TasNat 1925 No1 Vol2 Pp1-9 Lewis OutlinesGeology

7/28/2019 TasNat 1925 No1 Vol2 Pp1-9 Lewis OutlinesGeology

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~ b emasmanian j}aturalistTH E JOURNAL OF TH E

Tasmanian Field Naturalists' Club

New Series-Vo!. 1. / \ PRIL. 1925 No. 11.

Outlines of Tasmanian GeologySection 1 1 - ( Continued)

EROSION BY RUNNING WATER(Continned) .

DESTRUCTIVE WORI< OF MAN.Bush Fires.

Man comes along, and. in W!lat he mistakes as his interests alters the wholeface of the country and does so without; t moment's though t to the geology or tothe ultimate results of his actions. Thegreatest mischief-maker is the uncontrol

le d bush fire. Port ions of the countryrequired fo r agriculture are carefullywatched, bu t bush fires spread over wideareas of unoccupied land, and one firemay turn a ferti le hillside supporting aluxur iant forest growth into a bare, stollYwaste. The fires destroy the vegetation,an d the next shower wears out gutters,often down to solid rock, their size devending chiefly on the !dope. Before the, 'egetation can get a s tar t again the surface has been scoured into gl'eat channels, which ,vill g t ' O W from year to year,an d the land ruined fo r many lifetimes.

This is occuring everywhere ronnd Hobart . 'Ve should be particula.rb' careful to keep as much vegetation as! possible on onr steep hillsides, unt i l theyar e ready for the clm;;e attention of agricu1ture. Instead. fire after fire has ravished them, until in many places, notably on the S!lopes of l\iollnt 'Vellingtoll,all the soil has gone, and the hillside i ~bare of all b u t the most firmly established trees. The process may go further. and remove the whole of the soil.leaving only barren rock in the place ofa flourishing jungle. On La Perow;;;e.a bush fire, wantonly Hghted in a beautiful beech forest, even consumed thepeaty Boil. until now the surface of themountain is a veritable desert . Tt wasthe same destructive agent t ha t madeQueenstown and the Linda Valley resem-

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l)le a Somme battlefield. There the trees

\vere cut down fo r firewood, and the llntlergl'owth bur n t by a cuccessioll of bushfire:il deliberatel), lighted. Very shortly the excessive rainfall had removedl!very vestige of soil from the steep hillsides and given what thi r ty years ag o\Vas a valley of ferns and 1110::;se8 an aspect of ut te r desolation.

Clearings.Clearing of land should be under taken

with great circumspection, as most farmers in hilly country know. As soon

as the natu ra l coating of vegetation isremoved, rain water tends to wear deepchannels, which will, if not prevented,SOon ruin a fertile selection. Throughout Tasmania , which, with i t s steep slopesand heavy rainfall is particularly liableto this form of "pest ," you can se e farmFltha t have been ruined through the develovment of these channelt':o The remedy's Nature 's drainage and plant ing.

Road Cuttings.:Mllch damage is often Can se d by the

careless construction of roads runningacross a steep slope. Often a grasFlyhillside can jUt:rt drain off the surplusrainfall withou t disturbing the snrface.A careless engineer, with two little keenness to th ink of details, makes. a roarlacross the bot tom of the hillside. Sucha, road natural ly has a high cllttting onit s upper side.

Over this the water t h a t has beendraining quietly off the gl'assy sloperuns with a velocity increased f. hundredfold. In no t ime channel-s are cut in theroad bank, and theRe .extend rapidly Hpthe hillside. soon ru in ing i t for ever. Asimple drain to catch the water on tl.etop of the cutting, and AO prevent itfrom developing momentum by faHil1g'over the steep slope, 'would have prevent ed this. There are numerous examples


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THE TASMANIAN NATURALIST

of paddocks thus ruined round Hobart,notably on e just below the lower reservoir.

Another frequent form of stream erosion which has very damaging results istha t originating from the cart t rack in

hill country. A rough track or unmadel'oad is run across a s1ope. The surface is worn down sufficiently to intercept the natural drainage of th e hillsideand the check imposed by vegetation isremoved. This track then collects manystreams of water, an..d very soon becomesa miniature gorge, increasing in sizeevery winter. :Many of the streamcourses on the lower slopes of Mount\Vellington were once cart tracks, andmany of the prospectors' tracks throughthe West Coast are now mere grooves

worn six feet deep or more in solid rock."Bad Lands."When an otherwise level or e v e n l y ~

sloping piece of country b ~ c o m e sthusscoured an d seamed with a bewilderingmaze of deep gutters and s h a r p ~d i v i d ~ing ridges, It is termed "bad lands."Such are frequent when there has beena sudden alteration of drainage in th enear past. They may extend over apaddock or cover thousands of acres.They are merely an exaggerated exampleof erosions by the small tributaries at thehead of a drainage system, and illustratewhat is going on over the whole land vscape continually, j f imperceptibly.

