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Historical background: Early exploration in the East African Rift—the Gregory Rift valley

In relation to modern lines of communication it seems surprising that the Gregory Rift Valley was the last part of the system to become known. Much of the earlier exploration had however been centred on the problem of the sources of the Nile, and in consequence the Western or Albertine Rift was explored by Samuel Baker as early as 1862/63 (Baker 1866). Additionally there was a strong tendency to use the convenient base at Zanzibar Island for journeys inland by the Arab slave trading routes from Pangani and Bagamoyo; these led to the Tanganyika Rift and Nyasaland rather than to the area of modern Kenya. The first penetrations into the Gregory Rift area were in 1883; Joseph Thomson made an extensive journey into Central Kenya which he described in his book of I887, ‘Through Masai Land’ which had as a subtitle, ‘a journey of exploration among the snowclad volcanic mountains and strange tribes of Eastern Equatorial Africa—being the narrative of the Royal Geographical Society’s Expedition to Mount Kenya and Lake Victoria Nyanza 1883–84’. In his classic journey Thomson practically encircled the lower slopes of Mount Kilimanjaro and reached the Gregory Rift wall near the Ngong Hills. He then went north to Lake Baringo and westwards to Lake Victoria, before returning to his starting point at Mombasa. His observations on the geology were of good standard for the time. For example he referred to ‘enormous masses of porphyritic sanidine rock forming a lava cap to the underlying metamorphic rocks’ on the