Chilhowee metasedimentary rocks
Eric Butler - NPS Photo
The youngest rocks within
The earliest sediments were deposited directly on top of the still-fresh lava flows of the Catoctin Formation, whose eruption marked the beginning of the great rifting event that formed the
This story can be read in the sedimentary sequence capping the rocks in Shenandoah. Directly above the greenstone lava flows, beds of coarse gravel and sand are found. Above this, several thousand feet of finely bedded silt and sand preserve evidence of the ancient lagoons, slowly drowning in the advancing waters. Capping the sequence are cliffs and boulder fields of pure white quartz sandstone, the beach deposits heralding the arrival of the Iapetus shoreline. Further west, in the Page and
Together, this collection of sedimentary rocks is known as the Chilhowee Group, which is split into three distinct Formations based on rock type and history. The early river deposits compose the Weverton Formation, the thick lagoonal deposits compose the Hampton Formation, and the quartz sand deposits dominate the Erwin Formation. These designations help geologists define and describe the geologic history more clearly.
There is a final chapter to the story of these sedimentary rocks, however. Long after their deposition, and subsequent hardening into rock, these units were deformed by the great heat and pressure associated with the formation of the
There are few fossils in these rocks, as they are older than most complex life on earth. When these sediments were being deposited, life was just beginning to develop in the oceans. The one observable fossil can be seen in the Erwin Formation, a preserved worm burrow known as skolithos, which appears as long, straight tubes within the white quartzite.
The Chilhowee Group is best exposed in the South District of the Park. It rarely appears in the Central, but is exposed in a few places in the North, especially around Knob and
Did You Know?
Shenandoah National Park is the only known location in the world of the Shenandoah Salamander. More...