Aloes and Lilies of Ethiopia and Eritrea

8 ALOES AND LILIES OF ETHIOPIA
Geology
Ethiopia’s and Eritrea’s geology is based on an old
crystalline block, originally part of an immense area
that stretched from India to Brazil and formed part of
the ancient super­continent Gondwanaland. The hard
crystalline rocks are mainly granites and gneisses, and
contain many valuable mineral deposits.
Precambrian rocks with ages of over 600 million
years, found in parts of Tigray, Gonder, Gojam, Harerge,
Sidamo, Bale Illubabor and Wellega floristic regions, are
the oldest rocks in the country and form the basement
on which younger formations lie (Ethiopian Mapping
Authority, 1988). These include a wide variety of
sedimentary, volcanic and intrusive rocks that have been
metamorphosed to varying degrees. In the southern and
western parts of the country, these have been more strongly
metamorphosed than their counterparts in the north. This
is mainly due to the fact that the rocks in the north have
been subjected to relatively lower temperatures.
Towards the end of the Precambrian, a major uplift
followed by a long period of erosion took place. Sediments
deposited during the Paleozoic interval (which lasted
some 375 million years) have been largely removed by
erosion.
During the Mesozoic (starting about 225 million years
ago), subsidence occurred and the sea initially spread
over the Ogaden then gradually extended further north
and west. As the depth of the water increased, sandstone,
mudstone and limestone were deposited. As the land mass
was again uplifted, sedimentation ended in the western
parts of the country with the deposition of clay, silt, sand,
and conglomerate from the land as the sea receded. In the
south­eastern parts, gypsum and anhydrite deposits were
precipitated.
In the Early Cenozoic (which began 65 million years
ago), extensive faulting took place. However, the major
displacement along the fault systems of the Red Sea,
Gulf of Aden and East African Rifts occurred later during
the Tertiary. Faulting was accompanied by widespread
volcanic activity which led to the deposition of vast
quantities of basalt, especially over the western half of
the country. The great table­lands of the centre, northwest
and east were built up by the spreading of deep
basalts. Their erosion has now produced the spectacular