Peach retired from the Geological Survey in September 1905 after serving for forty-three years. His retirement gave him time to pursue at his leisure a line of research that had always fascinated him since his early days with Huxley at the Royal School of Mines the technical description and illustration of fossils, and in particular the Scottish Carboniferous crustaceans. Peach was a very competent palaeontologist, a fact that tends to be overshadowed by his more famous Highland work. It was he who identified most of the fossils in the Survey Memoirs, the most notable being the Lower Cambrian tribolite fauna of the Northwest Highlands and it was to be eighty years before they were redescribed. His friend and colleague Edward Greenly records how, even in the euphoric days of the Moine Thrust discovery, Peach had growled, but give me something that has once been alive!
As a man of much sympathy for all living things, he found their dead remains a source of endless fascination, especially those of crustaceans. At various horizons in the Scottish Carboniferous there occur sporadically, isolated but very well preserved faunas of shrimps, probably of freshwater origin. Throughout the 1880s and 90s collections of these crustaceans had accumulated and Peach, being Acting Palaeontologist for the Survey, became custodian of these treasures as he called them. Several detailed papers emerged, culminating in his Monograph of 1908, with page after page of technical description and twenty plates executed with his usual artistic flair. He has been criticized for over-interpretation and for drawing structures which were not really there. But he knew a great deal about modern crustaceans and theory may have led him to draw in structures which were not there in fact. However, comparison of his drawings with the original specimens shows that these lapses were few and in all other respects the drawings are executed with meticulous care and accuracy.
This fine Monograph of 1908 was Peachs last major work yet he remained as enthusiastic as ever with all aspects of geology until the end of his days. Greenly, who visited him six months before his death in 1926, tells how Peach, then a sick old man, became so excited about the opportunity to discuss geological theories with his visitor that the grim-faced landlady had to eject poor Greenly while Peachs voice, still declaiming geology, pursued them down the stairs!