EROSION BY RIVERS.In the previous sections dealing with

the wearing away of th e landsca.pe byrunning water, it was shown how t h ~bulk of the actu q 1 work of erosion wa's doneon the sides and a t th e heads of th evalleys by the small rills of storm waterthat come into exist-erlce after everyshower. But below these run the streamsan d often great rivers which an havetheir bearing on the erosion of the land vscape. Although they can only work intlheir channels an d aife'ct a very narrowarea compared with the streamlets already described, they really govern th ework of these smaller agents. In th efirst place tbey provide a local base levelfor th e rain wa t r channels. The river,although its work is restricted in area,is fa r more powerful than an y tributaryin its basin and can wear the bottom ofits valley deeper an d more rapidly thanthe water in any of the smaller streams.Thus the bottom of th e main v,alley inan y region is continually kept the lowestportion of that region. To this level thestreamlets can be alwqys reducing thesurrounding country and they al'e continually a t work widening this deeP"St valley. Were it no t for vhis work of th e

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river in providing an ever lower base levelthe small streamlets would very soon re duce the valley side to a level beyondwhich water would be unable to flow.In the second place the river removesand distributes th e material worn fromthe land surface by the smaller streams.Were this work not done th e streamletswould soon cease to flow OD account ofthe accumulation of sediments in thecourse.

MECHANISM OF RIVER EROSION.Trhe principles of river erosion are simi

lar for a vast volume of water like theDerwent or the Gordon, and for the8m!Olllest mountain stream and even forthe storm water runnel formed after aheavy showe'r, and these remarks holdgood for them all.

In th e first place, while a small stream vlet n:ay be aLle to cu t it s course ,;alelyin sLft eoil and wear ou t a course bymerely moving particles of this soil downtIle hill, a ;>p-rmanent stream or ::-iverve!'y soon ~ u t s d o w n ~in plac es if notthroughout its whole course tt. solid rock,When i t flows over hard rock, mereweight of water, or even volume, or speedof flow can accomplish nothing, and ariver cannot erode it s be d by th e s-im)iemethod of pu.hing or tloating porti0Dsof -;-,1e landscane down its course in fine~ r a ]ns. The river ca n only deepe,} its\,,;11iey by ruhb,ing boulders, p e b b l e ~ .cndS ; ' l l d over li'xposed bede of rock tu ;t sc n ~ T e ( ' As W ~ saw in the case ot !.hesh'ams, th e work a river can a co l1p1ishdeppnds on (1) '"he volume of w a t e ~andt re regularity of th e flow, (2) th e sl.)pe,(3) tbe hardness of the rock .ver \Vbi. hi t flows an d (4) the amount of sedimentit c ~ r r i e sover it s b.d

FLOOD TIME.1t is during flood time that th e l'i\"er

dr;c.s most of t h ~ work of erOSlOTI. Thenaq ~ t s tr ibutar::s arli' dischargIng ~ r . ~ ! ' I ,\'o:umes of sedb,eJJt into it , aud th e vii i t '"nal volume of water CS)!} tran'3pot't:.:trger bouldefl!l. 'DhHile poundi!:tg on an dsel'aping over '-,.e :-ork of its bed abvayslpRve "heir md ..;r in deepened chau ;(.'1


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a floqd may iD scour out the river channel that the ~ l o p eis insufficient to permitthe streams during the succeeding periodof Iow water to transport an y sediment.'Vhen a river has reached this stage it canonly erode it s valley when its volumehas been abnormally increased by floods.

