Being the adventures of a very amateur geologist.
You can read about my recent trip to Cwm Idwal here
Geology shopping links:
The OneGeology website is pulling together a number of resources for geology that are available online.
There are some fantastic images available over at the USGS website.
The predominant geology of Colwyn Bay seems to be limestone or possibly Mudstone.
A recent visit to the Great Orme gave some 'interesting' images. I have a little wander around the open quarry area, near the car park, and made a couple of observations. I didn't take a picture of the overall quarry area but here is a segment of it.
As you can see the exposed rock is very layered. The gaps between the layers seem to be filled with very crumbly material that is also wet. It must be more permeable to water so any ground water finds it's way down through the fissures in the layers and gets soaked up in the crumbly layers. This may be a method of forming faults and fractures. If a layer is dissolved away then the rock above will have a void beneath to collapse down into.
I assume these crumbly layers correspond to a different depositional mode/material than the more solid, fossil bearing, layers.
It would appear that near the bottom of the exposed layers there were certainly fossil indentations and evidence of shells etc.(there is a picture below) But the fossils were less numerous and less distinct than the ones at the top just underneath the topsoil. These next pictures were taken at the top of the quarry (presumably the youngest layer)
The texture of the rock is very rough with numerous shells and other fossil types. The shells seemed to have more 'original' material in them and have also retained more detail in the fossil.
You can almost get a feel for what the original sea floor must have looked like. Piles upon piles of shells all nestled together.
At the bottom of the exposed layers the shells appear to be less well formed and not as numerous. The matrix also seems to be smoother.
I am not sure why this is but potentially there may have been fewer shell- bearing creatures around at the time of deposition or the longer time period and presumably greater heat and pressure has resulted in the other material being transformed or dissolved in some way. You can clearly see the friable layers in the above image. Click on it to see a larger version.
(click on the image for a much larger picture ~3.3MB)
On Sunday the 17th September 2006 C.E. I went for a walk/geological field trip around Cwm Idwal.
With trusty hammer in rucksack I attacked the mighty mountains. Cwm Idwal is an example of a geological formation formed by glacial erosion. I was hoping to identify a few formations that I have seen on TV (Earth Story) or read about in books.
Arrived at Cwm Idwal ~9a.m after listening to Radiohead all the way from home on an overcast and drizzly day. £3 pounds bought me all the parking I could handle and I set off, on my own, up the nicely paved path.
I've read that veins of milky quartz are formed when very hot water containing dissolved silicon dioxide (quartz) seeps into cracks and fissures in rocks and cools. When the water cools the dissolved quartz drops out of solution forming the white streaks. It is likely that this happens deep underground where rocks are undergoing metamorphic transformation under high temperatures and pressures. Presumably the water is present within the rocks and so is the quartz. One part of this that concerns me is that the dissolved quartz should not fill the entire void when it comes out of solution. Unless of course the milkiness of the quartz means that it is heavily contaminated with water and dropped out of solution like a gel. More research required on that one.
Anyway I noticed that some of the sheer rock walls around Idwal have traces of quartz planes that would have been squeezed into any fault planes present during the metamorphic stages where the quartz veins were formed. It isn't overly surprising that the rocks would have fallen off exposing these layers as they are at natural fault planes and they don't seem to act as a particularly strong glue between the two rock surfaces.
The arrow indicates an easily seen example of a quartz plane remaining behind after the rest fell off. The blocks underneath also had traces of the quartz veins on exposed surfaces. Although I didn't look very hard there didn't seem to be much evidence of the fallen rocks having quartz veins within them. This would suggest that where veins exist they provide a fundamental weak point that allows weathering and gravity to bring the stone down.
This next image is of a medium sized boulder dropped by the glacier on it's retreat (probably). If this was in the middle of a valley miles away from the mountains it might be called an erratic.
(click on the image for a larger picture)
I think the dominant feature in Cwm Idwal (apart from the lake) is a geological feature evident on the back wall of the Cwm. At the top you can see a gash in the rock and beneath is a lot of scree. The cut seems to be in the centre of a geological fold.
(click on the image for a larger picture without the arrows and lines)
I'm not sure what it means. It may even be artificial. By the time I got around to that side my knee was hurting so I didn't get very high up. I'll have to go back to investigate another time.
This odd looking boulder caught my eye for two reasons:
(click on the image for a larger picture)
The first reason is it's overall pattern of it's faults. It's vaguely suggestive of something, some stress or movement. The second is the speed stripes that run along the top. This feature is (at least I think it is) an example of scoring from glacial erosion. I was hoping to see an example of this in the area. By the end of the day I was surprised how little there was of this type of markings. This rock may have been part of the rock wall or may have been one of the rocks suspended in the glacier as it gouged out the land. I favour the latter position.
What, I hear you ask, appears to be the predominant geology type of Cwm Idwal?
Good question! And one I can only guess at being a very amateur geologist.
By the time I'd made it to the above rock I had come across a few boulders that were suggesting to me that the area is largely made up from sedimentary and sedimentary metamorphic rock types. Up to this point I hadn't seen much that suggested any igneous rocks knockin' about.
(click on the image for a larger picture)
This massive boulder part of which can be seen in the above image appears to be a conglomerate. I chipped a couple of bits off and had a good old amateur stare. It appears to be a conglomerate that has undergone metamorphic conditions that partly re-melted it. Also some of the larger grains appeared to have weathered a lot quicker than the matrix as there were a number of little dips in the surface.
The only sandy looking block I found was just next door to the above.
(click on the image for a larger picture)
This is a super-macro image of Lichen on the sandy looking boulder. There was an awful lot of lichen on all the boulders around Cwm Idwal. I believe they are probably highly active in the erosion of the rocks. Here some of the little nodules in the lichen are actual quartz grains.
What settled the geology for me was the section next to the climbers:
Notice the quartz vein bottom left.
(click on the image for a larger picture)
(click on the image for a larger picture)
Lines added for clarity.
There are whole sections that represent depositional layering were soft rocks have been deposited and then buried and the soft rocks that have weathered more than the surrounding material leaving gaps that delineate distinct layers.
This next picture appears to show a fairly substantial deposited rock which has mostly worn away. Although the colour makes it look like sand stone I believe that it is mainly due to lichen or other organisms and not the actual rock colour. I had a bit of a scrape of the soft rock and it was indeed noticeably softer than the matrix and quite white-ish.
There was some evidence of igneous rocks (particularly a little further on from this section) but I didn't take any pictures of those. There were a few blocks that seemed to have large-ish quartz crystals embedded within but the majority of the rocks available seem to be sedimentary and/or metamorphic. Where the streams cascaded down the sides there also appeared to be some greenish stones (of which I took a sample) I hope to identify this in due course.
So there you go. My first geology field trip. I used my hammer and my little lenses and everything. It was all most invigorating.