RAINDROP IMPRINTS
Raindrop imprints are scarce, but highly diagnostic,
sedimentary structures. They absolutely demonstrate that a particular
bedding plane was subaerially exposed - it had to be land. Raindrop
imprints are expected to be small circular to subcircular pits or craters on
fine-grained siliciclastic sediment surfaces. They may be associated with
mudcracks and terrestrial fossils (such as fossil ferns).
Modern raindrop spots on bedrock surface, composed of quartzose sandstone
(= sand dune deposit) of the Cedar Mesa Sandstone (Cutler Group, Wolfcampian
Series, lower Lower Permian) near Congressman Bridge (ÒOwachomo BridgeÓ),
Natural Bridges National Monument, southeastern Utah, USA (6:15 PM local time,
5 September 2011).
Small pits or craters do not form when raindrops
impact on hard rock surfaces. Raindrop imprints will only form when loose
sediments are impacted (see examples below).
Raindrop imprints (above & below) on mudshale.
Raindrop imprints (above & below; above: 5.2 cm across; below:
4.6 cm across) on hard, slightly metamorphosed clayshale from the Rhode Island
Coal Measures (Pennsylvanian) of Attleboro, Massachusetts, USA.
