PHYSICAL GEOLOGY. 



15 



To the south the plateau merges into that of the Southern Shan 

 States, the geology of which is only imper- 



Southern boundary. . , , , ^ , , . , -, 



tectly known. i<or a long distance eastwards 

 from the edge of the plateau near Mandalay, the boundary of the 

 Northern States follows the profound gorge of the Nam-Tu, to the 

 point where this river turns northwards towards Hsipaw. Beyond 

 this there is no well-defined physical boundary, the line being 

 drawn entirely for political purposes. 



The river system of the plateau is not very complicated. Nearly 



the whole area is drained by one river, the 

 River system, the Myitnge of the Burmese, or Nam-Tu of the 



Shans, and its affluents. This river takes 

 its rise in the State of North Hsenwi, in Lat. 23° 20', Long. 98° 15', 

 within 20 miles of the Salween. It at first flows westwards to 

 Mong-tat, in about Long. 97° 30', where it turns abruptly to the 

 south, and traverses the hilly country north of the plateau in a 

 deep and very narrow valley, issuing from these hills a few miles 

 to the north of Hsipaw, in Lat. 22° 37'. It is then joined by a 

 large river from the north-east, the Nam-ma, which unites the waters 

 of three considerable streams, the Nam-mi, draining the hills to the 

 north, the Nam-yau, and the Nam-pawng, both of which rise among 

 the hills to the ea&t of the plateau, within a short distance of the 

 Salween. Below the junction of the Nam-ma with the Nam-Tu, 

 above Hsipaw, the combined rivers follow the direction of the 

 former, i.e., south of east, to Bawgyo (F 2), where another large 

 tributary, the Nam-hsim, coming from the State of Tawng-peng to 



the north, joins in. There is, however, some 



^Change in course of eyidence to ghow that j n former timeg the Mm _ 



Tu, instead of joining the Nam-ma above 

 Hsipaw, as it does at present, followed an independent course 

 from the point where it leaves the hills at Ta-ti ferry, 6 miles 

 north of that place, to Bawgyo. An account of the supposed change 

 in the course of the river, and the causes that led to it, will be 

 found in the Records of the Geological Survey (Vol. XXXIII, Pt. 1). 

 The original bed of the river seems to have been raised by an 

 accumulation of boulders and gravel in front of the point where 

 it leaves the hills at Ta-ti, until it reached such a height that the 

 river broke through the sandstone hills to the south-east along a 

 ravine excavated by two small side streams flowing into the Nam-Tu 

 and Nam-ma respectively, and deserted its former channel. 



