BIRDS OF ETHIOPIA AND KENYA COLONY 9 



as a typical section of it. It forms a troughlike valley about 3,000 

 feet deep, a sheer cut in the elevated plateau of eastern Ethiopia. The 

 eastern escarpment of the highlands in that region averages about 

 7,500 feet in height. Just south of the Hawash a range of mountains, 

 the Harrar highlands, extends eastward and was probably originally 

 connected with the mountains of Yemen in southwestern Arabia. 



In the northern portion of the highlands is an extensive basin of 

 lesser altitude. This depression contains Lake Tsana. The northern 

 escarpment is deeply cut by narrow chasms of great depth and has, 

 generally speaking, more abrupt, rugged peaks than the southern 

 highlands, which are more like tableland savannahs broken only here 

 and there by deep gorges. 



The drainage systems of the highland region are as follows: By 

 far the greatest portion of the territory drains into the Nile system, 

 chiefly by means of three tributary rivers — the Taccaze in the north, 

 the Sobat in the south, and the Blue Nile, or Abbai, in the middle. 

 This takes care of the great northern mountains and plateau. Drain- 

 age to the east is conducted by the Hawash River; to the southeast 

 (into Gallaland) by the Webi Shebelli (pi. 11) and the Juba; while 

 the waters of the southwestern corner of the highlands are carried by 

 the Omo to Lake Rudolf (pi. 12). 



As far as vegetational features are concerned, a glance at the floral 

 map (fig. 1) will show that the highlands north and west of the Rift 

 Valley are largely mountain grasslands with intertwining strips of 

 temperate rain forest, the actual escarpments being covered with 

 thorn forest. The highlands to the south and east of the Rift Valley 

 are covered with mountain grass only in their western parts, the 

 eastern areas being largely acacia-tall grass savannahs; both broken 

 by thorn forest and, in Arussi Gallaland (pi. 4), by temperate rain 

 forest. The Rift Valley itself is largely covered with the acacia-tall 

 grass association, although at its northern end it acquires a more arid, 

 desert grass vegetation. 



On the whole, the climate is temperate, ranging from decidedly 

 subtropical in the lower, southern areas to alpine in the very high 

 localities. Some of the highest peaks of the Simien Range are said 

 to retain some snow the year through. The rainfall is heavier than 

 in the more arid lowlands of Somaliland and northern Kenya Colony, 

 varying, according to region, from 20-30 to 60-70 inches a year. It 

 is a rather remarkable fact, but the rainfall is consistently very 

 definitely less in the Rift Valley than in the highlands on either 

 side (fig. 2). 



The rainy season, roughly, may be said to last from the middle of 

 June to the end of September, the rest of the year being fairly 

 dry. There is, however, a period of lesser rains during March. The 

 actual time of the start and finish of the rains varies somewhat in 



