I made an observation about the rivers and valleys around the Talyllyn valley whilst looking at Bryneglwys. At this point any connection with mining ends, but maybe somebody on here knows about geology so they might know.

I observed that there are at least 3 rivers which look like they once flowed north-west across the Talyllyn valley at a higher level:
Glyn Iago on the south side aligns perfectly with Nant yr eira.
Nant Gwernol (which comes down from Bryn Eglwys) still cuts across to Nant myniawyd.
Nant dolgoch aligns with a wide high level valley on the north side.
So it looks like the Talyllyn valley cut through a number of north-west flowing rivers, cutting two off completely, and probably Nant Gwernol still follows the original path because of moraine around Abergynolwyn.

BUT, the Talyllyn valley is on the line of the Bala Fault, which must predate the formation of the rivers that I refer to, so the Talyllyn valley must also pre-date the rivers?!

I can't understand what sequence of events made the landforms that we see today.

Any experts care to comment?

Chris.

https://maps.nls.uk/geo/explore/side-by-side/?fbclid=IwAR34Qt2Gu0Ml0wPtArCekbWAHyou_VEvomjxbE--nWd3h-8NxmQHRiUMoi4#zoom=13&lat=52.63069&lon=-3.96808&layers=10&right=LIDAR_DSM_1m 

I am probably barking up the wrong tree (or valley), but didn't Tom Rolt mention something about at least one of the rivers in that area being blocked off and diverted? By nature not man!

I seem to remember from Geography the process is called 'River Capture'. Definition:
'The natural diversion of the headwaters of one stream into the channel of another, typically resulting from rapid headward erosion by the latter stream.'
I remember an example of this was in mid- Wales.

Chris' observation of the SE-NW rivers being truncated by the fault valley looks spot on. I wonder though if the Bala Fault wasn't actually reflected by a valley until glaciation. So you could have three rivers crossing the fault at altitude, then glaciation digs it out along a line of weakness, truncating the rivers?

The Dysynni flipping valleys through a 300m high ridge, and the 'misfit' Afon Fathew suggests that something rather dramatic has occured here. I've seen some glacial overspills at Abersoch and above Bethesda, but this one is a monster.

I wonder though if the Bala Fault wasn't actually reflected by a valley until glaciation. So you could have three rivers crossing the fault at altitude, then glaciation digs it out along a line of weakness, truncating the rivers?

Indeed, that is the only explanation I can see - but it just seems so unlikely that the fault would exist but not be the dominant feature for 10's of millions of years, then very recently be cut down through everything else!

I can get the Dysynni cutting through to the next valley as I suspect there is an immense terminal moraine at that point, damming up the valley, and the Dysynni took an alternative route through the pre-existing valley as the moraine lifted it sufficiently high to reach it.

But the other two...

Chris.

Maybe the arrangement of the Dysynni has been repeated in the past.
Previous moraines have permitted rivers to flow across the valley for extended periods before subsequent glaciations have deepened and cut them off.

Chris.

Multiple glaciations have affected Wales over the past 2.5 million years, so you're looking at a superimposition of events.

Possibly the most impressive relic from the last glacial episode is the Graig Goch landslide. It's just downstream from the lake at Talyllyn and is marked by the vast area of hummocky ground. If you look over to the southern side of the valley you will notice the scar - best viewed on satellite images, it's so massive! It's why the lake is there.

I'd not put those two together. I presumed the deposit at the foot of the lake was a terminal moraine.
I also presumed that there was a 2nd one at Cedris, a 3rd one at Abergynolwyn (still there), pushing the Gwernol across the valley into the Dysynni.
And maybe there had been a 4th one at Dolgoch after a previous glaciation.

I guess there will be small terminal moraines dotted about but you need to reflect that at last glacial maximum the Welsh mountains ice-cap drained through the valleys and met the Irish Sea ice-river in Cardigan Bay. That ice-river extended all the way past the Scilly Isles at its peak.