In reply to davidwright:
> (In reply to pec)
>
> BGS has the Tremadoc crags as "microgabbro". The Intrusion that forms Bwch-y-moch and Pant Ifan is clearly visible as a pink band on their online geological map.
>
Thanks for that clarification, I did do an online search for maps etc but came up with nothing. I was hesitant to dismiss the Tremadoc is Dolerite theory outright as although I did a geology degree I actually don't really think about it much when I'm climbing. I've never consciously thought about it whilst climbing there and I've not been back since I read it was dolerite to look properly. Going from memory however I knew that it has none of the characteristics of dolerite which is why I was surprised.
> From Wikipedia it seems that the difference between a Gabbro and a Dolerite is mainly grain size so it would seem its either a small grained Gabbro or a large grained Dolerite depending on personal preference.
>
If its microgabbro that would explain a bit. Although chemically and mineralogically gabbro and dolerite are similar (as is basalt), rate and location of cooling, which is what results in the difference, are critical to physical appearance and can result in rocks which appear totally different.
As an example, the Rhyolite of say, Dinas Cromlech, will have a similar chemistry to the Granite of Cornwall or even Yosemite but in terms of appearance and characteristics to climb on they are totally different.
If Tremadoc is microgabbro it should really be described as such and not dolerite. As anyone who has climbed on Crag Lough will tell you, they are definitely not the same!