Diving the Cape Peninsula and False Bay/Bruce's Mark

1 Bruce's Mark: S34°14.880' E18°34.880'. The 17 m pinnacles are about 1.7 km east of Whittle Rock

This site is not in a Marine Protected Area. A permit is not required.

The name "Bruce's Mark" is derived from the position mentioned in the first known dive report.

Maximum depth is about 44 m on the sand, and the top of the highest ridge is about 17 m. Average depth of a dive is likely to be about 30 m, excluding ascent.

Visibility is likely to be similar to Whittle Rock on the same day, but the water circulation in this area is not well understood.

The reef appears to be made up of three lobes of similar north to south dimension, a wider one to the east, with hogsback ridge pinnacles to the south end, and the other two narrower and deeper, to the west with the shallow area below 20 m to the centre of the east lobe, and a more blocky profile in the area visited. Most of the ref has probably never been dived. The centre lobe runs more northwest-southeast and connects the other two like the middle stroke of an "N".

Geology:

Granite corestone outcrop of the Peninsula pluton, surrounded by sand.

The site is exposed to wind and waves from all directions, and swells from the southwest, however it is quite deep, so short period waves will not affect conditions on the bottom greatly. Low short swell and light wind is best. The prevailing long period swell is from the southwest, and if long enough can cause surge even on the deep reef. Other waves are generally short period wind waves and chop, but the reef is far enough offshore for a vicious chop to develop in a strong wind, which can be very uncomfortable on the ride back to Simon's Town. The area is protected from north-westerly swell, and to a lesser extent south-westerly swell by the Cape Peninsula, but longer period south-westerly swell will refract round Cape Point, and though attenuated, will reach this area.

Surface currents have been measured at up to about 1.5 kilometers per hour, caused by recent wind, and in a similar direction, offset in an anticlockwise Ekman spiral by the Coriolis effect (the current is offset more at greater depth, but proportionately weaker). The skipper should drop divers off a bit up-current of the shotline, which may not be quite the same as upwind of it, and the drift direction of a boat with significant windage is not quite the same as the drift direction of an almost completely immersed diver. Occasionally the current will extend right down to the deep reef, but usually it is shallow.

Water temperature may vary with depth. A thermocline develops in midsummer, gets deeper in autumn, and dissipates in winter. The visibility may also change significantly below the thermocline. The surface at Whittle Rock nearby can be 18 or 19°C with 10 or 11°C at the bottom, but the difference is more likely to be 5°C or less. Conditions at depth are not easily predictable, and may be better or worse than near the surface. There can be a plankton bloom in the surface layers and a sudden improvement in visibility from 3 m or less to over 10 m in the cold bottom water, or, less often, fairly clean surface water, but dirty at depth. This dirty bottom water is more common at deeper offshore sites in False Bay, such as the wrecks of the Fleur (40 m), General Botha (54 m), and Bloemfontein (57 m). The depth of the thermocline is also not very predictable, but has been known to be between 12 and 20 m in late summer.

In winter the water may be the same temperature from top to bottom, and as there is less sunlight to power the phytoplankton blooms, the visibility and natural illumination can be better even though there is less direct sunlight. There is no specific time of year for diving this site, you just have to wait for low swell and light winds, and take your chances with visibility.

1 Bruce's Mark: S34°14.880' E18°34.880'. The 17-m pinnacles at the southern end of the eastern lobe

The Southeast Pinnacles area has two hogsback ridges aligned north-south magnetic with a steep narrow gully between them, and a third, smaller, parallel ridge a bit deeper to the west, rising to 21 m. The high point is at about 17 m on the middle ridge. To the east of the ridges the reef falls away steeply to a bit over 27 m, then a little further east, another steep slope reaches the bottom sand at about 43 m. There are at least two places on the eastern side of the 30 m contour where the sand is very close to the contour. At one place the depth was measured at 36 m. To the west and north the slope is flatter and quite a large area lies above the 27 m contour. On the day of the first survey the surface water was very murky from a red tide, but below 15 m it opened up and was dark but clear and cold, with visibility estimated at about 15 m. Echo soundings indicate a large area of reef below 30 m to the west. The eastern ridge's southern end is shaped somewhat like the bow of a ship, with a triangular flattish top, peaking at about 22 m, so it is called forecastle ridge. The drop from the top of this ridge to the forecastle area is abrupt, with a large, but not very deep overhang on the southeastern side, and a steep sloping ridge drop at the northern end. The western ridge is much larger, and extends over 100 m to the northwest. At 21 m it is quite a bit wider at the northwest end, and there is a section to the west that barely reaches 21m, separated by an irregular narrow gap. The reef close to the ridges is mainly massive outcrops of bedrock with weathered gullies, but further north there are large boulders and flatter outcrops. The 30 m contour is fairly narrow below the pinnacles to the south, and gets much wider to the north. The profile of the northern sector is relatively high on the east side, and a lot flatter to the west.

2 Western Pinnacle: S34°14.835' E18°34.610'. The 23 m pinnacle near the middle of on the western lobe.

A group of large angular boulders scattered over an area of granite outcrop with sandy bottomed gullies and a pinnacle reaching a shallow point of 23 m near the sand edge at about 38 m depth, some 30 m to the northeast of the pinnacle. About 23 to 33 m on bearing 250° true from the pinnacle the boulders form a couple of swim-throughs. The first recorded dive was through a murky surface layer to clearer water below 20 m

The reef life is very similar to that on other granite reefs in the same depth range in central False Bay.

The reef is mostly fairly deep, and while the visibility can be relatively good in this part of False Bay, natural illumination is seldom going to be really good, and artificial lighting will almost always be desirable for colour. There are some impressive vistas due to the topography, which will work best with wide angle, and there will always be subjects for macro work.

The site is not well enough known to suggest specific routes.

No known site-specific hazards.

The site is quite deep. it is almost entirely below the recommended depth range for entry level divers, but there is a large area above 30 m, the depth generally accepted for ordinary recreational diving.

A DSMB is recommended to alert the boat crew to your position when surfacing, and to help fishing boats keep away. Nitrox can significantly extend no-stop dive time in this depth range.