In 1935–1936, he accompanied
Lawrence Wager as a petrologist on the 1935–1936 British East Greenland Expedition. They over-wintered on Greenland, with a party that included Wager's wife, Phyllis Wager (née Worthington), and Wager's brother (Dr Harold Geoffrey Wager; Hal) and sister-in-law (Elizabeth Mary Wager; Kit). Other members of the party included P. B. Chambers, Dr E. C. Fountaine, and fourteen
Inuit, from two families. They built and stayed in a camp with a three-roomed central house, including a physiological laboratory and mess room, on the Skaergaard Peninsula, at the entrance to
Kangerdlugssuak Fjord. In 1939, Wager and Deer published their report of the expedition. This treatise on the
Skaergaard intrusion is regarded as one of the most significant contributions to the science of igneous petrology of the time. In 1948 Deer led the NE Coast Baffin Land Expedition with
Chris Brasher, also of St John's College, to see whether the igneous activity of the Kangerdlugssuaq region of East Greenland and the
Disko Island area of West Greenland continued westward into Baffin Island. The reconnaissance was hampered by rough seas. In 1953, Deer and
Wager jointly led a British Greenland Geological Expedition back to Kangerdlugssuaq, where Wager had worked in 1930–31 and 1932, and Wager and Deer had overwintered in 1935–36. Five other geologists from Oxford and Manchester universities took part including C. J. Hughes (Oxford), G. D. Nicholls (Manchester)
G. M. Brown (Oxford), D. S. Weedon (Oxford) and P. E. Brown (Manchester). In 1966 Deer led another British Greenland Geological Expedition, with a party including G. A. Chinner (Cambridge), C. G. G. Born (Cambridge) R. Elsdon (Cambridge) P. D. Burnford (Cambridge) I. A. D. Sweetman (Cambridge) B. Atkins (Oxford) C. Kent-Brookes (Oxford) J. D. Bell (Oxford) D. G. Parrish (Oxford) J. D. Gunner (Oxford) P. E. Brown (Sheffield) C. D. Curtis (Sheffield) D. C. Dunn, Medical officer (Sheffield) D. Abbott, Research and Productivity Corporation (Sheffield) and N. McKinnon, P. A. Stirling, T. J. Sweeney, drillers. They carried out a programme of drilling and geological survey in Kangerdlugssuaq, extending the work of the 1953 British East Greenland Geological Expedition. ==Family==