The Watchman is an erosion-resistant extruded dome of dacite in the middle of Karekare crater.
The rocks that form the cliffs and landforms at Karekare Beach were deposited and erupted on the eastern slopes of the Waitakere Volcano during the Early Miocene (19-16 million years ago). The older rocks are layered volcanic conglomerate (Piha Formation) that forms the cliffs (Karekau Pt) at the south end of Karekare Beach and the tip of Farley Pt at the north end of the beach (north end of Union Bay). These were deposited as lag deposits by passing submarine lahars and at Farley Pt include an unusual layer of pumice breccia.
After the conglomerate was deposited under the sea, the Waitakere Volcano was uplifted and eruptions broke out along a line on its eastern flanks. One of these vents was at Karekare and resulted in the explosive excavation of a 1 km wide crater.
The crater was partly filled with several extruded domes of viscous dacite lava and associated flows and breccias. Eroded remnants of these erosion-resistant domes form The Watchman and Paratahi Island, offshore to the south. Also erupting into the crater were andesite lava flows, which contributed to the crater fill and can be seen in the cliffs above the north end of the beach. Some of the andesite may have been fed to the surface up intrusive sheets through the conglomerate. These feeders later cooled to solid andesite dikes that can be seen in the cliffs at both ends of Karekare Bay.
Since the eruptions ceased, the Waitakere Ranges have been uplifted and tilted to the west and greatly eroded. This erosion has exhumed the crater and eroded out some of its contents and its wall to the west.
At the peak of the Last Ice Age (18,000 years ago) when sea level was 130 m lower than present, the Karekare River flowed west 10-30 m below what is now Karekare Beach. When sea level rose once more (7000 years ago), this part of the river valley first became a flooded embayment which has progressively filled with sand and sand dunes. At the present time a slug of excess sand is moving north up the coast and the sand in Karekare Bay has built up remarkably in the last few decades.