superstratum

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

And before she could stop him, he had pounced upon it and pulled it out, upsetting a superstratum of gowns in the process.

From Phebe, Her Profession A Sequel to Teddy: Her Book by Anna Chapin Ray

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

Anne said little to all these things, and preserved a superstratum of calmness on her countenance; but some inner voice seemed to whisper to her that Bob was no more. 

From The Trumpet-Major by Thomas Hardy

The soil is generally deep, more or less yellow, and somewhat clayey; the hollows having a thin superstratum of black mould. 

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