Mortimer's interest was stimulated by visits to the 1851 Great Exhibition, and the geological and archaeological exhibits in the British Museum, and by the private collection of Edward Tindall of Bridlington. Together with his brother Robert, Mortimer assembled a small collection of fossil and antiquarian specimens; they also trained and instructed many local farm workers to collect any likely specimens they found; locally such artefacts became known colloquially as 'Mortmers'. During the latter part of the 19th century the area became a magnet for collectors, in part due to publicity from his collection; most sought ancient stone, flint and bronze weapons. Competition arose for these artefacts, causing a rise in their monetary value. Through their contacts with local agricultural workers the Mortimers were able to collect a great many thousands of specimens.