site at all times was possible but whether this will change in the future remains to be seen due to recent mindless vandalism on the 600 year old Grade I chapel. If access isn't possible for any reason, please look at the cliff at the edge of the site to answer the questions - it is worth looking at the cliff in more than one place to make your observations in any case. Abbey Road is a single track road with limited parking opportunities. Parking elsewhere and walking to the chapel is recommended although you may get away with parking nearby for a brief time, especially at quieter times.

The Nidd Gorge at Knaresborough is a marginal glacial feature cut at the edge of the Devensian Ice sheet during the last Ice-Age. At the start of the last ice-age (Devensian) the Knaresborough would have looked very different to what we see today - largely as the river and gorge weren't here. At that time, the River Nidd ran to the north and east of the present town. It deviated from its present course at Nidd, ran through Brearton and past Farnham to the northern outskirts of Knaresborough before heading eastwards. 

During the advance of the Devensian ice-sheet a thick fan of sand and gravel was deposited, this had been pushed along in front of the ice-sheet. This deposit is currently worked in the gravel pits north of Knaresborough. If you approach Knaresborough from the north via the B6166 from Boroughbridge, the extent of this buried valley, and its associated sand and gravel deposits, can be appreciated from the road. As the ice advanced further to the south and west it overrode the sand and gravel completely blocking the River Nidd and and diverting it westwards. After this had happen the river had no option but to find another route - so it exploited the lowest, softest rocks resulting in the present Nidd Gorge being carved out of the rock. The River Nidd has cut down through the Permian and Carboniferous sequence. 

If you can see well defined layers or bedding in the rock then it is quite likely you are looking at a sedimentary rock. Sedimentary rocks can be formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. An example might be millions of years ago a flood caused a layer of sand to be deposited. This might create one layer of the rock you see here. Then maybe a few years or many years later another flood left another layer of sand. Again, another layer is created. This process continued for millions of years, the lower layers gradually getting squeezed more and more by the heavy deposits above them. The thickness of the layer is determined by the amount of sediment deposited, therefore a bigger and longer flood would deposit more sediment creating a thicker layer.

The above photo shows a great example of sedimentary rock found elsewhere in the world. The sedimentary rocks you see before you here were deposited in a shallow sea during the Carboniferous period – about 320 million years ago. Sedimentary rock is not always consistent because each layer is formed at a different time and from a different 'batch' of sediment. This means some layers may be strong whilst some are more weak and therefore more susceptible to erosion forces many years later and could be formed from different materials.

Sandstone and limestone are both sedimentary rocks and usually are fairly easy to tell apart. Let's be honest, most people go by the colour. Sandstones are often red or yellow in colour whilst limestone is often grey or white. So when you arrive here and see a yellowish rock with a bit of a sandy texture it's easy to jump to a conclusion. That's why in most EarthCaches the descriptions are a bit more complex - you don't have to be a geologist to be able to identify a rock but you do need to consider multiple things to make your decision.

In general, limestone is defined as being composed of calcium carbonate. Calcium carbonate often comes from plant and animal skeletons and organisms and shells include mollusks, echinoids, and corals. Sandstone, on the other hand, is not described as coming from any one substance. It consists of sand sized debris, which variety from .0063 mm to 2 mm in size, and often (but not always) includes quartz. Sandstone is often composed of different elements that make up one large rock or surface like feldspar, mica, lithic fragments, and biogenic debris like shells.

Limestone is found in beds, and most limestone beds form in marine environments in which big deposits of organisms and carbonate precipitation build up over the years, like an ocean or large lake. Sandstone is shaped from the breakdown of larger rocks due to weathering and erosion as well as from processes that occur inside the rock, usually biologic but now and again chemical in nature. Many sedimentary rocks, including sandstone, display a visible stratification into layers. This visual cue can help determine how a rock came to be primarily based at the size and intensity of each layer. Limestone does not have the stratification pattern that sandstone does. Some limestone is composed completely of organic matter that is impossible to see with just the naked eye.

Below are some basic steps geologists might take to identify whether this stone is sandstone or limestone. In order to log a find on this EarthCache you will need to follow these steps and make some observations. Only brief notes are needed, don't feel you have to write an essay - but I do want to see enough detail to know you have been here. Please examine areas of cliff away from the chapel, ensure you are studying the cliff where it is untouched by man. Do not look at the carved areas as preservation work may have altered their appearance. Please send me notes on what you observed for the below points.

Step 1 - Does the cliff contain layers or beds?

Observe what you see in terms of layers/bedding and make brief notes. Please mention thickness of layers/beds you see.

Sandstone is created in layers (left). If you can see horizontal lines through the stone indicating these layers, you will likely find it is sandstone and not limestone. Sometimes the layers may not be horizontal if the rock has been tilted at some point in it's history but the layers should be clear to see. Limestone is made mostly of calcium deposits from large collections of shell, bone and coral and creates a more solid rock. It tends to form in thicker denser beds rather than layers (right). Remember to look at the whole cliff, not just the immediate chapel. 

Step 2 - Close study of what makes up the rock

Look really closely at the rock - a magnifying glass would help. Observe whether the rock is made of grains or something more dense such as organic remains or oolites.

