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Saturday, 21 March 2015

Geology, Calderdale, Dry Stone Walls, Millstone Grit, Coal Measure.

Posted by Gordon Jackson

Hello happy readers,

Spring is on the way. In another week or two, there will be plenty of things in the fields and woods to examine and enjoy. Get your cameras ready and batteries charged.

In my last post, I said I had one or two other posts lined up. Soon after that, three out of our four computers started to act up. In the end, I had to send one for repair, order a new 500Gb solid state drive and double the memory for another, and reformat / rebuild the third. It's been a long job, with daily life having to carry on as well. So, at last, here is another post. It will be a short one this time.

As I was walking through Robber Dodge with Charlie Streets, almost two weeks ago, I told him about how the stone in Stainland changes from Millstone Grit to Coal Measure. Coal Measure is the thin, slate/shale like stone and Millstone Grit the large, rough shaped pieces. Geologically, Coal Measure lies on the top of Millstone Grit. This difference in stone round Calderdale is a feature that many people don't notice, assuming it all to be Millstone Grit. There are lots of places about where the walls are mixture of both types. You can see the stone changing if you walk from the top of Stainland down to Holywell Green. It is also apparent in many other areas around Halifax and Brighouse. There is a striking example of this change in stone down Robber Dodge. I'll Show the images now and talk a little more after.

Millstone Grit is on the left of the 
gatepost, Coal Measure on the right.

You can see that the top right-hand side of the wall has some Millstone Grit at the top, just under the capstones, or coping stones, which top the wall off, preventing it breaking apart. Also, the left side has a similar mixture of stone, with less Coal Measure at the bottom. In the image below, I've removed the gatepost to show the comparison more clearly.

Side by side stone comparison.

The next shot is the top of a wall which is being repaired. Dry Stone walls are composed of two outer layers with a filling or hearting  layer in the middle. (See the illustration below). The walls are built up to the desired height layer-by-layer (course by course), becoming narrower as the wall height increase. Some walls have large, flat tie-stones, or through stones, going right through them, these strengthen the wall. Some may stick out at both sides, and can be used as styles.The construction of a dry stone wall and fact that they have no mortar binding them, makes them so durable. Some of these walls can be centuries old.

Note the hearting, or filling of smaller stones
between the two outer face of the wall.

Still on the subject of the types of stone found in this part of Halifax,

A geologist many years ago told me that the area in West Vale is of great interest to geologists. If you stand at Salterhebble Guillotine Lock, facing towards Halifax, you have the Copley Road to your left and Halifax road to your right. Now, here my memory fails me and I've not yet been able to sort out which way round
 what I'm about to tell you is correct. I'm sorry about this. If anyone knows the truth, please add the correct information at the end this post, in the comments section.

However, the point is, at one side of the valley, (say for example, the Copley road side of the valley) the stone is Coal Measure and the other side, (in this example, the Halifax road of the valley) is Millstone Grit. It was explained to me that this divide is an important geological feature which geologists will travel some distance to see.

Still loosely on the dry stone wall theme, to finish off this post is a photo I took on the same walk. It caught my eye in the way fellow photographers will be well aware of. It reminded me of an Otter and is an unintended sculpture, but a pleasant surprise for me.

Steel Otter sculpture - (gate catch).

Well, I hope this post will encouraged some of you to go out and examine the stone around you. 
Until the next time, be safe. Gordon.

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