First designated as a Site of Special Scientific Interest (SSSI) in 1971 for its range of wetland habitats; it is an important feeding station for migrating and wintering birds such as common snipe and water rail.

The second British record for great blue heron also occurred on Lower Moor and, was also found by Cavell Smith.

[7] The site can be entered via a short lane at Old Town, the original medieval settlement on St Mary's, or from the A3111 near Porth Mellon.

[3] Since grazing was withdrawn some of the marsh is overgrown with abundant common reed (Phragmites australis), rushes (Juncus spp.)

The drainage ditches, flow southward to Old Town Bay and divide the marsh into wet meadows with abundant soft rush (Juncus effusus), yellow iris and some ragged robin (Lychnis flos-cuculi) and greater bird's-foot trefoil (Lotus pedunculatus).