The Geology and Physioghaphy of the Lawnswood Area. 
ol 
Dift'erential oi'OKion has been greatly aided by deep and long con- 
tinued weathering which took place towards the end. ol! the previous cycle 
of erosion. Granite and basic rocks alike were reduced to a whitish clay 
down to the base (»f the kaolinised zone, oil to 10b feet below the laterite 
which was fonning at that time. This extreme weathering is very unusual 
in Western Australia and is ]_>ossibly due to moderate to heavy rainfall 
during the time of laterite formation forming unusually acid ground waters 
which altered all the rocks, with the exception of (juartzite, into the 
residual clay deposit. 
In the [n'csent cycle of erttsiou, dihcvential erosion ol th»‘so deeply 
W(uithered rocks has produced a dissccli'd peneplain with latcndte-capped 
mesas and buttes, overlying soft kaidinisial rock, rising steeply above the 
unweathered rock to a height of 50 to 100 feet, n.'he laterite mesas and 
buttes invarialdy s!o]ie at 4° to 8° towards the centre of the valleV 
(>ee text tig. 12) and small streams conse'pnml on this slope, which Hoav 
into a larger stream in the eentre of the mature valley, have, in most 
l)laces stripped off the kaolinised r<)ck leaving the nnweathered rock 
exposed. \ ery immature hubsei[uent streams are dissecting narrow gorges 
in the soft hornblende and mica-schist bands in between the ({nartzite 
ridges. 
W. 
E. 
Scale of Feet 
0 500 
O lOOO 2000 
=3 VERTICAL 
1000 
=3 HORIZONTAL. 
3000 
La ten te 
S 
Do!e rite 
G 
net ss 
Hornblende Schist Quortzi te Etc. 
Text Pig. 2. 
Cross section of tho Lawnswood Area along a line bearing 73° at 
23 chains injrth of Lawnswood siding, showing the laterite capped 
mesas which slope down towaj-ds the central valley. The okt ju'O' 
file upon which the laterite was formed, is shown l>y broken fines. 
III. (JEOLOGV. 
a. OCCEHREXCK of the rogks. 
1. Metasediments and Hornblende Schists. 
There are two broad hands of metasediments in the area both Avith 
the characteristic norlh-north-west strike. Tlie western band Avhich dips 
west at 25° to 35° in the northern ])arl, and west at 50° to 05° in the 
southern ]>art of tlie area, is corrclalcd with the upper metasedinieiit of 
the To'odyay Area, (23, p. 88) because it is on tli<‘ same line of strike 
(see text bg. 3) and petrologically tin' (piartzites are id(‘n1ical, both con- 
taining idioblastic palc-grcvn cln’onii'-niTiscovitc orienied parallel to the 
bedding and poikilobbistically included in ihe (juartz grains. The eastern 
meiasedinu'uts, which Inno been coniu'ctcd in tiu' Held to the lower 
quartzites’ of the Toodyay Ar(‘a (see text fig, 3), have dips varying 
from 40° to 50° to the east. 
