The Cambridge Expedition to East Greenland in 1929 set sail from Aberdeen on 2nd July in the sealing-sloop Heimland, which had been chartered from Tromsö, Norway. Taking the Fair Isle passage, the edge of the pack-ice was reached six days later, and entered in the region of latitude 71° 30′ N., longitude 15° 30′ W. Last summer proved unusually difficult for navigation in the East Greenland ice-current, and it was not until 4th August after a month in the ice that we reached land-water and anchored in Mackenzie Bay. From the Bay a course was steered west-wards along Franz Josef Fjord to the foot of the Riddar Valley on the west side of Kjerulf Fjord. Here the party divided; the ship-party with two geologists during the next fortnight studied the tectonics shown in the great cliff-sections bounding Franz Josef Fjord and King Oscar Fjord (Davy Sound) and their branches (Fig. 1), while a second party of six was landed near Riddar Valley in order to map the tract of country lying immediately to the west of the head of Franz Josef Fjord (Fig. 2). Their ultimate objective was Petermann Peak, discovered by Payer and Copeland of the German Arctic Expedition of 1869–70, and estimated to be 11,000 feet high. A hurried reconnaissance in 1926 to the Cambridge Peaks had shown that the most likely route to the interior would be westwards along the watershed between the head of Kjerulf Fjord and the Nordenskiöld Glacier. The land-party left Kjerulf Fjord on 6th August, reached Petermann Peak on 15th August, and returned to base on 18th August. The present paper embodies the geological work carried out during the mountain journey, and is based on collections made by Mr. J. M. Wordie and Mr. V. E. Fuchs.