The picture to the right is of a brown algal biscuit collected for study.
Algal Biscuits are a form of stromatolite, which grow on rocks and are often found rolling loose in the sediments at certain times of year. They occur in water up to 1 m deep, and are primarily formed laminated algal hemispheres of Phormidium hendersonii. The area to best observe these algal biscuits is between Castle Island and Old Castle Island. The algal mat is bound by layers which accumulate every 24 hours. In daylight, the growth of the algal filaments produces vertical fibers which trap sediments, and at night growth of horizontal fibers bind the sediment to the structure. This process repeats itself creating the laminated hemisphere observed below.