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Kiln-baked brick appeart in the reign of Augustus (r. 27 B.C.-14 A.D.), long;: and narrower than ours and used both for facir.: concrete and independently. For their monuments. buildings, the Romans turned to concrete; pozzno-lana, a volcanic ash, provided a natural cement. Regular ashlar masonry, opus quadratum, needed no facing, but the Romans believed that concrete did. Opus incertum, perhaps the earliest type, consisted of irregular blocks embedded in the surface. Opus reticulatum, square blocks arranged t diamond pattern, was standard under the empire through the reign of Hadrian. In later times, alternate hands of stone and brick, opus mixtum, iormecl the facing.

A glacier is one of the most powerful agents of erosion. Its ice erodes by abrasion and by plucking away at the bedrock. Blocks embedded in the ice are scraped along the bottom, producing grooved (striated) rocks, and resistant rocks are polished into roches moutonnees. The source area is enlarged into an amphitheatre known as a cirque or corrie, and where two such cirques meet they are separated by a sharp ridge called an arete.

A large area of paving blocks likely to receive considerable traffic should be bedded on mortar on a prepared foundation. Start to lay the blocks in one corner of the patio and work diagonally across the surface. This makes it easier to ensure that they are laid consistently flat. Place i/zin thick offcuts of wood between the blocks as consistent joint spaces, or simply butt up the blocks for finer joints. As you work across the surface, kneel on a piece of board to distribute your weight.