The Morphology of Sinkhole Land – the Mitzpe Shalem Shore

המורפולוגיה של ארץ הבולענים – אזור חוף מצפה שלם

The site lies on the Dead Sea shore, on a soft plain of clay sediments exposed as the lake retreated eastward. From this viewpoint you can see, alongside the clay plains, high alluvial fans made of sand, pebbles and conglomerate. The alluvium is brought and deposited on the shore by the flood waters of the streams (Darga, Arugot, Hever and others), while the clay that makes up the clay plains settled directly from the lake water. The clay was suspended in the lake water and settled out only in areas of calm water without mixing.

Types of sinkholes

Sinkholes that form in the clay plains are wide, open and usually flooded with groundwater. Sinkholes that form in the alluvial fans are usually dry and remain covered by a thin roof until the dramatic moment of collapse. After collapsing they have steep walls, unlike the clay sinkholes, whose walls are terraced and gentle. For this reason, alluvial-fan sinkholes are much more dangerous than those in the clay plains.

How the sinkholes formed

Layers of sediment are constantly deposited on the floor of the Dead Sea – usually clay, sometimes thin layers of aragonite crystals, and when the sea is very salty, layers of salt.
Drilling and seismic surveys have shown that along the Dead Sea shores, within the clay layers at a depth of about 30 m, there is a salt layer about 7–10 m thick – a remnant of a short period when the lake was extremely salty.
As long as the lake level was high, this salt layer was immersed in salty groundwater and did not dissolve. As the lake level dropped, so did the level of the salty groundwater. It moved east following the lake, and its place was taken by fresh groundwater originating in the springs of the rift escarpment to the west.
This fresh water flowed through long cracks in the sequence of clay and salt, dissolving the salt layer along the cracks and creating a line of dissolution cavities.
The cavities along the cracks grew over time: the salt dissolved and was carried away with the water, while the cavity roof began to collapse inward until only a thin roof remained that could collapse under load. In the clay area's sinkholes, usually no roof remains and the collapse is gradual.