[QUOTE 3431793, member: 9609"]Thanks for that, fascinating stuff even though I am struggling with it. Any descent websites you can suggest that might help me identify my own ?
One thing that puzzles me - Since Dolerite does not seem to form any of the bed rock in the area they were sourced (should have been Andesite) and since the stones are lifted from fields rather than quarried, is it likely they have been dumped here from glaciers during the last ice age and have just broken up in the fields over the millennia?
Even though the wall does look a multitude of colours, I reckon it is all pretty much the Dolerite that you have identified, although there are occasional (1-2%) pieces of a very red sandstone.
edit; (one final thing about this bloody rock) it must contain some iron, or at least one of those very powerful magnets found in hard drives, sticks to it.[/QUOTE]
I did the Open University geology courses - there's nothing like learning from experts who do this sort of thing every day! If you don't want to go to all that expense, time and hard work (understandable!) try
this web page - it seems well thought out with a whole set of questions to lead you through to an identification.
The dolerite will have come from a fairly narrow dyke. You've probably seen videos of those Icelandic eruptions which have lava erupting from long fissures in the ground. That lava will cool rapidly to form basalt. The rapid cooling means that large crystals don't have a chance to form so you end up with a fine grained rock. The lava's being fed to the surface through a feeder dyke - once the eruption ends, the magma in that dyke will cool slower slower and form dolerite.
Your rocks are rounded. That suggests to me that they've been reworked by water. Possibly they were then transported to their final position by glaciation and dumped in a moraine - if they're amongst a whole range of unsorted and mostly angular material with a wide range in sizes, that would suggest glacial transportation. Otherwise, it's quite possible that they were moved by some ancient stream or river that is long gone.