THE SLOPE OF THE VALLEY.A ~ explained before an increase in th e

,;lope of twice gives water a carryingcapacity nine times as great. Therefore,i t is where the slope is greatest - W1herethe river is running the quickest - thatit is eroding the most. Should a river,after flowing along a very gently slopingvalley. come to a place where, from al'P(,pnt fault or other cause, the slope ishtceper, it will cut back from this place~ n ,", deep gorge. This is tihe simple explanation of the Cataract Gorge a t Laun-ceston, for example. Here the South Eskflowing for many miles over a very levelplain came to a sudden sharp drop toth ' ramar va 1ley, The increased velocityenabled it to cut a considerable gorgein "ery solid diabase here, when fur therinlanJ it haR hardly cut at an into loosepebble drifts. 'Dhe same explanation ncC O U l l t s tor most of ou r great gorges,

This feature is very noticeable a t thl;":ertge of the Central Plateau, and to a l e s

s(:'} extent on al l our flat topped mouutains. Streams which have hardly wornOU T a defined channel for themselves onthp to p of the plateaux and at th e footwander in wide, shallow, twisting rivers,fiGW in tremendous gorges, over fallsand rapids as they change in level fromth e mountain top to the plains below.'T'he riYer c u t ~ down it s bed largel,rhJ working th e rapids backwards up stream.

THE HARONESS OF THE ROCK.Naturally a valley takes longer to

evacuate in hard rock than it does insc fter rock. Thus two streams runnint(pnrallel :-,ud with equal volume and slopeway in time erode very different typesd valleys in different kind of rock. Butin Tasmania variations L! rock beds arcsc frequent tha t no considerable streamflews fo r it s whole course over rock ofone kind. The result of this is that th eriver's bed C'onsists of alternating beds ofdiffering hardness. The river naturallywears out a wide, well graded valley il lth(> softer bed, bu t finds considerable dil.ficulty in cutting through the harder rock.I t will wear out i ts valley in soft rocken the downstream side of a hard bedwhile this hard bar is scarcely affected.Th. drop from the hard rock to the softer

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bed will be ma.rked and rapids will result. This lP the chief cause of tb e man,\'rupids in our big rivers. Take th e Del", 'rent, for example. From above NewNcrfolk downwards i t has evacuated itsbeci in relatively soft mudstone an d her!;'fI(;ws in deep reaches. At Falls it hashad to cross a b ar of glasRY trachyte (alaya), Bud has been unable to make muchimpression on i t . The change of level ismarked and the river runs in a series ofrapids. The additional power given by thewcrease of rate of flow does not cornpensate fo r the greater hardness of ther o ~ ka t this point. The South Rsk, Merscy, an d indeed all our larger rivers sho,,,"the same process at work. \Vhere rapidsoccur usually i t will be found tha t the riveris flowing over a bar of rock harder thanthe average met with on its course.

Above such a hard bar, arr area of softerrcek may be found. The river cannot c u tinto this to a depth greater than i t is cuttmg into the hard bar below, or i t wou\tlcease to flow and have cutting power, sothe erosion of this portion of its valley dep E ' r ~ d son the rate of erosion Of the hardrock below. Often yo u ca n find exam ph.,;where a river has had its rate of ermu()nin a soft rock delayed for a very longpHiod by such a hard bar lower in itll.c(urse, but m time it has cut right throughtiJC hard rock, as for example when thishard rock consists of a relatively thinsheet of lava overlying softer rock.

As .soon as the hard bar disappearstl!c. r l v ~ rstarts to cut with great raVldlty m to the softer rock higher un .FJ'('viously i t had worn a broad valley inthis area. Now i t s tarts to cut a n a l ' row valley in the bottom of the olderbrcad on e and gives us an example ofwhat is known as a valley within a valJey-a feature common in al l basalt coun.try, especially along the North Coast ofTasmania. On the Nor th East Coast, t inan d gold deposits, which were once depc'sited in the bed of a river , ~ a l l e y ,art'no w found a hundred or more feet abovethe river which, having cut through ahard layer, ha s deepened its valley veryfsst. When a river appears to cut out anarrow valley a t an abnormally fast rateaf ter a long period of gentle erosion thisis often th e cause. '

Another aspect of i ts rock bed tha timparts distinctive features to a river'scourse is the way in which this rock isb e d d ~ d .! f i t is a. massive, igneous rockbeddmg IS nonexlstant, but if it is astra tified rock its tendency will be towear away along the bedding planes.


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TH E TASMANIAN NATURALIST------------------------ -- --------

The rue!,: will tend to break in slabs andLht' water to work uown joints and cutthE' s t ra ta mto blocks goven,lecl by thesejoints. A river's course is often contl'(,dled a t i t ~ infancy by these joints an dlater it c,tnIlut move from th e course snfermed. Again, if th e strata ar e clippll1g down stream a t ('xactiy the slope ofth", valley the water will flow o\'el' it\vithout hin(lnuH'E'. I f i t is dipping a t ( f harder rock overhanging, thus I!;ivingli S a waterfalL l\ lany of our falls on~ a : 1 a l lmountain streams are so formed,e ~ p e c i a l l yin sandstone ~ o u n t r y ,where

,ve find mm'lt ,)f these picturesque s c e n i ~oeantieR.

