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The chalk itself is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. It forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from trillions of micro-organisms called coccolithophores. It is common to find flint nodules embedded in chalk which are thought to be the silicon remains of sponge beds. The appearance of the chalk from the promontory at Old Harry is quite regular with the bedding planes and joints almost looking like laid breeze blocks.
The chalk strata at Old Harry are on a horizontal plane as clearly shown by the the sand and grit line that runs through No Mans Land, Old Harry and on through the Needles, whilst within two killometres south the chalk strata flollows and vertical plane at Ballard fault. The cause of this fault is in dispute but it is probally the result of the African and European plate collision some 30 million years age that also caused the Lulworth Crumple.
The creation of Old Harry starts at a headland with the sea exploiting a crack in the chalk joints and slowly creating a cave. As the sea gradually eroded along the joints and bedding planes the cave eventually created an arch which then collapsed to leave the stack of Old Harry, No Mans Land and the gap of St Lucas' Leap. In the 1770's, people could still walk from the headland of Handfast Point to Old Harry. Old Harry's wife was still a stack until her eventual collapse in