TACOMA DELTA. USL 
Tucoma delta.—Among the formations which I may provisionally place 
in the Puyallup group is, in part at least, a voluminous delta underlying 
the city of Tacoma. A portion of the structure belongs to the Vashon 
epoch, another part underlies the Vashon till and probably was formed 
during the Puyallup epoch. The whole is dependent upon broad topo- 
graphic relations for its development. 
South of Tacoma expand the Steilacoom plains. North of Tacoma is 
the deep hollow of Admiralty inlet. The general elevation of the plains 
is 350 to 400 feet (100 to 120 meters) above sea; but about their eastern, 
northern, and northwestern margins they are bounded by higher zones, 
rising to 500 feet (150 meters) or more above sea. Their general slope 
near Tacoma is northward. Thus their topographic configuration is 
that of a shallow basin. The plains are diversified by numerous low 
delta terraces, which prove that the basin once held water. At the 
northern point is an outlet, a depression marked by abrupt terraces, 
which turns eastward and opens upon a deep gulch that leads down to 
sealevel. The railroad follows the outlet and the gulch. Sections of the 
gravels exposed in the guich exhibit the cross-stratification characteristic 
of deposits from swift streams, and the steep bedding of the deposit con- 
firms the inference, from its general relations, that it is the delta of a 
river which flowed from the Steilacoom lake. All of this belongs to 
the latest (the Vashon) epoch. 
The broad slope on which the city is chiefly built lies north of this 
gulch and is covered with Vashon till. Beneath the till is a remarkable 
mass of sands and gravels, exhibiting the structure shown in plate 9. 
The photograph was taken on Pacific avenue, a short distance north of 
tue city hall. Near the top of the section is a line of coarse pebbles 
which mark the base of the Vashon till. Beneath this unconformity is 
a lenticular body of gravel steeply bedded and spread over the edges of 
the underlying layers. The lower and greater part of the section is of 
sandy clays, which exhibit the plunge structure peculiar to deposits from 
swift currents, overloaded with sediment, entering a still-water body. 
Structure of this type may be traced along the bluff for a half a mile. 
The delta structure is complicated by dips in a direction opposed to 
the general inclination, which result iu synclinal depressions, one of 
which is shown in plate 9. The subsidence is exaggerated by faulting, 
with inclination of the fault plains toward the downthrow. Small faults 
of this type give a zigzag section to some of the beds seen at the right of 
plate 9. These peculiarities, taken in connection with the very steep 
dips of the strata, appear to indicate that the mass was deposited on or 
included in ice, which, melting, allowed the layers to subside locally. 
The Tacoma delta is a complex structure, which should be studied in 
XXII—Butu. Grou. Soc. Am., Vou. 9, 1897 
