182 GEOLOGY OF QUEENSLAND COAST, 



Pleistocene a<]je. This subsidence determined the present Barrier 

 Reef. What the immediately antecedent stage to the early- 

 Tertiary uplift of the Cretaceous peneplain may have been is not 

 known at present. Secular movement is, however, doubtless the 

 key to the initial stages, while subsidence with concomitant sedi- 

 mentation was merely an after factor in the evolution of the main 

 features of the present shore line topography. The presence of 

 small cliffs only on the markedl}^ irregular Queensland coast, 

 combined with the even seabottom of interinsular areas, and the 

 great extent of coastal plains, argues a gentle subsidence combined 

 with filling of sunken coast valleys, while marked depression 

 succeeded by more stable conditions during which marine erosion 

 could cut cliffs is hinted at b}" a study of N.S. Wales, Victoria, 

 and Tasmanian coasts. Shore-grade"* was attained in places, and 

 island-tying was effected by the complicated processes of along- 

 shore action. Splendid examples of this action are illustrated at 

 Challenger Bay in the Palms (marine erosion), at Hinchinbrook 

 Island (complication of tide, along shore current, marine erosion, 

 fluviatile action, and elevation), Stradbroke, Moreton, Frazer, 

 Curtis and other islands. Gain to the coast was accomplished 

 by the development of coastal plains. Bay bars were produced 

 by wave and current action, while similar processes resulted in 

 the foi'mation of lagoons, lagoon-marsh meadows, and successive 

 aggradation lines curving sympathetically with the shore line,t 

 while bays were becoming silted uj^ b}' fluviatile and {eolian 

 agencies after the formation of bay bars.| During this period 



* An adolescent feature — attained when transportation occurs along shore, 

 with island tying and formation of bay bars. 



t Line of intersection of the sea with the land. Gulliver, " Shoreline 

 Topography." 



X Port Jackson is an example of a ria or submerged subaerially curved 

 valley, which owes its commercial importance to its steep off- shores, the 

 absence of large streams discharging either into it or into the sea immediately 

 south of the Heads, thus preventing silting up by river action, or the redis- 

 tribution of river loads by wave and dominant along shore current (south) 

 action to form spits and bay bars. 



Botany Bay is an instructive lesson in the filling up of a submerged valley 

 by river and ieolian action aided by recent elevation. Aggradation lines are 

 seen on the large flat near Lady llobinson's Beach. 



