Complex sentence: The mudstone is only visible at that quarry site, across the stream from Bradgate House, but covers much of the valley floor around and beneath Cropston Reservoir,--demonstrating that the present Lin valley was also there 220 million years ago.
Split into simpler sentences: The mudstone is only visible where it was extracted, across the stream from Bradgate House. However, it covers much of the valley floor around and beneath Cropston Reservoir,--demonstrating that the present Lin valley was also there 220 million years ago.