Geology in the Lakes

Part of the life of a guide is undertaking continuous professional development (CPD) - either on our own or as part of a group. This week saw me in Cumbria to attend a Geology CPD in the northern Lake District. It was one of the few fabulously sunny days of recent weeks so to spend it inside was a little galling!  However, the morning lecture was excellent, telling us the story of the rocks of the Lake District and its wider county.  After lunch we visited the Threlkeld Mining Museum which is a treasure trove of every possible rock and mineral found in Cumbria.  Although the Lake District is thought of as a predominantly sheep-farming landscape today, for hundreds of years it was an industrial hive of activity.  In fact, we could say thousands of years if we go back to the production of hand axes in Langdale.  This became a significant site 6000 years ago during the Neolithic period and continued for perhaps a couple of thousand years - exporting axes right across Britain.

Langdale axes in the Threlkeld Mining Museum

But geology is a massive topic so today I just want to focus on one rock which is the one we went to see at the quarry behind the mining museum.  It might seem strange to have a favourite rock - but I do.  And it’s granite.  That is probably because it is the predominant rock found in the south west of England, in a place which is very special to me. The Cornish granite is black and white with huge crystals of quartz and feldspar and is either rough and angular or been worn into smooth pebbles by the sea. Either way, it sparkles and glistens and is just a joy to hold and look at.  When I moved to Cumbria I came across Shap Granite. This is a very special rock because the feldspar crystals are pink - it’s really very unusual and beautiful.  Granite takes a polish very well so, unsurprisingly both Cornish and Shap granite have been used as decorative building stones.

Polished rocks in the Museum

What I hadn’t come across before is microgranite. Like the name suggests, this is granite - but smaller.  So it really doesn’t look like granite at all and the individual crystals are only properly visible with a hand lens, not the naked eye.  However, it has the same chemical composition so who am I to argue with the geologists!

Granite is an igneous rock that forms from magma cooling slowly beneath the Earth’s crust. The size of the crystals in granite is determined by the cooling rate: larger crystals indicate slower cooling at greater depths, while finer-grained varieties like microgranite form closer to the surface, where cooling is faster in geological terms.

What we went to see however, was not the microgranite itself but its contact with the country rock. In other words, the rock which the granite forced its way up into.  The area around Threlkeld is interesting because it is more or less a boundary between the Skiddaw Group of rocks and the Borrowdale Volcanic Group.  Although the country rock in the quarry is BVG it has quite a slaty, flaky appearance.

Contact between the microgranite (left) and the country rock (BVG) on the right

In very basic terms, what happened is that the granite forced its way through the rocks which were already there and, as you might expect when boiling hot liquid rock comes into contact with another rock, it ‘cooked’ the rocks it was close to. How close isn’t quite clear!  In other words, that ‘cooking’ could have continued for many metres or hundreds of metres.  But what it touches, it changes.  So the flaky nature of the country BVG rock could be what has happened through metamorphosis due to the granite’s intrusion (or may have happened earlier).

What I found really interesting were the little bits of country rock that were broken off and became trapped in the granite.  It was like seeing a moment in time, suspended forever.  These little fragments have the fabulously useful Scrabble name of xenoliths.

The quarry itself also seemed like it was in suspended animation:  various bits of machinery were dotted around as if they had just been left there at the end of the day, ready to be started up again the following morning. It closed in 1982 after just over 100 years of providing ballast - initially for the Penrith-Keswick railway.

To top off a fantastic day, we were treated to some gorgeous light over Blencathra which looms over Threlkeld and is an ever-present backdrop in the area.

Evening light over Blencathra from the Threlkeld Mining Museum