Tertiary Sedimentary Terrains of Tigrinyaland and Semien – Part 1

It’s not our intension to describe each of the Tertiary plateaus in detail. Instead we will present an outline of the main cross-sections we have noted in various localities where the Tertiary terrains are found.

The uniformity of Abyssinia’s¹ Tertiary strata and their composition indicate deposits formed in situ, by means of surrounding rocks that were reworked into vast, generally elevated lakes, of which numerous representatives still remain. In the middle of these lakes, volcanic vents later opened up, and their products modified the sedimentary rocks that were still soft at the bottom of the lakes, and the waters were subsequently discharged by the change in level and relief of the ground. But what’s astonishing is that, despite our research, we haven’t been able to discover any fossils in these Tertiary deposits.

From Asmara to Adi-Ghebray, we see a plateau which is formed of tertiary clays and sandstones, interrupted only by basaltic lavas. It is generally a quartz clay, reddish white, hardened, passing to porcellanite; or else a clayey sandstone, compact, red, spotted with white dots, hardened and changing to tripoli. These rocks have been modified by the presence of volcanic products. Moreover, we see numerous examples of a similar modification elsewhere.

In Gulzobo, we saw a deposit of red sandstone or whitish flaxen sandstone, with white cement which appears to be kaolinic clay. A deposit covers the top of the phyllites, and we find it, with the same composition, at Debre-Damo. We also see, in the vicinity of Goulzobo, deposits of limonite which could be exploited to advantage.

At Neggot, we find, above the phyllites, a tertiary sedimentary terrain which is composed of the following deposits, going from bottom to top:

1. Whitish gray kaolinic and ferruginous clayey sandstone passing to arkose;
2. Clayey-ferruginous sandstone;
3. Fine, white, hardened clayey sandstone, more or less jaspoid;
4. Pudding-shaped ferruginous sandstone.

All these layers are approximately horizontal, but they are very modified.

In Intetcho, we saw a bed of white porcellanite clay, marbled with red and purple, and which rests on the phyllites. Between Intetcho and Debre-Damo, there is a plateau which crowns the phyllites, which is composed of friable ferruginous sandstone, and whose base contains veins of coarse ferruginous sandstone and dividing the upper sandstones into tiles. This division into tiles results, in our opinion, from the shrinkage experienced by the sandstone when the surrounding basalts emerged, and from subsequent infiltration in the cracks thus produced.

At Debre-Damo, the phyllites are crowned by a plateau of 4 to 500 meters in diameter, about thirty meters high, and composed as follows: at the base, a fine whitish sandstone, with matt white cement, which appears to be kaolin clay, or a more or less pudding-shaped ferruginous sandstone, and, in the upper part, a compact, yellowish and whitish clay or clayey tripoli. Towards the lower part, the sandstone is further divided into tiles by a coarse blackish clay-ferruginous sandstone. All these layers of clay, sandstone, etc, rest on the phyllites, without any noticeable inclination. This mountain is naturally fortified by sandstone.

The Adigrat plateau extends towards the Atsbi side. However, at Bélessa, we see the tertiary sandstone still covering the talcschist; and there, as elsewhere, we see no perceptible inclination.

In all the localities of which we have spoken above, sandstone is the fundamental rock of the tertiary deposits. The clay usually only forms a fairly thin layer which covers the sandstone.

This installment is the fourteenth part of the geological description of Tigrinyaland² and Semien,³ which has been adapted from Messrs. Ferret and Galinier’s work published in 1847.⁴

Reference Notes

1. Abyssinia is a reference to the region which encompasses the modern-day states of Ethiopia, Eritrea, Djibouti, parts of Sudan and parts of Somalia. ↩︎
2. 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. ↩︎
3. 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. ↩︎
4. 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. ↩︎