The phrase "are underlain by a" is correct and usable in written English.
It can be used in contexts discussing geological or structural features, indicating that one layer or element is supported or situated beneath another.
Example: "The sedimentary rocks are underlain by a layer of granite, which provides stability to the formation."
Alternatives: "are supported by a" or "are based on a".
Small, shallow lakes that freeze to the bottom each winter are underlain by a zone of thawed material, but the thawed zone does not completely penetrate permafrost except near the southern border of permafrost.
In 1766 Goldsmith revealed himself as a novelist with The Vicar of Wakefield (written in 1762), a portrait of village life whose idealization of the countryside, sentimental moralizing, and melodramatic incidents are underlain by a sharp but good-natured irony.
The lowlands of the catchment are underlain by a major sandstone aquifer, and mainly have freely draining, fertile soils that support intensive arable farming under a drier climate with around 800 mm of precipitation.
Other countries in continental collision zones are underlain by a nightmare of widely dispersed faults, splintering across thousands of kilometres of land.
Many acid crater lakes are underlain by a pool of molten sulfur which interacts with the crater-lake water to produce a complex sulfur speciation that includes polythionates.
These Tertiary sediments are underlain by a Mesozoic sedimentary succession, including the Upper Jurassic aquifer (Malm) (Birner et al. 2011, 2012), which is today intensively used for geothermal energy production, and a crystalline crust of Paleozoic age (Lemcke 1973; Freudenberger and Schwerd 1996; Schmid et al. 2004a, b; Berge and Veal 2005; Schmid et al. 2008).
Parts of the lowland are underlain by an extensive oil field that also has large natural gas deposits.
The STP is underlain by a clay till, which is in turn underlain by the Wood Creek Sand Channel (WCSC).
The coal seam is underlain by a seat-earth (underclay).
A large part of Accra is underlain by a complex distribution of shallow soft soils.
"That was the thick of it right there," says Dave. Letterman has an acerbic humour, but one that's underlain by a genuine warmth.
