Brick continued to be the main building-material in Assyria too, for that country derived its culture from the lower Euphrates valley.[12] But in the north soft limestone quarries were accessible. So in Assyria they lined their mud-brick walls with slabs of limestone, carved in low relief and brightly coloured; and they set up huge stone colossi to flank their palace entrances. This use of stone, both as a wall-lining and in wall-foundations, constitutes the main difference between Babylonian and Assyrian architectural design. Incidentally it explains how the earlier excavators were so much more successful in Assyria than in Babylonia; for in both countries they drove their tunnels and trenches into most of the larger mounds. They could tunnel with perfect certainty when they had these stone linings of the walls to guide them. But to follow out the ground-plan of a building constructed only of unburnt brick, with mud or clay for mortar, necessitates a slower and more systematic process of examination. For unburnt brick becomes welded into a solid mass, scarcely to be distinguished from the surrounding soil, and the lines of a building in this material can only be recovered by complete excavation. An idea of the labour this sometimes entails may be gained from the work which preceded the identification of E-sagila, the great temple of Marduk, the city-god of Babylon. The temple lies at a depth of no less than twenty-one metres below the upper level of the hill of _débris_; and portions of two of its massive mud-brick walls, together with the neighbouring pavements, were uncovered by bodily removing the great depth of soil truck by truck. But here even German patience and thoroughness have been beaten, and tunnelling was eventually adopted to establish the outer limits of the ground-plan, much of the interior of which still remains unexplored.[13]