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The Precambrian western core area, known geologically as a shield or craton, is subdivided by long, straight (or only slightly bowed) fractures called lineaments. Those fractures, most obvious in the north and west, delineate prominent rectangular or rhomboidal blocks, some of which have been raised to form uplands; others have been depressed to form lowlands or topographic basins. The lineaments display strong northwest-southeast and northeast-southwest trends in the northern, northwestern, and southeastern parts of the shield, but east-west alignments are prominent in the centre, and major structural lines are more nearly longitudinal in the west and southwest. In all areas, ...(100 of 51163 words)