Geological Survey Professional Paper 644F
History of Snake River Canyon Indicated by Revised
Stratigraphy of Snake River Group Near Hagerman and
King Hill, Idaho
AGE OF THE McKINNEY AND WENDELL GRADE BASALTS
Surveys between the years 1954 and 1961 established
that the McKinney Basalt erupted from McKinney Butte 8 miles northwest of Gooding and that the
Wendell Grade Basalt came from Notch Butte 4 miles south of Shoshone
(Malde and others, 1963). As these events were then understood, the
McKinney Basalt flowed south toward the Snake River, then west in the
upland north of the canyon rim, and finally cascaded from the upland to
present river level near King Hill;
the Wendell Grade Basalt flowed west and reached the
canyon rim at several places near Hagerman (fig. 3). Exposures 5 miles
northeast of Hagerman along the Malad River (which is the local name for
the lower reach of the Big Wood River) showed that the McKinney
overlies the Wendell Grade. Because both the McKinney and the Wendell
Grade lavas retain rough surfaces that are only partly mantled by windblown
surficial material (fig. 4)in contrast to contiguous older lavas
for which the original surface roughness has been almost entirely
obliteratedand because of erroneous interpretations explained
below, the McKinney and Wendell Grade were assigned to the Recent.
FIGURE 3.Generalized geologic map of area between Hagerman and
King Hill. Geology modified from Malde, Powers. and Marshall (1963).
Base from U.S. Geological Survey Twin Falls quadrangle. 1955-63,
and Halley quadrangle, 1955-62.
(click on image for an enlargement in a new window)
FIGURE 4.Pressure ridges of McKinney Basalt, which are partly
covered with surficial material. View is north toward McKinney Butte
from the railroad 5-1/2 miles east of Bliss.
A discovery made in 1962 by George Stone, who was
then collecting petrographic samples with Howard Powers, showed that the
McKinney Basalt, and hence the Wendell Grade Basalt, is older than the
Melon Gravel and is therefore of Pleistocene age, in spite of its
youthful appearance. The decisive geology was found along the Union
Pacific Railroad 4 miles south east of King Hill at a site 200 feet
above the Snake River. There, the Bonneville Flood scoured McKinney
Basalt to make the intricate anomalous topography known as scabland
(fig. 5), and it left well-rounded boulders of Melon Gravel on the
eroded McKinney, some of which were more than 5 feet long (fig. 6).
Prior to this discovery, I had recognized stream erosion of nearby
McKinney Basalt no higher than 125 feet above the river and thought that
this erosion could have been produced by overflow where the McKinney had
dammed the Snake. My other previous interpretations about the relation
of the McKinney Basalt to Melon Gravel, which I will now summarize, were
also wrong. I had erred in two places.
FIGURE 5.Scabland surface of McKinney Basalt about 200 feet above
the Snake River 4 miles southeast of King Hill. Part of the basalt was
removed by the Bonneville Flood, and the remaining lava was extensively
corroded, rounded, and polished.
FIGURE 6.Basalt boulder of Melon Gravel more than 5 feet long on
McKinney Basalt 175 feet above the Snake River 4 miles southeast of King
Hill. Another boulder, nearly buried by surficial sand, is at the
First, along the Snake River 3 miles southeast of
King Hill, the McKinney Basalt overlies basaltic cobbles and boulders
formerly interpreted to be Melon Gravel (Malde and Powers, 1962, p.
1217)the only local gravel deposit then known to consist solely of
basaltic debris. The length of outcrop of this gravel amounts to less
than a mile, much of it being obscured by talus from the overlying
basalt, but sections of the gravel several feet thick are locally well
exposed where protected by an overhang of lava (fig. 7). I now
discern subtle differences between this deposit and
typical Melon Gravel. The basaltic gravel under the McKinney is
subangular, rather well sorted, more cobbly than bouldery, and lacks
interstitial sand. In contrast, the nearby Melon Gravel is rounded,
poorly sorted, more bouldery than cobbly, and contains much basaltic
sand. Because of the unequivocal evidence found by Stone and Powers
(that is, boulders of Melon Gravel lying on McKinney Basalt), this
gravel below the McKinney must be older than the Melon Gravel.
FIGURE 7.Basaltic gravel under McKinney Basalt on right bank of
the Snake River 3 miles southeast of King Hill. Pieces of the gravel are
frozen in the base of the lava.
Second, the terminus of the McKinney Basalt near King
Hill, in the area known as The Pasture (fig. 8), was formerly
interpreted as several small tongues of lava that had descended shallow
troughs in the Melon Gravel (Malde and Powers, 1962, p. 1217), but the
physiography is now understood as subdued bars of Melon Gravel on the
FIGURE 8.Generalized geologic map of area along Snake River
southeast of King Hill showing inferred route of former canyon
of Snake River, which is filled with McKinney Basalt. Cross sections
(A-A' to K-K') are shown in figure 10. (click on image
for an enlargement in a new window)
Observations by Powers (in Malde and Powers, 1962, p.
1217), about the stratigraphic position of the Wendell Grade Basalt
with respect to Melon Gravel, in the light of present knowledge, were
also deceptive. Powers found a place 2 miles east of Hagerman where
Wendell Grade Basalt had cascaded from the canyon rim onto landslide
debris. Because this landslide closely resembles another landslide
2-3 miles northwest that overlaps Melon Gravel (Malde, 1968, fig.
19), Powers decided that the Wendell Grade Basalt was also younger than
Melon Gravel. From present evidence, this conclusion is false. The
McKinney Basalt clearly overlies Wendell Grade Basalt on the Malad
River, as previously mentioned, and both are older than the Melon
Last Updated: 28-Mar-2006