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4

442728 0-58

CHART |.

EXPOSED ROCK UNITS IN THE CENTRALIA-CHEHALIS DISTRICT

Formation
‘ond membe

Thickness
(feet)

Character of rocks

Pleistocene ond
Recent

Alluyium and
landslide debris

0-30

Gravel, sand, and silt along stream
courses; low-level terraces and
fan material; swamp deposits locally

Debris composed of Quaternary and
Tertiary rocks

‘QUATERNARY

Vashon drift

0-300

Outwash deposits of sand and gravel,
‘and till; composed chiefly of meta~
morphic and igneous rocks of
foreign origin. Deltaic deposits
of sand and gravel locally. De
posits containing abundant locally
derived basalt and andesite in a
few places

Pleistocene

Terrace deposits

0-50

Gravel and sand with interbedded silt
and clay

Logon Hill formation

0-200

Glaciofluvial ueposits consisting
of gravel, sand, and silt, with
‘some till. Gravel is composed
chiefly of locally derived por
phyritic voleanic rocks; unit
weathered and iron stained in
upper 30 to 60 ft

Unconformity

Pliocene

Miocene

Nonmarine sedimentary
frocks

0-100

‘Thin-bedded to massive tuffaceous,
‘arkosic, and carbonaceous sand-
stone and siltstone, chiefly of
lacustrine and fluvial origin.
Basaltic sandstone and conglomer-
ate near contact with underlying
basalt. Upper part of section
less consolidated, consisting
chiefly of lake deposits of clay,
silt, sand lenses, and sbundant
fragments and logs of fossil wood

Columbia River(?) basalt

Black aphanitic to finely porphyritic
basalt, vesicular in part, jointed

| — unconformity

Astoria(?) formation

‘Massive and crossbedded carbon-
faceous, arkosic sandstone with
thin interbeds of siltstone;
locally thick basaltic and ande-
sitic conglomerate beds and
pebbly sandstone. Fossil wood
‘and leaves commonly present.

A few fossiliferous marine inter-
beds

Oligocene

TERTIARY

Eocene

re
L

Unconformity

member

Lincoln
formation
of Weaver

Tuffaceous siltstone
\
iu

Bosaltic sandstone
‘member

0-200

Massive gray tuffaceocs marine silt-
stone and fine-grained tuffaceous
sandstone; few persistent calcareous
beds and tuff beds. Basal part inter-
fingers with basaltic sandstone
member in western part of area

+

‘Massive dark-greenish-gray fossil-
Merous basaltic sandstone with
a few tuffaceous siltstone beds
and basaltic conglomerate beds.
Persistent pumiceous lapilli tuff
beds near base. Calcareous
nodules and beds locally present

Skookumchuck formation

Massive to thin-bedded arkosic
sandstone and siltstone contain-
ing interbedded carbonaceous
material; coal beds near top
and base of formation. Sandstone
locally fossiliferous, grading
laterally into marine siltstone.
Basaltic sandstone and conglomer-
ate beds locally present in lower
part of formation, Inclides fissile

it

|— Local unconformity

Northcraft. formation

0-1500+

pyroclastic
rocks, basaltic conglomerate and
sandstone, Rock sequence thickens
from west to east

Mcintosh formation

Base not exposed

Dark-gray foraminiferal tuffaceous
siltstone and claystone. Contains
interbeds of massive pebbly arko-
sic and basaltic sandstone in upper
and lower parts. Caleareous nod-
ules and thin tuff beds present lo-
cally in the siltstone. Pyroclastic
rocks and lava flows in lower part
of sequence in western part of area