They consist of clusters of earthen tumuli or burial mounds. It is assumed that most portal tombs were originally covered by earthen mounds. A ghotul is a spacious tribal hut surrounded by earthen or wooden walls. Trail surfaces vary from concrete sidewalks to wood mulch, gravel and earthen construction. Mesopotamian towns were protected from flooding with high earthen walls. Its walls were thick, with log facings surrounding an earthen filling. The walls at the bottom were often sloping, and protected with earthen barriers. Individual residences were separated by earthen walls. Its walls were thick, with log facings around an earthen filling. With the replacement of masonry fortifications with earthen works, mortars became more important. The dam is high and long, flanked by earthen wing dams. The town has concrete roads, BT roads, of WBM roads and earthen roads. Spurs off the main line ran into temporary storage areas protected by earthen barricades. Water came primarily from large earthen tanks, rivers and tube wells. Sand provides structure, strength, and bulk to earthen plasters.