The southern Balga, in Dalai Tala basin on North-eastern Mongolian Platean, has a number of sand dune regions. Their forms are classified into the following three types ; the first is built by alluvial sand sent by the present rivers : the second lines up at the edges of eminences gradually eroded by the wind : the third type - the subject of this study - is crowded in the spindle shape, based on the lower-level surface of the Steppe. This lower base is nearer to the underground water than the higher surface of the Steppe covered with lots of Stipa baicalensis var. mongolica. Woody plants, such as Pmus sylvestris vor. mongolica are growing on the lower base, but there are scarcely any kinds of grasses. Fig 2 shows the distribution of the third type sand dunes.
These dunes of the socaled Mankha type are 5 to 10 metres high. Some people who had engaged in the study of inner Asia concluded that the sand of these dunes was originated in the sand beds accumlated by the water at the present location. Now this opinion is a general view. But we have some doubt about this view, because G. N. Potanin and L. I. Praslov had reported that the sand beds were found on the west slopes of the foothills and passes of both the Southern part of Great Khingan and the Mountains of Zabaikalia, and that they were regarded as the aeolian deposit. No false beds and gravels have been found in these beds. These sand beds were not recognized at the east slope of Great Khingan. This has been seen too by the auther at the middle part of Greaf Khingan, as shown in Fig. 3. On the Mountains of Zabaicalia the sand beds are always very low on the east slope. Are these the facts contradictory against the beformentiond general view ?
The subject of this study is to settle this problem by the data collected by the auther during the World War. The higher surface of this Steppe is generally covered with light chestnut soils. It has about 0.5-0.7 metre thickness at the eastern part. Its hardness can hold a motor-truck making nodepression of its tires, This hardness comes from illuviation of saline in B horizon of the chestnut soils, and the root-mat of grasses in Ahorizon. The thickness of this soil towerd the west of Balga diminishes to 0.2 metre or so, then the soil will be apt to be broken by the wind or other actions.
On the lower surface the sand band are so weak that a motretruk can hardly pass across the region of the sand dune area. In this region the desalininzation by rain seems to be vivacious.
There are great many water pools or little lakes on the lower surface of the sand dunes, which are supplied the underground water. Most of them are not parched in the midsummer, in spite of the evapolation exceed over the precipitation at this Steppe. Therefor we can guess the average surface of the underground water by the bottoms of the lakes. The depths of all lakes do not exceed by 1 or 2 metres. Thus the surface may be determined as shown in Fig. 2.
By the chemical composition of water of these lakes they are roughly classified into the following three types; fresh water, salt water, and sods. They are arranged in this order as shown in Fig. 4 from east to west, In adition to that, the textur of soils increase the clayly ingredients from east to west and decrease the coarse rand. All of these data support the following assumption regards to the origin of the sand beds of this steppe, That is to say, these distributions of the sand beds proved that they are the deposits of the old rivers which had flown from the eastern mountain land, or the old Khingan, and that they may be correlated with the river terraces in the inner part of the present Great Khingan. The fine sand of those beds were blown toward the west slope at the foot of those mountains by westery storms and it built up the sand beds of aeolan deposit.
