Work executed with more or less large stones put together without any attempt at courses is called rubble walling. Where similar work is laid in courses, it is known as coursed rubble. Dry-stone walling is somewhat similar work done without the use of mortar. It is bound together by the fit of the stones and the regular placement of stones which extend through the thickness of the wall. A rubble wall built with mortar will be stronger if assembled in this way.
Rubble Walls - In Malta Rubble walls are found all over the island. Similar walls are also frequently found in Sicily and the Arab countries. The various shapes and sizes of the stones used to build these walls look like stones that were found in the area lying on the ground or in the soil. It is most probable that the practice of building these walls around the field was inspired by the Arabs during their rule in Malta, as in Sicily who were also ruled by the Arabs around the same period. The Maltese farmer found that the technique of these walls was very useful especially during an epic where resources were limited. Rubble walls are used to serve as borders between the property of one farm from the other. A great advantage that rubble walls offered is the fact that when heavy rain fall their structure would allow excessive water to pass through and therefore, excess water will not ruin the products. It is also a known fact that water carries with it anything in its path including the soil in the field, however the wall structure allows the water to pass through but it traps the soil and it will prevented it from being carried away from the field. One can view frequent rubble walls on the side of the hills and in valleys where the land slops down and consequently the soil is in greater danger of being carried away. The following web site gives you an image of a rubble wall. http://www.searchmalta.com/cgi-local/photos/main.cgi?id=3398&type=3&brand=0