The Department of Geology at Amgueddfa Cymru holds important biographical and historical documents related to the early development of the geological sciences. A major part of the collection comprises of the papers of Henry Thomas De la Beche (1796-1855). De la Beche was a geologist who was active during the first half of the 19th century when the foundations of the new science of geology were being laid. Initially pursuing geology as a gentlemanly hobby, financial difficulties led him to work as a geologist for the Ordnance Survey. This led to the establishment in 1835 of what became the British Geological Survey, with de la Beche as its first Director. He also established the Museum of Economic Geology (later the Geological Museum, and now part of the Natural History Museum, London), the Royal School of Mines, and the Mining Record Office. The Department holds an extensive archive of de la Beche's papers, including over 2000 items of correspondence, drawings, notebooks and photographs.

The Department holds one of the largest collections of the famous geological map of England, Wales, and part of Scotland by William Smith. Current research aims to produce a monograph of the geological maps of William Smith. The Department also houses the correspondence of George Robert Vine a Sheffield-based stay or corset maker and amateur naturalist who specialized in the study of fossil bryozoans and coal measure plants.