The photo of the week is one that I took in November on my glacial sedimentology class trip to the Buckingham sand pit and at first glance it looks pretty boring. Nothing could be further from the truth though when you consider the implications of this lonely old block sitting al by itself in these sand beds. Also, my apologies for not including a scale. You’ll just have to take my word for it that this block was around 30 cm long and 15 cm high.
This block is called a dropstone. Dropstones are a glacial feature that occur when a stone that is incorporated into an iceberg or ice sheet falls out as it melts, the block settles through the water column and lands on the sediment beneath. The sediment continues to deposit and covers the stone. Because of the block the sand beds warp and deform around the shape of the stone as you can see in the picture. Dropstones are a smoking gun for indicating a marine or lacustrine environment since they can only be deposited in water. In this case the glacier, which was located nearby, was flowing into a large pro-glacial lake called glacial lake Champlain and deposited a huge amount of sand and clay in the Ottawa region as well as numerous dropstones.
A beautiful dropstone in the Buckingham sandpit, Buckingham, Quebec. (Photo: Matt Herod)
Jim Baldwin
I found one of these in central, NH last year. You can see my image here http://www.flickr.com/
I’m just an amateur but I love unusual rocks.
Matt Herod
Thanks for the comment and the picture. Geology is a great science because it is not exclusive to those of us at universities. I started out as an amateur and all of my early geology mentors were also amateurs. Happy sleuthing!