The William Rivers Pitt is "layered," and the Depression-era stuff is about 35 feet down from the top (remember, the William Rivers Pitt, which began in a ravine in 1875, protrudes, like the Jungfrau); the stuff from the days of William McKinley is circa 12 feet lower than that.
There was a constant change in the number of pigs producing this stuff, but 1 foot of the stuff circa 1885 is about 2 feet of the stuff circa 1940, because of the slow compaction, and all the weight added on top.
There's two non-swine by-product layers; about 8 or 9 inches of silt from the Dust Bowl during the 1930s, and then about six feet deep on top, covering all of the William Rivers Pitt like icing.
This is charred (and now significantly decayed) lumber from when the barn burned down in 1950 (and the remains were tossed onto the William Rivers Pitt), and all the airborne soil added on since then.
So it's a mixture of a swine by-product, soil from Manitoba and North Dakota, and burned lumber, but easily 90% swine by-product.
There had been some concern that the William Rivers Pitt had also been used for disposal of household garbage, but apparently not, as that sort of thing was found by the prairie archaeologist (not the soil scientist) on the other side of the house, along with four distinct locations of outdoor plumbery. There were at least four outhouses, from 1875 through 1940; outhouses were cheap and easy to build, and so when the hole filled up, one simply dug a new hole somewhere else and propped a new wooden shed on top of it.
The outhouse locations showed this family used a lot of lime.
She found a higher-than-expected level of selenium in the William Rivers Pitt, but myself not being a chemist, I have no idea what this means.