Again, while th e ri,7er is ( 'utting downthE centre of its valley i t is the smallerstl 'eamlets. as we ha,-e explained, t h a tarc- widening the valley. These ca n cutmere easily in soft rock than in hard. HOin conntry where the rock is Roft wef t ml to get broad, open, rolling valleYR.\",Lile in country where the rock is hardertlit:' valleys become narrow. steep-i'!idf'(lhorges- the small strt:'amletR heing unahl('le do much towards w"idening: them.

THE SEDIMENT LOAD.This ha s an impol'tant bearing on the

amonnt of work a river ca n do. Th('riv(>1' (lerivs tlIP sediment i t pus:he!'l overi t ~ bed mainly flom i ts small tributaries.The amount it ('an carry uepends on thevolume of " .-ater and the rate of flow, hutwhether or Hot the river is supplied withthe maximum amonnt i t ca n carry elependl'l on th e nature of th e r O l l n t r y ~overwhich the t r ibutary streams ftO\"\', oftennn the recentness and Y01m11e of the rainfall and Oil the absence 01 ' otherwise ofobstructions to th e carriage of sediment.

I f the stream Jet tributaries ar e cutt ing:t wa y loose soil or soft rock, or if theabsence of vegetation makes their workens;v. the river ,vill have a considerable

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load of sediment, and otherwise if thet r ibutaries are cutt ing iuto h a r d ' rock.Also if the landscape i::l hilly al l tl wI'otl'earns \\'il1 be assisted by the slope----while the process will be reduced in proport ion as the live ing proceeds. I t i ~('ommon knowledge t ha t after recentrains the r iw'rs are charged heavily with, 'ediments, an d i t js then t h a t their greatest work is done.

As nn increase of pace gives an increaseof carrying power, the H'YerSC is alsotrue. Anything t h a t teuds to retardth e How of a l'iYer tends to reduce it seroding Dowel'. Thus, if its meets a lakei t ~ load il'l deposited in the still w a t e r ~ ,and the outlet is free from sediments.Tl w same applif's in a lesl'ler degree todeep, quiet reaches so often worn by a

fiYer at intervals in its course. Here theriver is scarcely cutt ing a t al l unless intimes of aunormal iloods the current isstrong enough to scour along these deeppools.

The Shape of the Valley.As the river ca n only cu t iu the bottom

of it s valley, and as i t i ~ 1 alw.1Ys largeran d more powerful than an y of its tributaries, the valley tends to assume a Yshape, with the bottom of the V in thecentre of the river bed. I f the river i::;

cutting rapidly, and the t r ibutaries slowly,the sid-eR will be steep, 01 ' even precipitous ,,,,hen the river has cu t down toa point beyond which i t ca n cut, hut thelittle tributaries will gradually widen thevalley.

A stream in newly elevated countrymay commence to flow in 11 perfect];'Rtraight course, but snon it ~ t r l . r t stohend. ~ - \ slight obstruction, a differencein hardness of the rock, et difference inI';]ope, will al l tend to make the streamdt'Yiat('_ On a bend th e current m o v e ~faster on the outFdde (a s on the c i r ~cumference of a wheel). and thus b e n d ~tend, by cutting into the outer bank, toenlarge themselves, until any stream wese e today moves in a series of curves. TheRame proceRR gives ll S high banks and often cliffs on the onter Ride ~ f each curve,and lo w banks-somet imes swamps-onthe inner side. Tributaries also flow intothe main stream. The faster the r iver('nts down its bed th e more power thespt r ibutaries have to cut valleys of theirown. The t\VO processes result in a serif'S of spurs. \\"hich r U l l out into th e mainvalley. first from one side nnd then fromthe other. generally ending- at the outerside of a bend_

A river valley, therefore. usually prAsents a V-shaped section with numerou5I:s.purs projecting into the valley. You ca n


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seldom see far along th e valley i i standin g lleal' the level of th e river, and th eview is from anywhere restricted by aH...djacent :5pur.

If, from any cause the river derivesadditional cutt ing power, i t only accentuates these features, as i t cannot moveout of the valley already cut, and merely cuts the sncees.sion of beds deeper.

