Rocks Plucked by the Sea 



by Edm t'M) 



lhe miiI lofti is PIl* 1,1 Natures 

 ,,«u-r tools. Waves oscdlale then 

 ujt .hkI surf uishes against the 

 shore. Main tjl the resultant processes 

 ' been described, along with the 

 jfofa 0 <i weathering, yel il is remark- 

 am thai although rocky shores have 

 m$i sludied lor over a century, their 

 processes ol formation are still little 

 understood. 



One accessor process, that is 

 ......climes mentioned hut given little 



..Mention, is that ol piutking by the 

 ,ti Many writers have described 

 , uKI pressure, the effects ol com- 

 pelled ail (as seen dramatically in 



hlowhole > . and the erosive power 

 jjj sand and rocks swept by the water 

 ,ero!>s the surface of a shore plal- 



, ni W i iters have likewise recorded 

 how the sea quart ics by working tire- 

 Bjlj along joint planes, bedding 

 li.ncs. /ones of decomposed mck, 

 ioss.1 soils and such, so as to gradu- 

 ,||v remove massive boulders that 

 mmhlc into the sea. or may he 

 thrown landw.ud during heavy 

 >ii.ii tns. 



Phtcklttg is a different ptocess, 

 whereby pieces of rock are lemoyed 

 •■ lugging, sucking, and such like 

 || iv closely allied to quarrying, but 

 fa difference will be apparent from 

 ihc iwo examples described below 

 ih.il were observed on the coast ol 

 Victoria. Australia. 



From St'HM arose Ro> k M wk 

 Plate 1 ItuulC I shows the holdfast 



.id adjacent pari ot lite kelp Lh„- 

 ,„ .,i,„ched to ji piece i»l I QWM 



Creuccous arkose. It was found on 



D. Git t * 



a rocky shore platform southwest of 

 Von Mueller ( reek on the Otway 

 coast of Victoria. The kelp pulled a 

 piece of rock from an outcrop at or 

 below low water level, because thai 

 is where Durvillcn grows Such a 

 piece of rock must have been more 

 or less angular. The area ol attach 

 incut is flat, The rock is now worn 

 lo the exact oulline of the holdt.'M, 

 and completely rounded. We can 

 infer thai- 



1. lhe rock was lounded between 

 the time il was plucked from the 

 outcrop and when it was found 

 on the shore on 12 Junc> 1V7I. 



2. As the Dinvillcu was eroded but 

 still alive, the tune interval could 

 not have been great, and there- 

 fore erosion of such rock ui that 

 environment must be rapid. 



The brown colouration of the rock 

 near the kelp is apparently due to 

 organic slain from the alga. On the 

 Il.it surface to which lhe holdfast was 

 attached there is a series of iaim 

 roughly concentric line ridges. Such 

 t have not seen before, and presume 

 that they are connected with the 

 growth of the nlgn, In facetious 

 mood I asked a fellow palaeontolo- 

 gist what fossil il was. but of course 

 he had never seen one like that be- 

 fore! 



In Augusi )«r>7 1 a series of kelp 

 fronds was examined on a beach a 

 short distance south of St. George "s 

 RjVCT on the Oiw.iv coast near Lome. 

 It was noicvl that about half of the 

 holdfasts had some ruck attached. A 



•Hcp»«,' ttWMr. NiCiuM" VWo. I Vumru 



cfe<*. 1971 



287 



