Geology

Geology

Co-ordinator:  Michael Regan

Tel:  01565 651406

Email: info@knutsfordu3a.co.uk

Regular meetings are currently taking place using Zoom. The first of these was on 20th April and included the showing of a Youtube video of the 2018 eruption of Anak Krakatoa, a talk on a free online course on geochemistry from Kyoto University and a presentation on the geology of part of the Northumberland coast.

Prior to the lockdown the group met in members’ homes on the third Monday of the month at 7.30.  Please contact Micheal Regan if you wish to join the group.

U3a geology field trip to Ashford-in-the-Water, October 2021

Six members of our geology group met up in Ashford-in-the-water on Wednesday 6th October- a glorious sunny day- and immediately went for coffee. After admiring the fine buildings of Ashford and the historic bridge over the River Wye we walked the short distance to the now abandoned quarry/mine that used to produce the famous Ashford black marble. This rock is not a marble at all but a very dark limestone which polishes to a deep black colour and was originally deposited in shallow tropical seas about 330 million years ago. We found the two entrances to the mine (now securely barred) and many samples of the black marble (photograph 1).

Photograph 1:- Members of the group examining samples of black marble. The top of one of the mine entrances can be seen on the left.

We then walked back to our cars and drove to the Cock and Pullet in Sheldon for lunch. After lunch we walked over to the nearby Magpie mine ruins (photograph 2). This was originally the site of several lead mines that were worked up to 1958 and the Magpie mine was the last working lead mine in Derbyshire. We found many fossils in the limestone blocks used in a stile on the walk to the mine as well as in the walls of the buildings (photograph 3).

Photographs 2/3:- The ruins of Magpie mine (left) and members of the group examining the stone work for fossils (right).

As well as crinoids (photograph 4), which are a primitive relative of the modern sea lily we also found masses of brachiopods including the impressively large gigantoproductids. We also found some small samples of lead ore on one of the spoil heaps near the buildings.

Photograph 4:- There are many crinoids in this limestone block forming the wall of the old engine house.

Mark Barley November 2021

U3a Geology field trip to Alderley Edge, September 2021

On Friday September 17th 8 of us met up in the National Trust car park on Alderley Edge for a geologically themed walk around the local area led by Mike Regan. We started in Church quarry (photograph 1) where we found smooth sandstone, which had been quarried for building stone (used in local churches, hence the name of the quarry), was covered by a substantial layer of hard conglomerate. This conglomerate (known as the Wood Mine conglomerate) contained many rounded pebbles up to a couple of inches across. This deposit had been laid down during flash floods in a desert environment between 250 and 240 million years ago. Whereas a foot of the underlying sandstone may have taken millions of years to accumulate a foot of the conglomerate could have been deposited in a single flash flood.

Photograph 1:- Inspecting the rock strata in Church Quarry. The line between the smooth sandstone and the overlying conglomerate is clearly visible.

Similar conglomerate rocks were seen at several other locations in our walk around Alderley Edge. The photograph below (2) shows a closeup of a conglomerate at Castle Rock, above sandstone that had been deposited as sand dunes in a Sahara-like desert. The fact that the conglomerate overhangs the sandstone (protecting it from the weather) suggests it is harder and more resistant than the sandstone. In fact this is an unconformity:- the sandstone layer was originally about 50 meters taller, but was eroded back to its present level before the conglomerate was deposited on top

At Stormy point we searched for small flecks of Malachite and Azurite (several were found), copper ores that had been extracted from the nearby mines. We also had a look at the entrance to Engine Vein mine (photograph 3) from outside the fenced off compound. Mineralization is often due to hot fluids containing minerals circulating through faults and other weaknesses in the rocks. This is clearly seen in the photograph of Engine Vein mine where the miners have followed a fault, hacking out the minerals as they went. Note the green patch (of malachite) on the side of the rock cleft. We completed our visit by having lunch in the nearby Tea Room.

Mark Barley November 2021

Photograph 2:- A close up of the unconformity between the sandstone below and the conglomerate (note the size of the pebbles) above.

Photograph 3:- The entrance to Engine Vein mine. Note the near linear cleft in the rocks (probably following a fault) and the green patch (Malachite) on the (otherwise) orange and black rock wall (Photo Roger Brown).

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