The most famous cave houses in the world are probably the Yaodong in the loess plains of China. They are sometimes called earth houses, although the fine, unconsolidated loess can hardly be called earth.
Loess is fine rock dust with a grain size of 0.002 mm to 0.063 mm and is called silt in geology. It is an aeolian, unconsolidated sediment which usually behaves like a soft rock. It is often of glacial origin.
Glaciers erode an enormous amount of rock by grinding it through their weight and movement. This creates a very fine rock dust that is blown into the foreland by the cold wind blowing down from the glaciers. The wind is slowed down by vegetation or hills and the silt is deposited as loess. In the current ice age, i.e. in the last 2.4 million years, large deposits of loess have been formed all over the world.
Loess has very unusual properties, mainly due to the shape of the grains. Due to mechanical weathering, the grains are sharp-edged and angular, and therefore strongly interlock, in contrast to sand, which is transported and crushed by water. The fine silt is easy to dig, you can scrape and shovel it. But although it is only very fine sand, it does not collapse or flow away like normal sand. In other words, the dug caves are exceptionally stable. That is the reason why people decided to dig caves in this loose sediment. It was much less work than building a house.
Another aspect is the high fertility of the soil on loess. The rock dust is easily dissolved due to its large surface area, which allows the water to absorb a variety of minerals. Therefore, the soils are constantly fertilised from below, and before the invention of mineral fertilisers, these were the most fertile soils known.
During the ice ages, the Gobi Desert was cold and windy, and the glaciers advancing from the north produced rock dust in large quantities. This dust was blown away by the wind into an area with a little vegetation, thus slowing down the wind and losing the dust. Over tens of thousands of years, huge layers of this dust were deposited, forming the Loess Plateau or Loess Highlands in northern China. It covers a vast area of 1000 km east-west and 700 km north-south, most of Shaanxi province, as well as parts of Hebei, Henan, Gansu, Qinghai and Inner Mongolia provinces. The Yellow River gets its name from the loess it carries; at almost 40 kilograms per cubic metre of water, it has the highest sediment load in the world. In addition, the loess is usually very thick; on the Yellow River they are up to 400 m thick.
The fertile plateau was an excellent settlement area. But due to the great thickness of the loess, stones were scarce in a vast area. Only limited amounts of wood were available for building houses. Therefore, the construction of cave houses was almost imperative. Due to the properties of the loess, a special style of building developed which is called Yaodong (house cave). Entire cities were built underground; 35 million people live in yaodongs in northwest China today. The following three types can be distinguished:
The Sunken Cave, also 地坑院 (De kēng yuàn, Sunken Courtyard), is common in flat regions. This involves first digging a square shaft from which rooms of the house have been dug into the walls in all four directions. Access is either via a staircase on one wall, or via a staircase from the plateau that ends at the bottom of the shaft.