1. Education

Antietam Quartzite, Virginia

Submit an Entry: My Metamorphic Rock

By Labradorite

Antietam Quartzite, Virginia

Antietam quartzite

Antietam Quartzite, Virginia

Antietam quartzite showing Skolithos fossils

Where This Rock Is From (place, type of locality, etc.)

I found this rock along with with other types of cobbles in a landscaping bed around a local restaurant in Norfolk, Virginia. I immediately recognized the rock by its unique appearance.

What This Rock Is

This rock is Antietam quartzite - a white to gray fine- to medium-grain quartzite. It is from a formation that makes up one of three different components of the Chilhowee group, part of the lower layers of the Blue Ridge Mountains. The Antietam formation is about 800 feet thick and extends from Pennsylvania southward into Virginia. Because of the intense deformation of this strata during the formation of the Blue Ridge mountains, broken and weathered pieces of Antietam quartzite were eroded out of the mountains and deposited as Cretaceous gravel and cobbles in the Piedmont region.

What I Like About This Rock

The long narrow lines in this rock are the fossil tubes of Skolithos linearis worms. While fossils are uncommon in metamorphic rocks, this is an exception. It is believed that these were the homes of phoronid worms living in an marine intertidal region. The age of this rock is early Cambrian - approximately 500-550 million years old. There is a lot of controversy surrounding this particular rock type - many sources will call this a metaquartzite or an altered sandstone. The Antietam formation does grade into sandstones in various locations however I feel confident that this is a quartzite. Because of its weathered condition, it was probably transported in one of Virginia's rivers to a bed where it was mined for its use as landscaping or construction material.

Advice

  • Look out for really interesting rocks used in landscaping, crush-and-run, construction, and places where loose rocks have dumped for erosion control.

Andrew Alden, About.com Geology, says:

Judging the status of a quartzite, sedimentary or metamorphic, can be tricky (see more on this page). But fossils can persist in a metamorphic rock if there is no strain (deformation) imposed during metamorphism. That is pretty rare in a place like the Appalachians, but not unknown.
  1. About.com
  2. Education
  3. Geology

©2014 About.com. All rights reserved.