Phyllite and Related Rocks, Part 1 – Intermediate Lands of Tigrinyaland and Semien

This is the fifth 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, the intermediate or transitional terrains of Tigrinyaland and Semien composed of phyllites and related rocks will be presented.

Phyllite is a schistose rock, composed essentially of talc, quartz and clay. The clay is itself of a very complex composition. It generally is formed of alumina, silica, iron oxide and water.

Phyllite plays a very large role in the geological constitution of Abyssinia.⁴ It is one of the rocks which appears in greatest abundance in the country. From the south-west slope of the Tarenta,⁵ to the surroundings of Gondar, we see the phyllite forming the majority of the valleys and sometimes also the mountains, as well as plateaus. But along with phyllite, we must also include various other rocks that are subordinate to it, such as graywacke, anagénite,⁶ sandstone, quartzite, talcschist, etc. We’ve already discussed talcschist, so we’ll only be talking about the other rocks subordinate to phyllite.

On the southwestern slope of the Tarenta mountains, at the limit of the protogine, we see a plateau which is almost entirely formed of phyllite. This phyllite is often altered and crossed by veins or grains of white quartz. Around Mai-Harassat, where there is a diorite typhon,⁷ it changes to phtanite; while around Digsa where we see a massif of eurite, it passes into talcschist and even gives rise to a white talcschist, more or less altered. In the vicinity of Digsa, the general direction of the layers appears to be from northeast to southwest, and the inclination to the northwest. At Adi-Hadid we find a grayish sandstone, passing to graywacke and phyllite. Further south, it is a grayish schistose graywacke, passing to phyllite and talcschist. Around Adi-Kuita, the graywacke is very fine-grained and passes into a light gray speckled phyllite. In Assira, it is a grayish phyllite that we observe. In Adi-Kuita, a massif of melaphyre appears in the middle of phyllitic rocks, while in Assira, a typhon of diorite partly covers the phyllite.

From the preceding facts, we can understand that all of the schistose rocks which form the soil from Tarenta to the south of Assira are composed of more or less modified phyllite and phyllitic graywacke. From Gulzobo to Adwa, most of the valleys which join that of Mereb or that of Tekezé are composed of phyllite. At Gulzobo, below the tertiary lands, we see the reappearance of the grayish phyllite from the surroundings of Assira. Then, between Gulzobo and Logo-Sarda, we find a fine graywacke, of a bluish green and which passes into talcschist or chloritoschist.⁸ While at Logo-Sarda it is a pink feldspathic talcschist, which passes into protogine. However, in Logo-Sarda, in the Belessa valley, is also seen a semi-schistoid serpentine rock dependent on diorites. It is then easy to see that the anomalous schistose rocks, of which we have just spoken, result from phyllite or graywacke modified by the presence of the outwash rock.

The gray phyllite continues to the north of Neggot, a village located on one of the highest points of the range which precedes the mountains of central Tigray. Neggot is also in one of the most remarkable positions from which to view the immense horizon. The inhabitants of this village use the compact phyllite in their churches to replace the bells. The sonority of certain rocks has been put to good use, not only in Abyssinia, but also among the peoples of Asia, and to all appearances, among the Gauls of Europe.

The Neggot phyllite, especially in the southern part, is covered in many places by the Tertiary terrain. But when leaving this village to head towards Eggela, you cross a series of valleys or ravines which still show it uncovered. In Eggela, you see the gray phyllite whose layering direction still takes place from the north-east to the south-west.

The phyllites of Gulzobo, Logo-Sarda and Neggot also appear in the valleys surrounding Debre-Damo. We can even say that the base of the soil of this country is formed of phyllite which is covered on the high points by plateaus of tertiary lands. This phyllite is generally an ash gray, and often it shows, parallel to the layers, veins or small layers of milky quartz, bluish quartz and grainy blackish quartz showing imprints which could well be traces of fossil plants.

The phyllite around Debre-Damo is also crossed by amphibolites, and in contact with this rock, it passes into talcschist. But what is most remarkable is that the phyllite, particularly at the limit of the ash gray phyllite, has been changed either into phyllitic tripoli, fine and pink, or into porcellaneous clayey tripoli, compact and pink. This probably is caused by the action of the surrounding volcanoes. The direction of the layers of this phyllite is roughly from north to south, with an almost vertical inclination.

Going towards the south-southwest, the phyllitic rocks appear with great development in the surroundings of Ghendebta and Adwa, and form the soil of the valleys of Assam and Wari. In the northern part of the Adwa country, we observed, at the limit of the tertiary sedimentary terrains, basalts, trachytes, diorites, etc. First a band of talcschist, generally purple and passing to phyllite. Then, a great development of phyllite. In the middle of these Phyllitic rocks, we find several others which are subordinate to them. We will list among others:

1. A red oligist iron deposit, clayey, passing to limonite;
2. Sandstone;
3. Quartzite;
4. Of anagénites.

Moreover, the talcschist, of which we have just spoken, also appears to be only a dependency of the phyllite, because it is intimately linked to it and is found at the limit of the phyllite towards the rocks of igneous origin.

The Semaiata mountain is composed of phyllite, sandstone anagénite and whitish-grey quartzite. These rocks are cut and covered by diorites, and, towards the east, by tertiary terrains. The layers they form are disturbed, but their general direction is approximately from north to south. The road that is used on the west side to climb the mountain, is manageable for pack animals up to a third, and even up to half of the height of the mountain. It has an inclination of 50°.

It was around halfway up the Semaiata mountain that Rasi Mikael Suhul established his camp when he rebelled against Emperor Iyasu II.⁹ Using schistose stones, he had fortified the surroundings by erecting them like battlements, and by making on a kind of platform circular wells where the riflemen hid. The camp was fortified on three lines. The first line of defense was entered only through a gate formed by the natural rapprochement of two blocks of rocks. When arriving above where the third line of defense was, which presents a sort of platform, one is obliged to turn towards the north and has to descend by a precipice until facing a bare and sheer rock. This rock dominates the head of the mountain and rises 200 meters above it.

You then return to the west, and you begin to climb again via an unmade path to a plateau bristling with detached blocks. It is located below the last cap. This cap, which forms the summit of the Semaiata mountain, approximately 100 meters high, is very dangerous to climb. It is almost always hidden in the clouds during the rainy season. You can only get there by hoisting yourself up and turning around at an angle of about 50° to 55°. But once arrived, the traveler soon forgets his fatigue while admiring the magnificent panorama that unfolds before his eyes. It dominates all of Tigray Proper,¹⁰ Temben, and Agamé. Its view extends as far as the Semien province, whose high mountains, imposing with their masses, strike the mind with astonishment and admiration.

In the upcoming part of this article, we will continue our discussion of the phyllites and related rocks of Tigrinyaland and Semien.

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. ↩︎
4. Abyssinia is a reference to the region which encompasses modern-day Ethiopia, Eritrea, Djibouti, parts of Sudan and parts of Somalia. ↩︎
5. 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. ↩︎
6. Anagénite is a French term for a conglomerate formed of fragments of granite, gneiss or schist. ↩︎
7. Typhon is a collective French geological term for the plutonic rocks boss, stock and batholith. ↩︎
8. Chloritoschist is greenschist. ↩︎
9. Emperor Iyasu II, whose thrown name was Alem-Seged, reigned from 1730 to 1755 A.D. ↩︎
10. Tigray Proper is the province that has given its name to the whole Tigray region, and is bounded on the north by the River Mereb, on the east by the province of Agame, on the south by the River Wari, and on the west by the province of Shire. ↩︎