Click the photo for the full-size version. Gas-escape structures are widespread in sedimentary rocks, just as gases like methane are common in various sediments. In this locality south of Golden, Colorado, they are seen from the underside, fossilized in shale of Cretaceous age (140–65 million years ago). For these features to be preserved, a mudflat must be at the right consistency first, then must be promptly covered by fresh sediment before the marks have a chance to degrade. The Cretaceous seaway that once lay across America's Midwest is one such setting.
You can see gases rising in many settings, from the jettings of clams at the beach to bubbles of swamp gas rising from mucky pools. Seeps of natural gas are another possibility. Earthquakes may disturb wide areas and release gas bubbles. So may the steps of large animals, like the dinosaurs whose footprints are preserved at this same site.
Features like these are also explained as raindrop impressions, but their concentric structure is more complex than the mark a raindrop might leave. Hailstones also leave marks in soft sediment. They might be distinguished by a wider range in size than raindrop impressions and an absence of splash marks.