John "Jack" Walter Gregory and the Great Rift Valley

John "Jack" Walter Gregory was born in London in the year 1864. Already in early age his interest in natural sciences and travels emerged - during his later career he will visit Europe, Africa, Australia, India, North- and South America, even the remote island of Spitsbergen.
His interest in geology spawned from the practical need to know where he actually was:

"… my attention was first directed to geology in order to understand the geography of the districts through which I rambled, and the, often, apparently erratic course of the rivers … and to understand local topography'."
Gregory in 1906

He studied natural sciences, working during day as wool merchant and studying at night - he even became accustomed to sleep only 4 hours per day. After graduation he found work at the British Museum for Natural History, where he worked on the collection of rocks, fossil echinoderms and corals.
In October 1892 Gregory was asked if he would take on an expedition to East Africa, comprising the area of today Kenya, Ethiopia and Somalia - regions at the time still poorly known or mapped, but geopolitically important.

Gregory however showed interest in the geology of Africa long before the possibility to join the expedition, impressed by the hypothesis of Austrian geologist Eduard Suess.
The origin of the mountains and depressions of the African continent were fiercely discussed by geologist, Suess summarized in 1891 the results of an expedition leading to Lake Rudolf, carried out by Count Samuel Teleki (de Szek) (1845-1916), and suggested that the depressions starting at the coasts of the Red Sea were the results of periodic tectonic movements.

Fig.1. "The Great Rift Valley: some associated fractions are marked by broken lines." The part of the valley system explored by Gregory in 1892-1893 (located east of Lake Victoria) is today also referred as Gregory Rift (GREGORY 1920).

20, November 1892 the ship of the expedition unloaded 300 tons of equipment in the harbour of Lamu, 110 camels and 40 donkeys were acquired to transport the material and 300 soldiers were hired to protect the caravan - planned destination of the expedition was Lake Rudolf, a destination that they however would never reach.
Despite the 300 tons of equipment soon problems arouse - food went bad and cooking pots and tents were missing or unsuitable. Fever and various diseases spread among the expedition members, Gregory was first plagued by ulcers on the legs that immobilized him for weeks and on 17, January 1893 he fell sick with malaria. He recovered only after days with very high fever - arrived to Mombasa the expedition was officially cancelled.
Gregory, still interested to see for himself the depressions of Central Africa, decided to take advantage of the situation. He was already in Africa and the equipment of the abandoned expedition could still be usefull - with the financial help of his family and the British Museum he organized a new expedition. On 23, March 1893 the expedition comprising this time 41 men left Mombasa. Gregory's old peculiarities emerged; he loved to walk alone for kilometres, collecting specimens of plants, animals and rocks, for most of the time he didn't sleep at night, sneaking trough the camp controlling if the sentries were on duty.

"… the geology was so tempting that I went off alone. By this time the men were accustomed to my going by myself, for I did so whenever the country was safe and the next camping-place easy to find. These solitary rambles were to me the most delightful incidents in the expedition. Free from the bother of the caravan, I could climb a mountain, track a river, visit a neighbouring lake, chase butterflies, and collect plants as careless as a schoolboy."
Gregory 1896

The main geological work was carried out from the village of Njemps on the shores of Lake Baringo, where they mapped the geology of the western wall of the Kamasin Scrap - today recognized as part of the Great Rift Valley. Gregory confirmed Suess interpretation of the tectonic origin of this valley and deduced from the weak erosion seen on the mapped faults that this process must have been very recent. He described his discoveries in an article published in the journal of the Royal Geographical Society in 1920, where he coined also the modern term of Rift Valley and connected it to tectonic movement of the earth (a controversial hypothesis at the time, when most landscapes were regarded as results of erosion).

Fig.2. Geological sketch map of British East Africa (Kenya) showing the locations of Lamu Island, Witu, Mombasa, Lake Baringo and Mount Kenya, all visited by Gregory in 1892 and 1893, plus an outline of the Rift Valley (GREGORY 1896).

Fig.3. Section across the Rift Valley (GREGORY 1896), F=faults, Gregory suggested that the faults, forming the characteristic elements in the section of the rift valley, were due to vertical movements - apparently also Scar from "Lion King" is singing in an area with active tectonic uplift…




"For this type of valley I suggested the term Rift Valley, not implying that the whole valley was formed by the two sides being simply pulled apart, but as a breach due to a subsidence between two series of rents."
GREGORY 1920

Gregory connected the African Rift Valley with the similar Red Sea in the north - the dimensions of this feature surprised him, clearly that was a mayor element of earth's crust, connected to mountain ranges and similar basins found around the globe. He proposed that the magmatic rocks filling the African rift erupted during the Tertiary from the lateral shear zones, evidence of vertical, rather than horizontal movements - before the advent of continental drift a common model for global tectonics.

Gregory visited also Mount Kenya, even if - despite his alpine climbing experience - he was not able to reach the summit. Here he mapped the moraines of the glaciers and noted that they had retreated previously to 1893 to their "actual" position. Mount Kenya was conquered by Sir Halford Mackinder in 1899, who also named one of the glaciers found on the mountain - Gregory Glacier.

Fig.4. Reproduction from his 1896 book of a view of Gregory and an African climbing on Mount Kenya.

Bibliography:

GREGORY, J. W. (1920): The African rift valleys. The Geographical Journal, Vol. 56 (1): 13-41
GREGORY, J. W. (1896): The Great Rift Valley: being the narrative of a journey to Mount Kenya and Lake Baringo : with some account of the geology, natural history, anthropology and future prospects of British East Africa. John Murray, London: 500
LEAKE, B.E. (2011): The Life and Work of Professor J.W. Gregory FRS (1864-1932). Geological Society Memoir, No. 34: 234