The photos above were taken of an unusual rock formation that I found among the weeds this summer. We had hired some landscapers to remove a lot of overgrown shrubs in the garden inside our oval driveway. A few months after the shrubs were gone, the weeds took over and I had just started to pull them up when I noticed a long, low rock sticking out of the dirt..
I had never noticed this particular rock before. All the other ones are just the generic “rock garden” boulders, solid shapes in grey or brown stone. This one caught my eye because of the regularly spaced holes along one edge and the layered and organic appearance of the rock in general. I did move it a foot or so away from where it was lying (semi-buried in about 4-6 inches of dirt).
It measures about 22 inches long by 11 inches deep by 9 inches high. The holes along one edge are roughly 1-1/4 inches long, but they vary a bit from one hole to another.
I reached out to a contact at the Adirondack Park Agency (APA) to try to determine the type of rock it was and why it would have formed as it did. This is the response I received:
Thanks for sharing the images. The holes in the rock were formed by erosive forces and the rock may have been taken from a nearby stream or river, or bedrock outcropping. A colleague of mine who has a background in geology believes the rock may be igneous, but without seeing it in person and splitting it open to see crystal formations she would not know for certain (the rock is very weathered and oxidized). You might find it interesting to visit nearby streams and rivers or roadsides which have exposed bedrock to see if you can find similar rocks in the area. However, you can also sometimes see rocks with somewhat similar weathering while hiking far away from streams and rivers as show in a photograph found here: http://www.wildernessphotographs.com/photo/granite-bedrock-on-hurricane-mountain/
I hope this helps.
What do you think? Do you agree with the APA assessment? Have you seen any similar rocks in the area?
I’m not at all convinced that his geologist friend is right. This looks sedimentary to me, but who am I to say?
What are your thoughts?
I would say that it has been folded and then folded again, as can happen with very old rocks in the Adirondacks. It could be one of the Grenville marbles I suppose, and you can do a simple scratch test with a pocket knife to test its hardness. Most sedimentary (or perhaps meta-sedimentary in this case) can be easily scratched whereas granite and gneiss are harder.
Thank you for your helpful response, Greg. I did the test you mentioned and found the surface of this rock very easy to scratch, so I guess it is what you describe, “one of the Grenville marbles.” I am especially intrigued by fact that the oblong holes are so evenly spaced and similar in size. Garden plants seem to get all the attention, but for me, the rocks are just as intriguing!
Hhhmmm… So does this rule out the possibility of petrified Swiss cheese gifted across the ages from our gourmet forbears? I certainly hope not! 😉