superstratum

After leaving the narrow valley which the river has cut for itself through a superstratum of yellowish clay, the country becomes nearly level--a dreary plain, covered with fern and the manuka bush.

From Narrative of the Voyage of H.M.S. Rattlesnake, Commanded By the Late Captain Owen Stanley, R.N., F.R.S. Etc. During the Years 1846-1850. Including Discoveries and Surveys in New Guinea, the Louisiade Archipelago, Etc. to Which Is Added the Account of Mr. E.B. Kennedy's Expedition for the Exploration of the Cape York Peninsula. By John Macgillivray, F.R.G.S. Naturalist to the Expedition. — Volume 2 by John MacGillivray

In what he has to say about the Indians, a subject that lies as a superstratum under his work, he is anxious to hear all that can be said.

From Personal Memoirs of a Residence of Thirty Years with the Indian Tribes on the American Frontiers by Henry Rowe Schoolcraft

The superstratum was very light, and brownish black, the remainder yellowish brown, the yellow tints as well as the stiffness increasing downwards. 

From Journals of Travels in Assam, Burma, Bhootan, Afghanistan and the Neighbouring Countries by William Griffith

After a time the superstratum of rock, which is full of cracks and seams, is undermined and precipitated into the chasm below.

From The Falls of Niagara and Other Famous Cataracts by George W. Holley

It was composed of granite at the base and capped with horizontal strata of sandstone, some of the beds containing large water-worn pebbles, and the superstratum highly ferruginous.

From Journals of Australian Explorations by Augustus Charles Gregory