You need to look really closely at the rock. Try and find an area where the rock has been recently exposed. Sandstones are composed of grains - 0063 mm to 2 mm in size, see the left photo. Limestones however are composed of calcite. The texture is usually different, often showing signs of organic matter, but it could have some carbonated grains. See the middle image for an example of limestone. A magnifying glass will help to look in detail - this is an oolite limestone so may have a concentrical structure as shown in the photo on the right.

Step 3 - How does the rock feel?

Does the rock feel coarse/sandy/gritty or does it feel smooth or silky?

Linking in with the previous step is the actual texture of the stone when you feel it. Sandstone does generally have a coarse touch, sandy or gritty where as limestone is often smooth and even silky to the touch. How does the rock feel here?

Step 4 - Colour

Describe the colour of the rock here. If the rock has been coloured by a mineral, try and look for areas that haven't had the colour altered. What are the colours you observe and what do you think is the original colour of the rock without mineral influence?

Colour isn't necessarily a reliable way to identify the rock but it's certainly worth considering. Sandstone may almost be any colour due to impurities within the minerals it contains, but the most common colors are tan, brown, yellow, red, grey, pink, white, and black. One online article states that limestone is always white due to it's calcium makeup but this is sadly untrue. As it is often wholly or in large part composed of calcium carbonate it could well be white but it may be coloured by impurities, iron oxide making it brown, yellow, or red and carbon making it blue, black, or gray. Gray limestone is quite common in this area.

Step 5 - Fossils

Can you see any obvious fossils or organic remains, if you do please describe them. Don't spend too long searching.

The presence of fossils doesn't necessarily help define the rock type but the type and quality of the fossil might and it is something that is always interesting to have a look for. Limestone often contains fossils of shelled sea creatures. Entire reef formations and communities of organisms are found preserved in limestone. The types of fossils found in limestone include coral, algae, clams, brachiopods, bryozoa and crinoids. Most limestone forms in shallow tropical or subtropical seas. In some cases, fossils make up the entire structure of limestone. The calcium carbonate of limestone allows very detailed and well preserved fossils, some may be large, some may be microscopic. It is not true to say the presence of a fossil automatically means it's limestone as sandstone can also contain fossils.  Since sandstone is a coarser material than shale or limestone, fossils found in them do not usually show as many details as fossils in shale and limestone and they are unlikely to appear in the quantity they could in limestone. Sandstone rarely contains delicate fossils. Sandstone can contain fossils of creatures such as trilobites, brachiopods, crustaceans, bryozoans and plants. Remains of land animals like mastodons and dinosaurs are much more likely to be found in sandstone. The Addlethorpe Grit (which is a type of hard sandstone) further up the gorge actually contains a fossilised tree-trunk so fossils are possible in the neighbouring type of sandstone, but they are unlikely to be marine creatures.  

Step 6 - Erosion/Weathering

Can you see any chemical weathering where the rock looks like it has dissolved and left harsh edges?

The one difference between sandstone and limestone weathering is the way it occurs. Sandstone is formed of grains that can be worn away by wind and water. This can form all kinds of shapes in the rock but the key thing is they will have smooth edges in most cases. The calcite in limestone reacts with the acidity of rainwater and this causes a reaction turning parts of the rock to a solution that runs away in the water. Both types of weathering can cause different shapes and holes in the rock but with limestone the process can cause some quite sharp or abrupt edges in the stone rather than the smoother edges of sandstone. This is by no means a definitive way of identifying a rock, but it may give you an indication.

Step 7 - Make your conclusion

Based on the evidence you have gathered, do you personally have enough proof to conclude that you believe this to be limestone? Alternatively you may have inconclusive evidence and believe an expert would be required to make more detailed studies. What is the main evidence that has made you come to your conclusion?

Step 8 - Please take a photograph

Please take a photograph of yourself or GPSr either outside the gate with the blue plaque or outside the chapel itself. Under revised guidelines this is now a logging requirement to prove you were there. Please avoid any close up photos of the rock that would give away the answers.

Please remember to send in your answers when you log or soon afterwards. I do appreciate these may need to be processed later but if no answers are received within a reasonable time frame your log may be deleted. No logs will be deleted where a reasonable attempt at the answers has been made so please do give it a go. I do read all answers and try to reply to them all but there may be a delay so please log immediately, don't wait for a response. Thank you for visiting the church micro here in the Nidd Gorge.

The tiny medieval Chapel of Our Lady of the Crag lies a stone's throw from the River Nidd, about half a mile from the centre of Knaresborough, North Yorkshire. It was carved from the cliff face by John the Mason in 1408, permission being granted by King Henry IV. The Chapel was probably built as a wayside shrine for pilgrims walking between the town and the Priory, and possibly going further afield. It would also have been a place of quiet and prayer for the quarrymen working in the quarry nearby. 

The Chapel lost its religious status at the Reformation but yet it has remained a popular site for visitors and pilgrims. The details of what the chapel contained in its early years are not known, nor what happened to the original statue of Madonna and Child, if there was one. The Chapel became a shrine again in 1916. The existing statue, which replaced one dating from 1919, was carved in Halifax in the year 2000 by sculptor Ian Judd from half a ton of Derbyshire gritstone, funded by a millennium grant from the Arts and Sacred Places organisation. 

Today the Chapel continues to be a place of pilgrimage and worship, as well as a place of gentle strength, peace and spiritual refreshment for visitors of all faiths and none. It retains its identity as a Marian shrine and is owned and maintained by a charitable trust, 'The Chapel of Our Lady of the Crag Trust'. Please see the website for a full history or further information.

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