November 13, 2011
W is for Weathered Rock
An old-school sound test demonstrates the difference between soil, core stones, and solid bedrock.
Early in my career, I worked on a project in the northern Virginia province of Piedmont. I was preparing a map and report showing the engineering properties of the soils of the area. As a geologist working in the Piedmont for the first time, I felt like W.C. Kerr, who wrote “…the most striking and novel feature of the geology is the great deal of earth which almost everywhere mantles and conceals the rocks.” (Kerr, 1881, “On the action of frost in the arrangement of superficial earth material,” American Journal of Science, v. 21, pg 354.)
That was quite a contrast to New England, where I had previously worked. There, the bedrock is hard, fresh, and exposed.
Al Froelich, my new project chief and mentor, took me out in the field to show me the ropes. We started at the top of a hill and worked our way down a small gully that grew in size as we descended. All the way downstream, Al kept whacking his geologic hammer on the bank of the gully, each whack producing a dull THUNK. He pointed out how the soil looked exactly like rock, but it was soft and much less dense than rock. As we continued down, we started to see little pieces of real rock. Al said they are called core stones. The lower we went, the more plentiful and larger the core stones became. Whacking the larger core stones produced a noise like hitting a metal pipe that you held in your hand — more pleasing that a THUNK, but still not a very pretty sound. About 20 vertical feet from where the gully joined a larger stream valley, Al whacked the bank one more time. BRINGGGGGGGG! Al’s face lit up like a kid in a candy store as he proclaimed, “This is solid bedrock. It rings like the anvil chorus.”
What we had done was work our way down through a section of clay-rich, weathered, decomposed rock, which, in that part of the world, was chemically weathered rock called saprolite (from Greek for rotten). Saprolite is formed from rock that has been exposed to so much chemical weathering that up to 60 percent of the rock mass has been removed, but the original volume is not affected. Saprolite weathers in place to a thickness ranging from 0 to 325 feet, but typically is 50- to 65-feet thick.
Weathering of bedrock is not just a curious geologic phenomenon — it directly impacts quarrying of crushed stone. Weathering of bedrock decreases the hardness, soundness, and strength of the rock, limiting its use as aggregate. Only relatively unweathered rock is suitable for use as crushed stone. Furthermore, weathering increases the proportion of soft material, increases the overall cost of separating the sound from the unsound rock, affects the blasting and extractive techniques, and reduces slope stability. Thus, weathered rock increases the overall cost of mining and processing.
So, while a geologist might hear BRINGGGG when encountering solid, unweathered rock, an aggregate producer will hear CA-CHINGGGG.
Author’s note: Al Froelich passed away too soon. But I can always imagine him whacking away on some outcrop with it ringing like the anvil chorus.
Bill Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey and can be reached at 303-236-1249 or via e-mail at firstname.lastname@example.org.