Section 12-EROSION BY ICEIn high latitumpcrate regions,Huch as Tasmania, the tops of the mOUlltains will first be aifectpd as a glacIalcycle approaches. On al l mountains thereare small hollows and pockets in which

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SllGW lies longer than elsewhere. A t tilUt'!olbanks of suow will persis t throughout amIld summer. These are very frequent on~ l !our mountains, and snow can always bpfot;ud in the same spots .l.t least up toChristmas t ime. The8e summer snow

Lanks on account of their persistence, gradually "dig themselves in." As has beenexplained, frost is a most powerful d6 en t01' erosion. Round these snow patchel!l ~ f ~J j naturally colder than on the res t ofthe mountain side, und here frost is ,LtwC'rk longer than eL:;f'v; here. Also the meltm g snow supplies l ( ' J'ltinl1al flow . ofwater W111.'h, soakh;:; l I l t \ ) t h ~ s.urroundlr.glock, is ready to \ . : x p n n ~ la! a t ear t l H ~particles ~ . 'ThHCRlf' usually filled with chiTlS of l'ork o rsnil, the products of thi-s frllRt (li . ;integm wtion. Gradually these sno,,,, lwnks will\,,"ork out fo r themselves an appreciable hollo w which, if the glacial cycle is advancing, wi U become the seat of th e ~ l a c i c n .

Th e Neve.

As snow accumulates in greater quanti t es than i t can be melted . t forms fir8ta permanent snow field. The weight ofa great depth of snow ; 1d the free7.ingad io n of the very lo w temperature soonttirn any considerable accnmulation intoie'. This ice cap covers the highest portions of the landscape, and in t ime, withthf ' increasing pressure of succeeding accu

IlIulations, it s tar t s to move. The directiOn depends on the slope, but in C a S f ~ , 3\If absolutely flat country pressure alonema y force the ice field to creep graduallyout from the centre of greatest accumulations. I f the slope is regular or noneXIstent, the ice will PlC;:. outwards untilit reaches a line where the t e m p e r a t u ] ' ~i high enough to melt each year as muchicf' as th e pressure behind forces intoth e locality. I f the slope is not regularthe ice will tend to accumulate in val-1f'Ys, where i t wi11 move as a frozen riverknown as a g1acier. The neve is theaccumulated snow bank or icefield tha tfeNh; the glncier, o r moving ice sheet.

.'\, glacier oc('upieF; a vttlkv and. therefor+>IJ ('n" i:-; lIsually exposed above it c r a g ~0: iee-free rocks. The neve, or an 1.('1'>sheet, on the o ther hand covers


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the highest portions of a given l a n d ~ ( ' . a p e-on ly occasional pinnacles J}rojel::~ i n g .

Clrques.T1hese snow b an k s gradual ly deepen

their seat , and as they do so th e forceof niv'ation grows s t r o n g e r ~and alsohas greater surface on whioh to work.The base of th e snow line is the pointa t which frost does most of its work.Hig h er UP. under a permanent snowcap, th e rocks are prote,cted from aUernat ing tempera tures , and from e r o ~ i o nby w a t e r or wind. Lower down thecold is less intense, but in the Z U l l i : ~

. where the sun is continually melt ingsnow a t nigh t , and set t ing free waterto saturate every crevice in th e rockface, and the frost a t night is expanding th is again as ice, t ea r in g the rocksa p a r t f ragment by f ragment , and enl a rg mg tlie network of t iny c r acks fo rthe w'ater from the nex t t haw to percolate th rough , we have t he maxImumwear and tear.

Al though th is may occur r ight rounda symmetr ica l peak , such fea tures O'Ccu r seldom, and th e most usua l pOSition for the zone of maximum nivationis a t th e head of th e val leys, j u s t,,,"here th e glaciers emerge f rom the ic esheet. The g lacier appea rs in thisway to gouge a rou te for its,elf in to theg ( ~ n e r a1 slope of the mounta in , andthese huge semi-circles of cliffs (might.:,'3rmchairR on the mounba.in side) a retermed cirques. They presept the mo'>ts t r ik ing features of our Tasmanianmountain count ry, and provide u s wi thour finest scenery. )1any large volumt:b would be required to describe thecil 'ques of TaSTIlania, b u t they arewell known to a ll who love the mountains. At the head of broad val leysa horsf'shoe of cliffs rise sometimes insteps, sometimes in unbroken wal l s fo ra thou sand ('Ir more feet to the narr-owridge of th e mo u n ta in crest . The flooris ~ l l w a y sf lat , and Ulsua,]:]y Clcyveredwi th t h e del ightful K i n g Billy pine an dpre t ty grass trees, while in m o s t repose th e jewels of the h igh lands , theazuri te blue rock t:lrn.

