Remapping the Builth district on a larger scale led to conclusions regarding the structure and history of the region fundamentally different from those of previous investigators. The general succession is summarized and the strata, chiefly ashes, lavas, and shales, are described in detail. The geologic history involves an interesting period marked by the presence of a rocky shore with boulder-strewn sands and high cliffs of volcanic rock, the whole later covered by sands. Subsequently three great belts of faulting were produced in the region, the country being shifted along great tear faults a quarter to a half mile, the aggregate movement being nearly a mile in a little more than three miles of strike.

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