The degree to which the original Cape Fold mountains (formed during the Carboniferous and early Permian Periods) have been eroded is attested to by the fact that the 1 km high Table Mountain on the Cape Peninsula is a syncline mountain, meaning that it once formed part of the "bottom" of a valley when the Cape Supergroup was initially folded. The anticline, or highest elevation of the fold between Table Mountain and the Hottentots-Holland Mountains to the east, on the opposite side of the isthmus connecting the Peninsula to the mainland, has been eroded away. The Malmesbury shale and granite basement on which this anticline mountain rested also formed an anticline; but being composed of much softer rocks, readily weathered into a 50 km wide sandy plain, called the "Cape Flats" (see diagram below, on the right).