The L a k e Judd cirque, on MountAnne, is a gul f cu t into th e h e a r t ofthe moullta.in three miles deep. andbounded i t s whole l eng th by a Circle

of cliffs t.hree thousand feet sheer f romthe top of the mounta in to the blue wate rs of th e lake. The so-caned Crate!'Lake, on Cradle );tountain, lies in acirque of whi te cliffs si x hundred feetperppndicular from th e lake 's Burfac('.Th e Lake Belcher eirque, in Nat ionalParI.::, has been shown b y several well-

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known photographs , and can er;ual f'4-mnus E:pots in the !Rocky Mountains fo rpIctureSque and grand mountain scenery.

Glaolers.From thei r source in th e cirques, an d

fed b y th e accu mu la t io n s of th e NeveIce, the glaciers are p u sh ed down theval leys until t.hey reach a poin t wheretht'} vrcssure from behind is insufficient to replenish th e wastage from ineltl IIg. These r ivers o f ict" have a c'onsic1erablf' bear ing on the erosion of thelandscape. In the first place, i,n th ecirql1es a ~ th e wate r, freed b y the mid-day ~ h a w ,freezes again a t night , th eic e of the glacier becomes cemented onto th e surfac:e of the ci rque wall . Theg I a ~ i e l 'Is continually, if v.ery gradual

ly . moving down th e val ley, an d i t exerts a t remendous pul l on th e blocksof rock a lready loosened, as described,by frost, and tears them bodily ou t oftheir beds. I t i s qui te common to findblocks so wrenched from cirque wallssca t te red through our upland valleys,and Dften they measure f i f t y cubic feet,

. In the second place, th e very weightof ie" does considera.ble work in erodi'hgth e valley over which i t t ravels . InthiA work i t is ass is ted by blocks of

rock t h a t find thei r w a y to i ts unders u r f a ~ e ,and to blocks which i t picksup as i t moves. These, with th e enormous weight and power of th e moving'ice, tear and sc ra tch away th e rocksvf t h t ~ valley floor. A glacier ca n alsoscoop hollows in i ts bed below thegeneral leveJ, which is impossi,bl. WIthwater. B u t to o mu ch st ress must notUt:' plE'ced on th e erosive power of glaciers, 'beca.use ~ h e y very easi ly r ideo v ~ ran obstacle.

U Valleys.A s h as been explained, a s t ream of

water erodes only the very bot tom ofi ts val ley. A glacier, however, usually ccverg a m u c h wider bottom than as t r eam does. Also the s t reams val leyis largely widened by t iny t r i bu ta ryrIlls. A glacier ha s nothing to oomparf ' with these, and any t r i bu ta ryg l a c i e r ~are themselves qui te main features. F o r th i s reason th e sides of aglacial vaI1ey do not tend to widen.Again, a glacier can cu t with i ts sides,

WhiCh a r iver cannot , and the processof nivat ion wh ich is responsible for th ec i r q u ( ' ~is a t work to some ex ten t alongtl w ~ i ( J e , qof th e glacie,r. Here Is maxlmnm fro-st act icn . and the moving icE'C3.11 tOHl ' blocke of the rock wal l ruway.So (l s a resu l t a glacia l val ley presentsthe general a.speoct of stra:]ght, often


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precipitous lSoidE:ls, w l ~ h a broad, flatbottom. They rese'mble in section aU, j u s t as a water-wQ.fJl valley resembles a V. Fur ther, the glacier seldombends, au u tends to shear off prot rudin g spurs , giving a glaciaj vaUey amore open aspect , an d al lowing a lunge rvie,v than is usual with water-wornvalleys.

Such valleys arc very commonthroughout our west coast. T h e well-In.own view looking from the Lyel lmine down the Linda Valley showsthese features to perfection. IIn manyplaces on the weRt coast , fo r example .3.1ong the head of the Pieman and I t ~grea t . tdbutar i ( ' s , the Husskinson,::.Hurchison, and Mclntosh, and fu r the rsouth, f rom Lake St. Clair, to theVale of Rasseb .s and r igh t down theUpper Huon, the val leys presen t thLSdistinctive feature.

Glaciated L a n d s c a p ~ .

