The Kenn Reef areas consists of four main reefs which cover an area of approximately 40 km² looking like a backward facing L-shaped structure (or boot) of 15 km in length, with a maximum width of 8 km along the southern edge.
The ankle or the South Eastern reef is the largest reef and with two sand cays on the SW part of the reef the only permanently emergent land the tiny Observatory Cay , which is about l00m by 50m and 2m high with a little vegetation.