The boulders at Shag Point (Matakaea)are similar to those at the more well known Moeraki site.

The boulders are grey-coloured septarian concretions, which have been eroded out by wave action from the cliffs of soft, black mudstone that back the beach.In places,partially exposed concretions can be seen in the cliffs.They originally formed on the sea floor when the mudstone was accumulating during the early Tertiary period some 60 million years ago. The largest concretions are traversed by cracks, filled by yellow calcite.In some the upper part is worn away; only a shell remains, looking like discarded segments of orange peel. The concretions weigh several tons and are up to 12 ft in circumference. Similar concretions are known on the north-facing foreshore of Shag Point, some 12 miles further south, but these are derived from older (upper Cretaceous) mudstone.

According to Maori legend, the origin of the boulders dates from the loss of the Arai-te-uru,one of the large sailing canoes that came from distant Hawaiki. On her quest south for the precious greenstone, the canoe was wrecked near Shag Point (Matakaea). The reef which today extends seawards is the canoe's petrified hull, while close by, in the shape of a prominent rock, stands the petrified body of her commander. Strewn along the beach are the boulders which represent the eel baskets, calabashes, and kumaras washed ashore from the wreck. The name Moeraki (Moerangi) means “drowsy day”.