Water i n c n ~ a s e 8m capacity for er()-sion as i t descends a hill. so the gener a l ~ l s p e c tof watet '-worn topographyi& convex, t h a t is, the s ~ o p e incl 'easestowards t , h ~ bottom of the hill unti l t h ' ~ ~zone of deposition is reached, when Itattains the level with a gentle, sweeping curve. Ice-eroded country, on theother hand, owing to the pe,culiar metho d ~ of erosion by nivatlon an d pluckmg, becomes ra ther conclave in genEralslope. The sides of a mounta in a reo.iren hollowed o:ut in perpendicularcliffs, which m ay occur anywhere.'l.'hesf' cliffs do not shade off in a curving slave to the valley floor, bu t , I l I ! lYmeet i t nearly a t a r igh t angle. L.;o-lated mountains assume a pointedshape, and valleys are bounded by ragged cliffs. Lakes and waterfall.:;abound. T h e ~ eare a sure indkat ionof an interruption of the orderly pro

cess of erosion. In general ice actiongives the landsca;pe beloved of thephotographer and tOUrIst, the wild,forbidding ranges of the w'st coast ' a tonce inspiring a s ~ c e n e r y ,and heartbreaking- a.s source of livelihood.

Section 1 4 - THE SEA COASTThe seashore, so well k l lo wn tt l 118 all,

stands by itself a.s a peculiar, if common,

{eatun' in th e landscape with chal'aeteristics all of i ts own. The border line be-tween land and sea assnmes: variedforms-high cliffs, sandy h e a c h ' ~ ,low,barren. roeky shores, range after rangeof sand dunes, swampy reaches ann mUllfiats, all common an d often n l t e r n a t i n ~within a short distance. Here we see

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intense erosion of a distinct type, an dwe often find deposited sediments, theforerunners of ne w fields.

The general trend of th e seashore isusualJy regulated by major fault lines,which bound th e elevated blocks of th egloue which rise as land masses. Theserault lines, when originally fixed by earthmovements, may extend in roughly astraight line fo r many miles, giving astraight coast line, or may consist ofseries of cross faults, giving a serratededge to the land (caUed a serrated coast),or again .may occur in a mosaic pattern,which give.s a repeated series of roughlyredangular salients and i n d e n t a t i o n s(termed a "lobate" coast). The ordinaryerosion agencies soon modify the coast asoutlined by these faults. The en d ofriver valleys tend to become bays an deidgcs to pit out as capes. Gradual sinkin g may produce swamps, and rising ahigh coast line. Alteration of th e levelof th e sea also has its effects. Eu t still,by looking over a long length of coastth e original trend of the main causingfudol's can be discerned.

Waves and Breakers.

vVaves are caused by factors (chieflyail' currents) tha t disturb the ocean's surface. Friction reduces their effect, an da wave does not move the water belowa certain depth-which depends on its size.[ t is only when the water is so shallowthat the wave effect reaches to th e bot-to m tha t the particular wave becomes anagent of erosion. This, of course, mustoccur sooner or l a t e r ~as the wave nearsth e shore. When the friction of theshore is great enough to retard the bot-Lom of th e wave, the to p turns over,often with great force, as a breaker.

I f the coast is gently sloping and sandyor muddy, the effect of this is merely to

{lisplace some of the sand ur mud. Themore abruptly the coast rises, th e strongerth e force of th e wave_ This force is tremendous. A t the Blowhole at } J a g l e ~hawk Neck iron stanchions, concretedinto solid rock were completely smashedout of their fastenings, and thick ironrailings, all many feet above high-watermark, were twisted like pieees of fenc-ing wire by on e storm. This mere forcehas eonsiderable e f f e c ~but just as wesaw was the case witn streams, wateralone has really little effect. These wave.

are charged with sand, pebbles, an d evenlarge boulders, which are hurled againsta high coast or cliff, and break down an dtear apart the rocks. The stronger theforce of the waves th e bigger the boulder s it can hurl at the cliffs, an d it is truly!'emarkable th e size of the rocks that aresometimes left by a storm high up on a


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clitf ledge. Half a to n boulders hurled;-sO feet above higlH,-ater mark are common n ( ~ C l l r r e n c : e A .Remembering thatth e solidest rocks have joints and linesof weakness, it can easily be understood

I L l t I l l ) c L H I U l ' e l ' a n withstand the buffet-in g of storlllY seas for ever.

