Penwith Downs Moors and Heaths in west Cornwall has been submitted for possible SSSI1 designation. NVC2 surveys have been completed, and some of the key features of interest for the proposed site are valley mires, the largest and most significant of which is Bostraze Bog (centred on NGR 139300 31950), located around 2.5 km east-north-east of St. Just.
The existing fen and bog SSSI selection guidelines advise that sufficient land should be included within the SSSI boundary to protect the interest feature and its hydrology. Guidance for determining boundaries for valley mire features states:
Wetlands in basins and valley bottoms can be supplied with water from various sources including surface run-off from within the catchment, groundwater seepage and direct rainfall.