Tertiary Sedimentary Terrains of Tigrinyaland and Semien – Part 2

A tertiary plateau starts from the surroundings of Aksum and extends to the Tekezé valley, and then reappears on the other side of the River. However, we notice, between Adwa and Aksum, on the surface, a fine, whitish sandstone, and, at the base, a fine, brown sandstone colored by manganese. Between Selekhlekha and Mai-Tuaro, sandstone, limonite and pudding-shaped oligist are observed. In the Shiré plain, various rocks exist, among which sandstone dominates, and to which fairly considerable deposits of oligist or limonite are subordinate. Thus, in the Murakat ravine, near Aber Semmaka, a tertiary deposit is formed, in the upper part, of jaspoid chalcedony millstone flint or compact jasper and very siliceous clay, often resulting from the alteration of jasper; in the middle part, a fine red psammitic sandstone; at the base, a fine white sandstone which rests on syenite and pegmatites.

In the Mai-Tenkat ravine, north of Adi-Qesti, the Tertiary deposit which still rests on the syenite is formed at the base of a layer of argillaceous sandstone 5 metres thick and covered with a layer of very thin, violet, marbled clay or porcellanite, which itself supports a layer of yellowish, marbled, clay-quartzite tripolean sandstone. Above it, we see sometimes a layer of white clayey sandstone, sometimes a layer of blond, cellular pyromorphitic flint, about 75 centimeters thick.

At Adi-Kesti mountain, we see the fine sandstone more or less weathered and nearly colored red.

Between Debre-Abbai and the Tekezé, very close to the valley where the river flows, a tertiary deposit which rests on talcschist and phyllite is formed, going from top to bottom:

1. Of a layer of jasper, oligist and limonite;
2. A layer of psammitic red sandstone.

In the Frefira, the tertiary deposit of the Shiré plain presents the following sections going down:

1. A marbled porcellanite, or a pink punctuated porcellanite 2 to 5 meters thick;
2. A jaspoid porcellanite marbled with purple measuring 33 centimeters;
3. A white porcellanite;
4. A 4 to 5 meter thick clay-ferruginous porcellanite;
5. A granular pink porcellanite 8 centimeters thick, passing to tripoli and resting on the syenite.

In Mai-Sergui, near Tukul, we notice, isolated on the talcschist, a small plateau of variegated red sandstone, which belongs to the tertiary terrain.

As we have said before, all Tertiary deposits are approximately horizontal, when examined over a small area. But if we take the general slopes of the plateaus, we see that they converge towards the Tekezé, heading approximately from north-northwest to south-southeast.

This is the fifteenth installment of the geological description of Tigrinyaland¹ and Semien,² which has been adapted from Messrs. Ferret and Galinier’s work published in 1847.³

Reference Notes

1. 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. ↩︎
2. 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. ↩︎
3. 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. ↩︎