Cliff Erosion.Chffs al'P,


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then ge t caught by th e wind and gra-duaJIy moved further from high water mark.In time, sufficient accumulates to form asandy beach; and if th e wind is strong,sand dunes begin to grow. High cliffsar e usually separated by fine beaches. Onsuch a cuast el'OEIion is active. I f currenk; run strongly and indentat ionsH bound, the best beaches will be found.I i th e prevailing winds are inshore youwill find high sand dunet'!o l 'hu5 thestormy \ V e ~ tCoast preBents Ub Tasmania'sfinest beaches, and th e rugged South-Easthas many good ones; while the relativelyquiet waters of Bass Strait an-d D'EntreC i l ~ t r e a n xChannel leave us with low rockcoasts and few stretches of sand.

The numerous "tied-islands," or pairs ofi.slallus connected by a narrow sand spitthat abound round out coast, e.g., MariaIsland. Brm.y, South Arm, Tasman'sPeninsula, probably have their origin

likewise, in the droJ)ping of sedimente bythe tide an d currents as they are checkedwhen passing th e tw o islands.

Wind piles up th e sand into dunes toa certain height; but unless protected thesame wind tends to blow the top sandlayers of f these dunes when they havereached a certain height. Thus th e dunescontinually tend to move inland, oftenspreading destruction over many miles ofcountry, The danger to th e railway linenear Strahan &rom overwhelming dunes isa case in point. The only thing to chet:kthe onward march of th e sand dunes isvegetation. 'Many pleasant fields havebeen ruined by remo\,.-.-ing vegetation nearthe seashore. Cutting trees or burninggrass on a sand dune is as much d crime,an d likely to be fraught with as dangerousresults of the lighting of wanton bushf i r e ~described earlier.

A. N. Lewis.

Some Tasmanian FernsIt does not require a person to be a

botanist in order to be interested I D th eform of a fern. Though they bea r nobeautiful flowers nor offer any ag)'t-eable frui ts , thel'13 is somc,thing in th : gracefulness of foliage wh ich appeals tothe sense of even the ind i fferen t passrrby . To the stUdent there is a muchgrentf:r attra,ction; there i s recal led tohis mind the t ime when the ancestorsof the ferns of the present day coveredthe ear th with a mighty vegetation. fUiug u p the marshy depressions with the: ~ p e n tspore caRes and decay ing lunbs tomake the coal seams of the presen tgelleration. There were then no flowering plants , no other t rees b u t pr imi t iveforms related to th e conifers, then th eern:s. Thei r relat ives, the clUb mOB-ses,grew to noble dimensions of a hundredfeet and more st imulated by the wa r mhumid atmosphere then obtaining.

Few scenes in nature ca n be morebeautiful than a fern gully. \Vith a background of shrubs and trees th e Old ~ l a l ltree ferns spread their long fronds filliug up every spa-ce where the light of th esu n ma y enter, Though the complete

Mosaic of these Fronds

ma y appear as though they must absorb every particle of light energy fallingabout them, yet beneath this canopy1:\ f' th e plan t beings of greatest interestto th e naturalist . Delicate filmy ferns ofmany forms, beautiful names and weirdfungi revel in the fern t runks and moists('il of the depths of th e gully, We al l

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note tbe beauty, but ho w few eal'e toknow more; yet a further acquaintaneewith the s t ructure and life of ferns wi IIadll so much pleasure to a walk in t h ~

bush t ha t the little amount of thinkingl 'equired will amply repay us.Take the Old Man fern, which ap-

peal'S by It s robustness as though Itn l 1 ~ 8 tlive throughout the world, aidedin this ambitious delSil'e by the number Jfspares it customarily sheds. for in t l 1 1 ~case of :;he average tree fern theseamount to about fifty million per annum, yet it s distribution appears to bevery restricteu. I t occurs in many partso ~ temperate Australia, also ill New Zealand, but nowhere as luxuriqntly as ~ n']'asmania.

Yes, th e average tree fern, after it hascome to maturity, drops from it s frondlSH, fewer than fifty millions of spores, an yone of which may, i f circumstances faVOl' it , eventually grow into an Old Manhee fern itself, But how many of thesefifty millions do really succeed't Examineth e average gully with perhaps a hundl'ed specimens, there are a few thous1-:nd young ones looking quite happy; outof these a few ma y re,ach th e stage ofb E g i ~ n i n gto form a t runk, bu t veryfe,,,' mdeed appear to 'Succeed in pushingup their heads, after

A Hundred Years of Effort,~ o t ~ k eup a position of maturity amongstI ts fellows. Perhaps on the average not

TasNat 1925 No1 Vol2 Pp1-9 Lewis OutlinesGeology

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