50 
J. ii. \L McWiiAt^ 
iaU'i'iU*, imlik(^ most ol^ llic laU'ritc ol Ausli'aLia which is hoi'i/oulal (51, p. 
o2), is roi'iiu'd (til a siui'ace sloping at 4 to 8'^ away ivoiu the resistant 
melast (liiiientai'y vi(.g‘es. 
Two parties of senior stiuhiits of the rniversity ot Western Australia 
(•aj’i'i(Ml out Ihi' tield work, under the guidanee oE Dr. U. T. Prider, during 
tlu' first vacation, of 19.'!0 and 1945. Cliain and compass traverses tied to 
a rranu'work ol‘ I.ands and Survey I )epai‘tnient -"subdivisions were em])loyed 
in tin* mapping. 
II. PllVSIOfJltAiMLV. 
T!u' (Ireat Plateau of AYestein Australia (15, )>. 5) i.s generally mature, 
especially in the inland part ttf the plat('a.u. Theia- are, however, ocea- 
sional latrrite-ca[)p(al mesas and buttes, the .summitH of wliich mark the 
level of' a former juaieplain, atid raic‘ monadnocdvs rising above the general 
])lateau levd {S, p. 11), whicli iH'come more numerous towards th{‘ (‘dgc 
of tlu' (derated peingdain pioducing an immature topography. 
In the Lawnswood Artai dii'lVn’ential erosion and weathering, botli 
in the pres('nt and juist cycle of ei'osion, have been responsible for the 
main topogra])hie iealures. As the (|uartzite& are resistant to both 
weathering and e]-osd>n tlu'v form two monadnocks elongated }>aral]el to 
tile regio}ial strike'. Tlu' broad, mature valley running through the centre 
of th(' area corresponds to llie less I'esistant granitic gneiss and possibly 
was formed by an ancient river whieh flowed in a south-east direction 
before it was (-aptni'ed by more vigorous west-tlowing streams (14, pp. 
1 . Doh'rite dykes ane intermediate in resistance forming valleys 
in tiu' (|uart/.it('s and low i‘idge< in th(‘ gi’anitie gneiss (text fig. 1). 
Text Kig. 1 . 
Tlirc^e doleritc dykes in the upper granite gneiss, 20 chains 
soutli of (l.ackliiio. The doleritc dykes, hoirig more resistant to 
erosion tlian the gneiss, form ildgea. Note the iJuralleMstn of the 
dykes. 
