This is the sixth part of the geological description of Tigrinyaland1 and Semien,2 which has been adapted from Messrs. Ferret and Galinier’s work published in 1847.3 In this installment, we will continue our discussion of the intermediate or transitional lands of Tigrinyaland and Semien composed of phyllites and related rocks.
On the slopes of the Mountain Abba-Gherima, the ground of the road is covered with white quartz coming from the veins which cross the phyllite. The phyllite rock is compact and bluish in the ravine that is on the left and which is called Adi-Mutareh. The blocks of hard and compact phyllites contain very thin veins of white quartz which cut the strata.
The loose soil cultivated around Adwa is made up of clayey detritus coming from the mountains which surround the basin where the town is built. Adwa presents one of the best localities for studying the phyllites which make up the majority of the transitional terrain, which is the basis of all the lands of Tigray. The general direction of the layers of the phyllites around Adwa is roughly from the northeast to the southwest.
The north-western part of Adwa is seated on sheer rocks bathed at their base by the Assam River. The Assam river flows in a rugged bed, and in the midst of detached blocks of phyllite. The phyllite in this place is hard and compact, bluish gray and veined with white quartz arranged irregularly with respect to the strata of the system. A little above the bed, this rock becomes more schistose, passes into talcschist, and forms a real slate which sometimes takes on a slightly more reddish color due to the decomposition of the cubic pyrites (marcasite) which are found scattered there.
The phyllites reaching at the level of the loose soils cultivated in front of Adwa, on the opposite side of the Assam River, become clayey-talcose and contain a fairly large quantity of cubic crystals of marcasite. We are going to mention a fairly important fact for the theory of the formation of marcasite crystals. This is because these crystals, in talcschist and in phyllite, are surrounded by a crust of quartz. They look like they are encased in quartz crystals. So that probably the silica was attracted to their surroundings during their formation, and more so towards the tops of the crystals.
At the Shelloda mountain, the phyllites tilt to the right and left of the slope at an angle of 55° to 40°. Towards the summit of the mountain, the phyllites are entirely vertical and cut in the northeast-southwest direction by a quartz vein whose thickness varies from 1 meter to 2.5 meters. The phyllites and the phyllitic sandstones are sometimes covered by amphibolites, in contact with which the phyllites are decomposed. This decomposition comes, in all appearance, from the reciprocal action of the iron oxides and the alumina. Finally, the whole is sometimes also covered by iron limonite in layers, in kidneys and in grains, the debris of which is spread on the slope of the mountain on the northwest side. But the summit of the mountain is formed by the amphibolite rock, the detached blocks of which make the approaches difficult. In all cases, by calculating the average of the directions of the layers and those of the mountains, we find that it takes place from the northeast to the southwest.
When we go from Adwa towards the Mereb, after leaving Shelloda, we still see phyllites, talc-schists and various other rocks belonging to phyllites. In the Mai-Terra valley, it is a more or less decomposed phyllite, inclined at 50 to 55°, and crossed by veins of white quartz measuring 1.5 to 2 meters thick. But at Debre-Sina and Nagah, the phyllite is cut and covered by diorites, amphibolites, etc. In the Mai-Khanaho stream, we can easily see the phyllite which is bare, since it is only covered here and there by blocks of amphibolite rocks detached from the heights.
The path that we follow to cross the mountain of Adi-Hedmot is made in the midst of phyllites, often very ferruginous and of an ocher-yellow color. But the edges of the road are still bristling with amphibolite blocks, detached from the summit of the mountain where these rocks point. Beyond Daero-Técli, going towards the Mereb, we also find phyllites crossed by veins of purplish-brown-red hyaline quartz. The direction of the layers of these phyllites is from north-northeast to south-southwest, with an inclination of approximately 55 degrees.
In the bed of Korzohro, we find diorite and amphibolite, with very distinct amphibole crystals. The pebbles rolled by this watercourse are debris of higher rocks, such as the amphibolite of the plateaus, white hyaline quartz, iron ores, loaded with a sparkling bluish-gray dust, which appears to be nigrine. This nigrine is also mixed with sand, with yellow, ductile flakes, which are probably gold.
We find phyllites at the beginning of the Hamado plain. In this part of the country, the direction of the layers is from north to south, with an inclination of approximately 57°. The Adi-Tsa’áda mountain is made up of phyllites which are often ferruginous and crossed by quartz veins of one meter in thickness. The village of Adi-Arba’été is also built on the phyllite, whose layers have an inclination of 60°, and which is further cut by quartz veins. Finally, the phyllites continue up to the Mereb, that is to say up to the limit of diorite, syenite and granite. Moreover, these latter rocks, notably the granite, are sometimes surmounted by layers of phyllites which are easily divided into slabs.
When we head from Adwa towards the Tekezé, we find the phyllitic rocks in the Firfira ravine. These are phyllites erected almost vertically, and the direction of the layers is from northeast to southwest. We then see the phyllitic rocks continuing towards Chilacheqnné and Debre-Abbay, on one side, and towards Abergellé, on the other side, either in the Tekezé valley, or in the ravines which lead to it.
In the vicinity of Chilacheqnné we observe that the phyllite rests on talcschist, that it is sometimes crossed by diorites and amphibolites, and that it is covered in some places either by these latter rocks, or by sandstones, clays and tertiary tripolises. The phyllitic rocks found in the vicinity of Chilacheqnné are sometimes a purple phyllite passing to talcschist, sometimes a gray phyllite passing to greywacke, and containing veins of milky quartz, and sometimes, a gray-greenish talc-schist, passing either into phyllite or greywacke, and resulting from the modification of the latter rocks by the action of igneous rocks. The phyllites that we have just talked about extend from the Tekezé valley to the village of Tukul, affecting all kinds of inclinations and directions, but presenting an average direction from northeast to southwest and giving rise to extremely picturesque sites.
Thus the phyllitic rocks are cut by diorites and amphibolites. They are covered, on the Shiré side, by Tertiary sandstones, clays, etc. They affect all sorts of directions and inclinations. They offer various varieties, among which satin-finish slate plays the main role; whereas, towards the Shiré plateau, a talcschist dominates. Between this plateau and the Tekezé there are veins of crystallized quartz.
When we go to the region closer to the south-southwest, we find, on the other side of the high mountains of Semien, the phyllites which extend towards Gondar. In the vicinity of Wi’á, located on the eastern slope of the Tarenta mountains,4 we see, above the primary rocks, a crystalline limestone, in which we find garnets the size of a pea. However, this limestone must probably be linked to the phyllites, or at least to the transitional terrains.
Reference Notes
- 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. ↩︎
- The Tarenta mountain chain forms the eastern escarpement of modern-day eritrea, and looks down on the Eritrean eastern lowlands which are close to the Red Sea. ↩︎