DY B. WILLIS—DRIFT PHENOMENA OF PUGET SOUND. 
volcanic cone,and which has been completely occupied during the glacial 
period by an ice-tongue from the mountain. In its latest phase this glacier 
exists at the head of the canyon. ‘The second section was apparently 
determined in its course by the accidental relations of glaciers, whose 
recession both to the southeast and northwest opened the way for its flow. 
It first pursued a channel in the drift, and in post-Glacial time has cut 
its canyon 200 feet (60 meters) into the coal-bearing strata and asso- 
ciated igneous rocks. It now enters the Puyallup valley at a point some- 
what remote from courses which it pursued during the earlier part of 
the episode of glacial retreat. 
The two rivers described represent cases of streams rising outside the 
area of extensive drift and flowing into it. They are well supplied with 
abrasive load and cut rapidly. Their canyons give a maximum meas- 
ure of postglacial corrasion. Cases of another degree are found in brooks 
which rise within the plateaus and descend to the alluvial plains in the 
major hollows. Such streams have their courses wholly within the drift, 
their waters accumulate in swamps, and their power of abrasion is prac- 
tically nil. They cut ravines in steep slopes, however, where they flow 
in such manner as to undermine the gravel banks. 
Fennel creek is a brook draining about 8 square miles (2,000 hectares) 
of swamp in the Tacoma quadrangle and emptying into the Puyallup 
river. ‘he course from the swamp to the alluvial plain is 4 miles (6.4 
kilometers) ; in this distance the brook falls 400 feet (122 meters), and 
seven-eighths of the total fall is concentrated in the lower mile and a 
-half (2.4 kilometers). The head of active corrasion is where this rapid 
descent begins, and is marked by a cascade 40 feet (12 meters) in height. 
The valley of the brook is determined by an interglacial delta facing 
northward and westward, around whose base it flows, and by a gentle 
southward slope of till on which lie the swamps. ‘The depression in the 
plateau surface is therefore of glacial age, and the effect of postglacial 
corrasion is represented in the ravine below the fall. The ravine is a 
notch in the steep slope from the plateau to the alluvial plain. The 
maximum depth of the ravine below, a contour restored in its probable 
former position, is 150 feet (45 meters), and the channel at the level of 
that contour has receded three-fourths of a mile (1.2 kilometers). The 
section thus far cut has the outline of a very low obtuse-angled triangle, 
with the stream on the long side. The work accomplished by Fennel 
ereek is a fair example of postglacial dissection of the plateau margins. 
The ravines of this type are as yet cut only in the margins; the in- 
teriors of the plateaus are not dissected, but are diversified by lakes and 
swamps. 
