Above Dirbah, in the valley of Mai-tchingua, we find a more or less cavernous travertine limestone deposit containing shells, plants, etc, which we see in the surroundings. This deposit of travertine limestone is not very considerable. At the bottom, it rests on the talcschist, and at the top, on the tertiary and oolitic terrains. It is well-known how these kinds of limestone are formed at the expense of the surrounding limestones, by means of sources of water very charged with carbonic acid. We will therefore not dwell on it.
In the valley of Tekezé we still find a similar deposit. But we also see there a crystalline limestone in the form of stalactites. The numerous mineral and thermal springs of Abyssinia1 give rise to deposits more or less analogous and more or less developed.
The deposit of sandy-clay alluvium and very often saliferous, which constitute a small plain to the west and south of Mitsiwa, is the result of a coastal formation. But the salt which these alluviums ordinarily contain could well be due to another cause. For it is probable that the salt plain of the Afar is exemplified on the side of Mitsiwa, below the sandy-clay alluvium. It is possible also that the Triassic terrain of the Afar plain extends to the environs of Mitssiwa, forming the eastern part of the high mountains on the shores of the Red Sea. In any case, the small plain around Mitssiwa is arid, yields only salt water, and produces salt by efflorescence.
Let us mention the large alluviums found in many parts of Abyssinia, at the bottom of the valleys, especially in those of Mareb and Tekezé. These alluviums are composed of clay, sand, gravel and blocks of rocks, often very large, which have been torn from the mountains that are observed not far from their deposit in the valleys. So that the idea of a diluvium, which comes to mind as soon as we see these alluviums, immediately disappears after a careful examination of the facts and local circumstances.
The periodic rains of this country, which are torrential, and the waters of the numerous lakes which crown the plateaux, must necessarily detach blocks of rock, gully the ground with great intensity at certain times, and then produce considerable transports. The high mountains of Abyssinia are also covered at certain times of the year with snow, which melts almost suddenly and which then produces impetuous torrents. Added to this is the rugged and cut configuration of the soil of Abyssinia, which unites all the most favorable elements for the formation of a transport terrain.
It is probable that several volcanoes, the products of which we see in the islands which border Abyssinia and on the shores of the sea, have existed before the time when the Ptolemies came to found establishments on the coasts of Abyssinia. Moreover, a general geological movement having taken place after the foundation of the cities of Adulis and others, all these establishments, the ruins of which we still find, were partly submerged. Similar volcanoes still exist in Abyssinia today. Some of the hot springs that are observed on the coasts of the Red Sea and elsewhere reach up to 65° and beyond. The volcanic phenomena which began in Abyssinia during the formation of the Tertiary strata, still continue to this day in certain parts of the country.
This is the twentieth installment of the geological description of Tigrinyaland2 and Semien3, which has been adapted from Messrs. Ferret and Galinier‘s work published in 18474.
Reference Notes
- Abyssinia refers to the region which encompasses the modern-day states of Eritrea, Ethiopia, Djibouti, parts of Sudan and parts of Somalia. ↩︎
- Tigrinyaland was a collective name of the Midri-Bahri (modern-day state of Eritrea) and Tigray (the northernmost region of modern-day Ethiopia). The term employed for Tigrinyaland by Messrs. Ferret and Galinier in their book is “Tigré”, which had been the designation used by the Amhara rulers of Abyssinia to refer both to the Tigrinya people and the Tigrinyaland. ↩︎
- Semien was historically the frontier province of the Tigrinya with the Amhara. However, since the reign of Emperor Susenyos, the province of Semien had been governed by members of the Amhara royalty and nobility. Following the death of Dejazmatch Sabagadis in 1831, Semien under its Amhara ruler Dejazmatch Wubbe turned from the frontier province of the Tigrinyaland to the power-center of the Tigrinyaland. This continued until the rise of Emperor Tewodros in 1855. ↩︎
- Ferret, Pierre Victor Adolphe et Galinier, Joseph Germain (1847) Description Géologique du Tigré et du Samen. Voyage en Abyssinie dans les provinces du tigre, du samen et de L’amhara. Tome troisième. Paulin: Paris. ↩︎