STRATIGRAPHY AND STRUCTURE OF THE PUGET GROUP. 3 
developed either on stagnant ice-sheets or in front of glaciers as morainic ridges or 
beneath ice of whose lower surface they present the casts. 
Tn the vicinity of the Sound these gravel deposits are deep, extending below sea- 
level probably several hundred feet, and even at distances of 20 to 30 miles eastward 
along the foothills of the Cascade range covering the older rocks locally to depths 
from 300 to 400 feet. They thus determine the topographic aspects of a wide area, 
_ almost obliterating the configuration of the solid rock surface upon which they rest. 
From the bluffs about the Sound the plateaus rise toward the mountains by ter- 
races, which are often disposed irregularly with reference to existing streams, but 
in a general way extend above the higher tracts between the rivers. Within these 
higher areas the deposit of gravel is thin or locally wanting above the older rocks. 
The canyons cut by the principal rivers flowing from the Cascades and mount 
Rainier also expose the underlying strata, and they may be seen in occasional iso- 
lated outcrops in the gravelly expanse nearer the sound. The topographic aspects 
are not markedly influenced by these outcrops, over which, as over the plateaus, 
extends the prevailing forest. 
The gravels occur at considerable heights on the Cascades, but their general sur- 
face probably does not extend to much more than 1,000 feet above sea. From it 
the mountains rise up boldly, sometimes with precipitous fronts. Between these 
bold foothills on the east and the all-concealing gravels on the west is the zone, 10 
to 20 miles wide, of terraces and transverse canyons throughout which the coal- 
bearing strata may be traced. It is worthy of note that the known productive coal 
fields all lie in the lower basin of Puget sound and not upon the higher western 
slopes of the Cascade range. . 
Valleys, canyons, and hills older than the present ones lie buried beneath the 
gravel deposits. They are so concealed that no clear conception can be formed of 
their distribution, but their relatively bold character is indicated by a few facts. 
In the vicinity of Renton, and between that town and Seattle, sharply defined 
hills of hard rock rise like islands from the alluvium of the Duwamish valley. The 
former canyon, now filled almost to the summits of buttes along its course, is in- 
ferred to have been deep and steep-sided. At Burnett, 20 miles from tidewater and 
335 feet above it, a gangway driven on a coal vein 200 feet below the outcrops 
passed into a channel filled with gravel and tree roots. The slope from the nearest 
outcrop to the point where the buried channel was struck descends at an angle of 
not less than 35 degrees. At Wilkeson a similar buried channel was encountered 
in a water-level gangway 2,250 feet from the entrance and 250 feet below the level 
of the overlying gravel terrace. This preglacial topography is of much interest as 
a phase of the history of the Sound Basin, and it is economically important as a 
factor which modifies the amount of coal available above any given level. It 
sometimes introduces difficulties in mining. The topographic surface of the gravel 
deposits bears no definite relation to that of the Coal Measures. 
STRATIGRAPHY 
The coal-bearing rocks of the Puget Sound basin have been designated the Puget 
formation.* They are prevailingly sandstones of variable composition, texture, 
and color, thinly interbedded, and frequently cross stratified. Their composition 
varies from that of a typical arkose, consisting of slightly washed granitic min- 
erals, to silicious clays. Beds of concentrated quartz sands or conglomerates have 
*C, A. White, Bull, U. S, Geological Survey, no, 51, 1889, pp. 49-63, 
