GEOLOGY OF NORTHERN SHAN STATES AND YUNNAN 207 



more open, less accentuated, topography of the Plateau limestone 

 over most of the eastern section. 



The ShweH in the North and the Nam Tu in the South both 

 flowing in a general southwesterly direction are the dominant 

 streams, carrying off all the drainage of the area, with the exception 

 of a narrow strip along the eastern edge, where the streams flow 

 east into the Salween. The Salween flows almost due south and, 

 as is characteristic of it for most of its course through Yunnan and 

 Burma, it has cut its way down far below the surrounding country 

 and consequently has a very limited lateral drainage. The 

 streams flowing into the Salween in this section have a length rarely 

 exceeding thirty miles. Notwithstanding this fact it far exceeds 

 any of the other streams in volume of water. 



The abrupt change in the character of the country at the 

 contact between the older Paleozoic sediments and the younger 

 limestones is most pronounced, the former rocks standing up as a 

 rampart across the whole area. In the isolated oval-shaped 

 exposure of these rocks in the northeast corner of the Northern 

 Shan States the same conditions hold, the older sediments rising 

 up as a rugged, intricately dissected network of mountains out of 

 the surrounding limestone country, which, by comparison, is of 

 moderate relief, with wide valleys and rounded hills. The char- 

 acteristic underground drainage of the limestone area has given 

 rise to large sections, in which the topography consists of numerous 

 sink holes of all sizes, inclosed basins in which the drainage 

 disappears into the limestone. The same characteristic has also 

 produced peculiar local topographic forms, such as hanging valleys 

 without visible outlets and valleys with two distinct valley floors 

 at different levels. 



Certain sections of the granite area, in which the rock is of an 

 extremely coarse-grained homogeneous nature, have weathered into 

 a multitude of small, rounded hillocks without any definite drainage 

 system. The streams wind sluggishly through the maze of small 

 hills, often forming marshy lands. In this section the granite has 

 weathered in situ often to a depth of several feet, leaving a soil 

 composed of all the constituents of granite arranged in their normal 

 position but unconsoHdated. 



