In Glenwood Canyon there is a rock. It sits on the north bank of the Colorado River between a parking lot and the trailhead to Hanging Lake. It sits alone with a concrete footpath parting and passing both sides, yet there is no interpretive plaque at its base, and it is ignored by those walking by. Interstate 70 crosses the river just to its west as if to give deference, but this is more likely being paid to Hanging Lake.
The rock is 18 feet tall and 10 feet wide. It is an anomaly. Amongst the omnipresent sedimentary rock surrounding it on the canyon walls, this rock is of granitic origin. It is uniformly grey and fine-grained. Further distinguishing it is a lovely pink four-inch-wide pegmatite dike running straight from bottom to top and showing large crystals of feldspar and quartz.
The rock has been moved to this spot. I think perhaps it is a glacial erratic having been transported by ice. But 10,000 years ago the Wisconsin Glacier only got to Montana. And then I spy an iron spike protruding from its side. Ah, the hand of man! But from where did the rock come?
I gaze across the river to the opposite canyon wall and the layer cake of multi-colors. The bottom layer is Late Cambrian Sawatch quartz sandstone and for over 100 million years or for the time dinosaurs roamed the earth, water, combined with iron oxides, carried silt, mud, clay and beach deposits of quartz sand over the bottom of an inland sea. This and the layers above have become 500 vertical feet of exceptional beauty.
But where does my rock fit in? This rock was not laid down over eons by deposition on the seafloor. Rather it welled up from below as an intrusive (never reaching the surface) igneous formation of hot magma from the earth's mantle and bulged into the overlying rock to intrude on the pretty sediments being deposited above.
And then as I study the wall opposite, I find the Unconformity. Here is the line between the Precambrian metamorphic and igneous basement rock below and
the Sawatch quartzite and sandstone above. Hiding below the spruce and fir on a low ledge, the rock takes on a pyramidal shape, changing form and becoming grey and unpatterned and widening to the canyon floor. I have found the rock of my rock and perhaps a northern outlier of the great Laccolith Triangle beginning 20 miles south at Carbon Peak.
Note the Red line of the Great Unconformity with Sedimentary above and Precambrian below. There is about a billion years of time missing at this line, wiped away by erosion. The Great Unconformity was first described in North America by John Wesley Powell in 1869 in relation to the same phenomenon he found in the Grand Canyon.
The Precambrian era spans time from the Earth’s creation and accounts for 88% of its geologic time. At its beginning earth was a molten rock cast off by the sun. Magma, swirling around its surface, eventually cooled and crusted over to fuse into the plates and continents we live upon today.
