Let us first examine the wall. There are three different layers of stone courses visible, see illustration below (a similar picture appears in Todd Bolen’s blog). The lowest courses are Herodian. They were built by King Herod the Great, who began building the new Temple Mount in 19 B.C. Seven courses can be seen above ground, but there are many more below the surface. Above these stones are four courses of large stones which were laid in the Umayyad period (around 700 A.D.). Both types of masonry are so strong and well built that, up till now, no earthquake has been able to move these walls. The big earthquake of 1927 caused the destruction of the El Aqsa mosque, but not even a crack appeared in the Herodian masonry! The upper third part of the Western Wall is made up of small stones. It is in this section that some stones are crumbling.
There are legends about this upper section of the wall, one of which claims that it was built in the 19th century by Sir Moses Montefiore, a Jewish philantropist. He indeed tried to buy the Western Wall and the adjacent area to facilitate Jewish prayers, but his efforts came to nothing.
It is difficult to say when this upper portion was built, as it has no distinctive masonry. It could date from any period after the Early Muslim time, i.e. Fatimid, Crusader, Mamluk or Turkish. The top three courses were added by the Muslim Religious Council as part of general repair work. This upper third part of the wall is, of course, the back wall of the western colonnade.
We must ask ourselves the question why the upper parts of the Temple Mount walls suffer so much from crumbling and/or bulging. Very large bulging parts in the southern and eastern walls had to be rebuilt a couple of years ago.
I believe that it is caused by the renewal and renovation programme undertaken on the Temple Mount by the Muslim Religious Council. Many walls and pavements have been “repaired” by pointing in between the stone courses. If this is not done properly, it can cause problems such as bulging and the subsequent collapse of walls.
Ancient walls were built with stone courses on both sides of the wall and a fill in between. This fill should have been made up of a hard core fill of broken stones set in lime, but often soil was mixed in with it. When it rains the walls get wet, including the inner fill. The ancient mortar, made of lime and an aggregate, such as sand, gravel or sometimes ground up pottery, is porous and lets the water seep out through the gaps in between the stones. Such walls can ‘breathe’ as it were.
Over the last 30 years or so, an extensive programme has been going on on the Temple Mount, where old walls were refaced with new stone and the joints in between stone courses of old buildings have been filled with Portland cement. This modern cement is very hard and seals off the spaces in between the stones. Walls still absorb rain water, because stone is porous. However, once the inner core is wet, the water cannot seep out, causing a swelling inside the wall itself. Over time, more water is absorbed until the swelling is so large that it causes one of the sides of the wall to bulge, usually the outer side, as there is no counter pressure. Individual stones can be crushed by the movement of the stones. This is the cause of the crumbling and bulging of the upper parts of the Temple Mount walls and also of other buildings on the mount itself. If this situation remains unchecked, more sections of the walls will bulge, crumble and eventually collapse. So, watch your head when you stand at the foot of the Western Wall!