HOLE : SOME INDIAN GRASSES AND THEIR (ECOLOGY. 39 



CHAPTER I. 



Description of Locality. 



38. The locality in which the species included Topography - 

 in the present paper have been chiefly studied comprises the Dehra Dun valley and the 

 northern slopes of the Siwalik range which bound that valley on the south, these together 

 forming a portion of the Dehra Dun Civil District, and also the southern slopes of the 

 Siwaliks which are included in the Saharanpur Civil District. This tract lies between 

 Long. E. 77° 40—78° 22' and Lat. N. 29' 55'— 30° 32', and the principal part of it is 

 shown in the map which forms Plate I. 



The elevation above mean sea level varies from 900' to 3,000'. The Dun valley is divided 

 into two approximately equal portions, the Eastern Dun lying to the south-east of Dehra 

 cantonment which drains into the Ganges, and the Western Dun lying to the north-west of 

 the cantonment which drains into the Jumna. The Asan River flowing close to the 

 foot of the northern slopes of the Siwaliks constitutes the main line of drainage in the 

 latter, while in the former the Suswa River occupies a corresponding position. The chief 

 tributaries of the former are the Tons, Suarna Rau, Chor Khala and Sitla Rau and of the 

 latter the Bindal, Rispana, Song and Jakhan Rau, all from the Himalayas. The upper 

 slopes of the Siwaliks consist mainly of sandstone and conglomerates with some clay beds, 

 the lower slopes and the entire Dun valley are covered with the debris brought down from 

 the Himalayas, or from the Siwaliks themselves, consisting of boulders and sand, mixed 

 with which are clay beds of varying thickness. 



Where the impermeable clay bands crop out, swamps are often found, but owing to 

 the great thickness of the pervious boulder and sand deposits the subsoil water level is 

 frequently at a great depth below the surface. In the Dun valley owing to the masses of 

 debris brought down from the Himalayas the greatest thickness is naturally in the north 

 of the valley near the foot of the Himalayas, and Fernandez notes : " It is for this reason 

 that the permanent level of water is at such a great depth, the well at the cutcherry at 

 Dehra being 228 feet deep." 1 Another striking feature of the locality consists in the pecu- 

 liar character of the streams by which it is drained. 



The tributaries of the Asan and Suswa rivers which rise in the Himalayas and Siwa- 

 liks and the streams which drain the southern slopes of the Siwaliks are, with the excep- 

 tion of one or two of the largest, such as the Song, dry during the greater part of the year 

 and their exposed beds of boulders and sand, often of great width, are then a conspicuous 

 feature of the landscape. 



l Working-Plan of the Dehra Dun Forests by E. E. Fernandez, 188P, p. 10. 



r 39 i 



