266 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
they have been seen, capped with coral, and were once barrier reels lying off 
a mountain coastline. 
Geological History —The history of the coast has been one of continual 
uplifting, the stages of which were as follows. 
At the time of the opening of the Red Sea Rift Valley the Archean rocks 
were covered more or less with a sedimentary sandstone, and the gypsum 
now found associated with it was already formed. The faulting and minor 
adjustments consequent resulted in these sedimentary rocks being thrown 
into folds parallel to the sides of the main faults, of which folds three main 
systems are now conspicuous, lying at successively lower levels *. The sea 
now filled the valley up to the bases of the great hills, probably not for the 
first time, the highest of the sandstone folds being more or less submerged. 
Fie. 1. 
| Small Island |Shall- Deep 
A Reef flat) undercut |Recf flat| Deep Lagoon|Reef| ow |Reef fault | Reef (Cliff 
Cl ile Lagoon depression 
ZL FF SESS So See a aay Wipf 
LZ FEE 
Zit; LLL 
LP, 
> = 
Lie ; 
a 28 tl 
Shallow 
Roef flat} Shallow L R 
tt allow Lagoon Ke ieee ee Salen 
Vv 
eee se C — — 
5 AAA EZ > 
EE Zo LLL fp 6 Ye 
SAEED 
Diacram of the leveiling of a range of partly submarine hills by erosion and coral 
growth, resulting in formation of a complex series of reefs and lagoons; sea-level 
shown by dashes. 
A. Thin line outline of range of hills of which two summits are shown, one above and one 
below sea-level, and a deep fault cleft between the summit and the rise towards a 
third. From these are formed an island surrounded by undercut cliffs and broad reef- 
flats, two deep lagoons, a miniature atoll with shallow lagoon. 
B. The same after elevation and further erosion and coral growth, starting from the results 
shown in A, We have the complete removal of the island, broad reef-flats and a 
lagoon occupying its site. The atoll ring is broader, but otherwise much as in A, while 
the deep lagoons become narrow passages and are slightly shallower. 
By the growth of coral on those portions over which the depth of water was 
not too great, and the cutting down of the islands, they were converted into 
a chain of barrier reefs (fig. 1). Assuming that the relative levels of the 
* Detailed soundings outside the barrier reefs might reveal several more—note, e. g., in 
the middle of the sea how closely the 500-fathom line approaches the narrow central 
trough with depths of 1000 fathoms. 
