Geology Reference
In-Depth Information
rocks. How thundereggs form isn't certain, but the
process probably begins with an internal cavity pro-
duced by gasses expanding in highly viscous lavas. Once
the lavas cooled, the chamber is filled in with agate or
opal. Thundereggs range in size from less than one
inch to over 4 feet in diameter, with the average geode
about the size of a baseball. Within a rough, knobby
and drab exterior, the thunderegg reveals a colorful,
star-shaped interior when cut open. Thundereggs can
be found in a number of localities in eastern Oregon,
but they are best-known from rocks of the John Day
Formation. According to Indian legends, angry thunder
spirits who lived in the craters of Mt. Hood and Mt.
Jefferson once hurled these spherical rocks at each
other accompanied by thunder and lightning.
Geothermal Resources
The potential for the commercial exploitation
of geothermal energy in the Blue Mountains is only
moderate compared to that in the High Lava Plateau
or the Basin and Range provinces to the south. Initially
thermal waters in the Blue Mountains were utilized by
pioneers, and a health spa was erected in 1880 at Hot
Lake near La Grande. At nearby Cove and Union,
public and commercial facilities were built to exploit
thermal springs and wells. Here water averages less
than 100 degrees Fahrenheit, while waters from Hot
Lake at 185 degrees Fahrenheit are the warmest.
Thermal waters follow narrow conduits along
fault zones by which they reach the surface. This has
the effect of localizing the hot waters. At Hot Lake, for
example, a cold water well is located less than 150 feet
from the hot springs.
Thundereggs, a popular term for agate-filled
geodes (photo courtesy of Oregon Dept. Geology
and Mineral Industries)
range. Spectacular, high triangular peaks with steep
walls are known as matterhorns and are cut out by the
headward erosion of three or more glaciers back to
back. Headward erosion by a glacier is primarily a
freezing-thawing process where rocks are plucked or
torn directly off valley walls by the ice. This style of
erosional attack leaves bare, vertical rock faces and
dramatic scenery at the heads of valleys.
A diversity of peaks in the Wallowas rise to
heights of nearly 2 miles above sea level, producing
both a sharply serrated as well as a rounded skyline.
Southwest of Wallowa Lake, Matterhorn Mountain,
composed of blue-white marble, rises to 10,004 feet just
opposite Sacajawea Peak at 9,880 feet. Other moun-
tains, as Aneroid, so named because an aneroid barom-
eter was used to determine the elevation of the nearby
lake, is more rounded and gives a different appearance
to the landscape.
The Wallowa Mountains contain some of the
finest scenery and recreational facilities in Oregon.
Three separate episodes of Ice Age glaciation have
shaped ridges, carved valleys into U-shaped troughs,
and left crystal clear glacial lakes to be enjoyed whet-
her driving, camping, or hiking here. There are close to
100 lakes of all shapes hollowed out of the granitic
Features of Geologic Interest
Wallowa Mountains
Formerly known by a number of names, the
rugged, glaciated Wallowa Mountains were called the
Granite Mountains, the Powder Mountains, or the
Eagle Mountains before an official decision was made
to use the current name. "Wallowa" is a Nez Perce
Indian name meaning a series of stakes arranged in a
triangular pattern in the river for catching fish. Traps
of this sort were used in the Wallowa River. Extending
over the northeast Oregon border, the Wallowa
Mountains are called the Seven Devils Mountains in
Idaho. Between these two ranges, Hells Canyon of the
Snake River has carved a gorge, 1,000 feet deeper than
the Grand Canyon of the Colorado River.
During the Ice Ages, glaciers atop Wallowa
Mountains radiated outward from the center of the
