They are extensive grassy plains, the lowest being the bed of an ancient inland lake about which is a broad terrace (mesa), the talus perhaps of the ancient encircling highlands.
In width, called the Dasht-i-Hamdamao, or Dasht-i-Ardewan, formed by the talus or drift of the higher mountains, which, washed down through centuries of denudation, now forms long sweeping spurs of gravel and sand, scantily clothed with wormwood scrub and almost destitute of water.
The talus zone of this region, especially atelevations of 1000 to 3000 ft., is noted for its great fertility and the luxuriance of its vegetation.
But in the " Drift " maps many other types of deposit are indicated, such, for instance, as the ordinary modern alluvium of rivers, and the older river terraces (River-drift of various ages), including gravels, brickearth and loam; old raised sea beaches and blown-sand (Aeolian-drift); the " Head " of Cornwall and Devon, an angular detritus consisting of stones with clay or loam; clay-with-flints, rainwash (landwash), scree and talus; the " Warp," a marine and estuarine silt and clay of the Humber; and also beds of peat and diatomite.
In the heights of Harden (2651 ft.) and Whitecoomb (2695), whence the Clyde, Tweed, Annan, and Moffat Water descend, the high moorlands have been scarped into gloomy corries, with crags and talus-slopes, which form a series of landscapes all the more striking from the abrupt and unexpected contrast which they offer to everything around them.
The long screes or talus-slopes at the foot of every crag and cliff bear witness to the continual waste.
The recent deposits, which cover so large a part of the depression between the Eastern and the Western Cordillera, appear to be partly of torrential origin, like the talus-fans at the foot of mountain ranges in other dry regions; but Lakes Titicaca and Pampa Aullaguas (Poopo) were undoubtedly at one time rather more extensive than they are to-day.