At our lake in Quebec, near Gracefield, we were out in the boat and noticed these odd formations in the rock. Along about 50 yards of shoreline, there are these little pockmarks underwater, making the rock look like a cross between a honeycomb and an egg carton. The holes are fairly large and could hold a small melon or a large apple. They appear to occur in regular lines and are right into the solid rock. I would love to know what caused them.—Frank Hutchingame, Lac Pemichangan, Que.
So would we. “It’s a little tricky to say without anything in the photo for scale, but melon- or apple-sized helps,” says Jason Venkiteswaran, an associate professor and geography undergraduate officer in the Geography & Environmental Studies department at Wilfrid Laurier University, in Waterloo, Ont. So your size estimates, along with the location details, are enough to suggest a theory.
Experts discover a mysterious shipwreck in Lake Ontario
Geology time! “Gracefield is due north of Ottawa, and this area is part of the Grenville Supergroup,” he says. The what? “It’s a band of highly metamorphosed and folded rocks made of gneiss, marble, and quartzite.” Of these types of rocks, marble (a.k.a. metamorphosed limestone) is prone to weathering under the right conditions. “It dissolves much more readily than gneiss or quartzite,” he says. “So, this makes me wonder if these depressions in the bedrock are ‘dissolution pits.’ They’re common on both limestone and marble surfaces. Essentially, the water ‘preferentially’ erodes pits into the rock.”
Preferentially means that for a number of reasons, “there is a slightly higher rate of weathering in one spot than in the areas around it,” says Venkiteswaran. “For example, there could have been a small dip in the rock where the water would pool due to gravity while the surrounding rock was dry.” Eventually, this could result in the honeycomb/egg carton-like look of the rock in your lake.
Okay, but how does lake water dissolve rock? It’s rock. “Rainwater is naturally acidic from the CO2 in the atmosphere. It has a pH around 5.6,” says Venkiteswaran. “This means that there is always weak acid being added to the lake. Marble and limestone will gradually dissolve in that weakly acidic water,” he says. “It’s like a very slow version of adding vinegar to water to remove the scale buildup from a kettle or coffee maker.”
This article was originally published in the Early Summer 2026 issue of Cottage Life.
Related Story New app uses AI to prevent human-wildlife conflicts
Related Story Invasive Prussian carp confirmed in Sylvan Lake, Alta.
Related Story Here’s what cottagers should know before the next thunderstorm rolls in