View to the
west-northwest with Separation Fault and its twin canyons
along the lower edge. The river turns sharply to the left at
242 Mile Canyon and then back to the right just before Spencer
Canyon enters from the left edge. Surprise Canyon enters from
the right just beyond the ridge.
The inner gorge (Lower Granite Gorge) is composed
of Precambrian schist capped by Tapeats Sandstone. Then the
Bright Angel Shale forms its characteristic slope. Cambrian,
Devonian, and Mississippian limestones form the major cliff.
The Supai Group (with its flat Esplanade Sandstone top) forms
Erosion along one of the many northeast to
southwest faults in the area has caused 242 Mile Canyon as
well as allowing the Colorado River to make a zigzag to the
View to the
northwest with 242 Mile Canyon joining the river where it
makes a sharp turn to the left. Spencer Canyon enters from the
left edge and joins the river after it makes another sharp
turn, but to the right. Surprise Canyon is the next major
canyon that enters from the right.
Before Lake Mead backed up to fill the lower end
of the Grand Canyon, there was another major rapid where
Spencer Canyon joins the river. The rapid was named Lava Cliff
Rapid after a large remnant of the old lava flows that is
still stuck to the north bank. The top of the flow is some 80
feet above the current river level. Since the old river
channel is still lower yet, it indicates there has been
moderate downward cutting by the river since the lava flow was
Surprise Canyon is significant as it is the place
name for the Surprise Valley Formation. After the Redwall
Limestone was deposited some 350 million years ago, this area
emerged above sea level. Rivers developed across the area and
formed small river valleys. Subsequently these valleys were
filled in with silt to form the Surprise Valley Formation.
Part of this was eroded again before the entire area was
covered with the Supai Group.
river miles 232 to 240
to river miles 248 to 256
the Index Page for the Grand Canyon Tour
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