Ever since I moved to Oregon I’ve been noticing random patterns of lighter colored spots on the sidewalk, like this. I am aware that salt can interact with concrete and eventually degrade it, and I have occasionally seen road salt being used. But we don’t need it very often here; our winters are are far milder than most other North American locations along the same latitude.
So are these marks from salt, or something else?
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That looks like salt to me, though a close-up would be useful. Not even necessarily a change to the concrete itself; just a bit of salt itself precipitated out on top of it. The next good (liquid) rain will probably clear that right up.
Looks like it could be lichen.
My thought was lichen as well. We need closer pics. Do the spots change when the sidewalk is wet?
Yeah, or what’s left of it after it’s been killed and mostly worn away.
Definitely lichens.
Not an ideal photo, but definitely looks like lichen. If they are still there after a rain shower then not likely to be salt or other soluble mineral.
No, it won’t rinse off. These markings are permanent discolorations. When I first saw them I thought someone had been careless with a paint bucket, but then noticed that the spots could also be seen at various other points throughout the neighborhood.
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Lichen. I see that on lots of sidewalks where I live in Western Washington which is probably identical to your environment in Oregon.
Even when it’s gone it leaves the sidewalk discolored. Here is a picture from a blog that talks about removing lichen, notice the grey/white spots that look identical to your picture.
I added some more photos which are available here.
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The white spots look very close indeed, but I’ve never seen the green lichens while they are still alive.
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ETA: Contrary to my earlier statement, I now believe these marks will eventually wash away, but it takes a long time, like a couple of years or more.
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There could be two things going on. The light gray lichen grows on the surface, contrasting with the concrete surrounding it, especially if the surrounding concrete is dirty with grime or mold. Also, if and when the lichen dies, even if it’s washed off or otherwise stripped away, the clean surface of the concrete is left underneath, which still contrasts with the dirty exposed surface. So assuming the latter situation, it’s not that the light spots need time to get washed off, instead they need time to get grimed up.
Looking more closely, I do see growing lichens around here. I didn’t notice them before because they are generally a very dark green unlike the brighter colors in the photos linked above. In this case I’m sure that’s largely because they are damp and tend to stay that way in this season, much like the soil.
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