The Caledonian Orogeny produced highlands (Southern Uplands, Anglo-Brabant, Welsh) by crustal shortening and uplift. Post orogenic granite blocks such as the Alston and Askrigg were also emergent. These persisted while crustal extension in the early Carboniferous created a series of rift basins around them. When rifting slowed the thermal relaxation of the crust led to thermal sag basins.
To put the Carboniferous into context I re-read the Open University book in the title of the post. I possess no artistic skills at all but have drawn the following cartoons which shows the main stages with no reference to paleolatitude. They are obviously not to scale or the correct shape either!
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| Scotland and the rest of the British Isles are on different landmasses |
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| Avalonia separates from Gondwana and begins its journey across the Iapetus |
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| The Iapetus closes, the Rheic opens |
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| Laurentia, Baltica and Avalonia collide in the Caledonian Orogeny as the Iapetus finally closes |
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| The Rheic closes and Pangea forms in the Variscan Orogeny |
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