In the early Palaeogene period between 63 and 52 Ma, the last volcanic rocks in the British Isles were formed. As North America and Greenland separated from Europe, the Atlantic Ocean slowly formed. This led to a chain of volcanic sites west of mainland Scotland including on Skye, the Small Isles and St. Kilda, in the Firth of Clyde on Arran and Ailsa Craig and at Ardnamurchan. Sea levels began to fall, and for the first time the general outline of the modern British Isles was revealed. At the beginning of this period the climate was sub-tropical and erosion was caused by chemical weathering, creating characteristic features of the Scottish landscape such as the topographical basin of the Howe of Alford near Aberdeen. The vegetation of the period is known from Palaeocene sedimentary deposits on Isle of Mull. The rich flora here included temperate-climate tree species such as plane, hazel, oak, "Cercidiphyllum", "Metasequoia" and ginkgo.