[1] At low tide a wide expanse of sand and rocks are exposed and a bar of fine shingle provides a barrier from the sea to form the largest area of (usually) fresh water on St Mary’s.

In its half-mile length it falls less than 25 feet and has barely cut a river bed through the marsh;[2] but feeds the pool with fresh water.

The area is an important feeding station for migrating and wintering birds and was designated as the Higher Moors and Porth Hellick Pool (St Mary's) Site of Special Scientific Interest (SSSI) for the ″wide diversity of habitats with several rare and notable plant species″ in 1971.

[3] All of the land designated as the Higher Moors and Porth Hellick Pool (St Mary's) SSSI is owned by the Duchy of Cornwall.

The moors were once grazed and the fields have a rich wet grassland flora with Greater Tussock-sedge, Hemlock Water-dropwort, Purple Loosestrife, Royal Fern, Water Mint and Yellow Iris.