The rocks along Big Creek tell a story of the earth's history at a very old stage and at a relatively young stage, but with a large gap in between in which no geological events are recorded.
The oldest rocks in the drainage are found near the Taylor Ranch Field Station, and are commonly either sedimentary rocks that were formerly silt beds, or limestone rocks that have been metamorphosed and altered.
These three rock types (below) are from the Precambrian (~700 mya), and were deposited beneath the sea at a time when there were no land plants or animals on the earth.
The first of these three rock types, the Yellowjacket Formation, consists of metamorphosed silts, sand, and limestone beds that have formed a distinctly striped rock, frequently tan, green, and orange or yellow-colored.
On top of the Yellowjacket Formation lies the massive Hoodoo Quartzite, a very resistant metamorphosed sandstone approximately 200 feet thick. It forms many of the high peaks in the region, including Horse Mountain and the highest parts of the Dave Lewis Peak ridge. The quartzite observed at Horse Mountain is also seen at Copper Camp, where the nearly-vertical beds are oriented north-south along the western edge of the Ramey Ridge Precambrian diorite. The Hoodoo Quartzite is capped by the silts and muds of the Apple Creek Formation, which consists of finely laminated siltites, sandstones, and argillites (mudstone).
These rocks were then intruded by three igneous bodies of diorite in the late Precambrian, "doming" them up into a laccolith. This diorite is now found on Rush Creek Point, Acorn Butte, and Ramey Ridge. At this stage, the story of what happened between 700 mya up until 70 mya is not recorded in the Big Creek rocks. Rocks were presumably deposited during this period, but have since eroded away. Far more recently, the various Precambrian rocks were intruded by the granite diorite of the Idaho Batholith, which is only found at the head of the Big Creek drainage and in minor outcrops by Coyote Springs.
In the Eocene epoch (~45 mya), the geologic activity in the region shifted towards large volcanic caldera eruptions. The eruption of the Van Horn Peaks Caldera, a collapsed volcanic vent located west of Challis, blasted an enormous mass of ashflow tuff into the region. The tuff was composed of lithic rock fragments torn from the vent walls, crystals that had formed within the magma chamber, and pumice (gassy magma that expanded upon the release of pressure when it was blown from the vent). This material rained to earth and formed a thick blanket (~1000 ft deep) that was hot enough and heavy enough to form a dense, "welded" tuff. It can now be observed at the mouth of Cave Creek (also forming the Cave itself) and Coxie Hole. These pumices were flattened during the welding process, so they appear thin in cross-section (1 mm), but are roughly the size of dimes and quarters when viewed from above, earning it the name "Dime-and-Quarter Tuff." The locus of volcanic activity then shifted northward, closer to the Big Creek area.
The Thunder Mountain volcanic center (near the head of Monumental Creek) became active shortly after the Van Horn Peaks Caldera. Its eruption produced a rhyolitic tuff known as the Sunnyside Tuff, which was a crystal-rich ashflow. However, the evacuation of its magma chamber led to the collapse of two northeast-trending faults, dropping down a block of the Dime-and-Quarter Tuff and the Sunnyside Tuff which overlies it into a graben, preserving them to this day.
The volcanic tuffs of the Dime-and-Quarter and the Sunnyside formations also have intrusive igneous equivalents, which represent magma that remained inside the magma chambers and cooled more slowly. This process formed two rock types: a Tertiary granodiorite and Tertiary granite, which outcrop in the heads of Cave Creek, Cow Creek, Cabin Creek, Crooked Creek, and on the Mormon Mountain ridge.
In the present day, a hiker walking west along Big Creek from the Middle Fork of the Salmon River would go from the ancient metamorphic rocks near Taylor Ranch into Precambrian igneous diorite, which outcrops north of Rush Creek Point and on the west end of Horse Mountain. The hiker would then cross a major fault at Cabin Creek and enter a zone of entirely young volcanic rocks: the Dime-and-Quarter Tuff and the Sunnyside Tuff. Continuing westward, the hiker would cross a second fault near Garden Creek and go from the uppermost Sunnyside volcanic tuff into the Dime-and-Quarter, before leaving the downdropped graben and returning to older rock formations.
The Cutting of the Big Creek Canyon
A pillar at Coxie Hole exhibits a 30-ft thick deposit of river cobbles included in the volcanic matrix of the Dime-and-Quarter tuff, on top of which lies welded deposits of the tuff itself. This demonstrates that the Dime-and-Quarter tuff once filled a river valley along the course of present-day Big Creek when it erupted 45 mya. There are also remnants of the volcanic tuffs on the canyon walls lying above the older Precambrian rocks, which confirms the suspicion that the Big Creek canyon existed (similar to its present state) prior to the volcanic eruptions. Since then, the stream's activity has re-excavated its former riverbed.
Another interesting feature of the Big Creek drainage is that in at least one place (and possibly two), large landslides have come off the canyon walls and dammed up the river for long periods of time (~10,000 years). The best-investigated slide is the Soldier Bar landslide (3 mi east of Taylor Ranch), which filled the canyon up to the level of Goat Basin and formed a lake extending upstream as far as Crooked Creek, and in which fine-grained deposits were left behind.
Written by Dave Stewart and Amie-June Brumble