Underground

The Sharon Springs Member beneath the topsoil.

See that dark green mass labeled "Kmss"? It's sitting up there on the top of the hill (and under the ten acres) and extends in a patchwork fashion over small areas here and there in the valley. It's a geological formation known as the Sharon Springs Member, and here, courtesy of the USGS (https://pubs.usgs.gov/pp/0728/report.pdf), is a description of what's down there:

"The member consists of black, slightly bituminous shale, with which is interbedded dark-gray shale in the lower portion and about in the middle of the member. The shale is full of the remains of small fishes, which were probably the source of the bituminous matter in it. The small scales and bones of fishes can be detected in nearly every piece of the black shale. 

"The Sharon Springs member can be conveniently subdivided into Upper and Lower Sharon Springs. The Upper Sharon Springs, which is about 65 feet thick, can be recognized by the abundance of concretions, many very large, whereas in the Lower Sharon Springs the concretions, none of which are large, are very scarce and in many places practically absent. The shale of the Upper Sharon Springs is also somewhat different from that of the Lower Sharon Springs. Some beds of the former resist weathering more than the ordinary shale of the formation and appear as slightly prominent bands in the outcrops. 

"For the present investigation a threefold division of the Sharon Springs is recognized: the lower unit, about 115 feet of dominantly dark-gray soft flaky shale having a few concretions of yellow chalky limestone, is referred to as the 'dark soft shale unit.' The middle unit, called the 'organic-rich shale unit,' consists of about 90 feet of hard buttress-weathering shale, is rich in organic material and in varied limestone concretions, and has yielded abundant vertebrate and a few invertebrate fossils. The upper unit, about 10 feet of hard-platy-weathering slightly phosphatic shale that contains numerous layers of soft highly weathered phosphate nodules, is referred to as the "phosphatic shale unit." 

So, what's "slightly bituminous shale"? Here's what another site (http://www.futura-sciences.us/dico/d/geology-bituminous-shale-50005403) has to say about it:

"Bituminous shales are rocks containing organic substances called kerogens. When there is a sufficient quantity of kerogens, they can be exploited to produce oil and gas fuel, however, it's a low profit, polluting operation, not to be confused with oil sands.

"Geologically speaking, bituminous shales are not 'true shales' because they are not the result of metamorphism -- modification in a solid state by means of temperature and pressure. Moreover, they have no precise composition. Nevertheless, they are divided into three families:

• carbonate-rich shales;
• siliceous shales;
• carbonaceous shales.

"Kerogen can be transformed into oil by a chemical process called pyrolysis. The rocks are heated to a temperature of 450 to 500 degrees in the absence of air. The vapour formed is then distilled to produce shale oil (a form of oil) and gas. On a worldwide scale, 2800 to 3100 billion barrels of oil could be produced from these rocks. Bituminous shale can also be burned as such but the energy quality of the fuel is then less."

OIL! There's black gold down there! And down there is where it's going to stay, unless some particularly diligent tap root or a particularly industrious prairie dog drills down that far.

"The exploitation of bituminous shale is not very profitable since the processes that must be applied to the rocks are long and costly. Furthermore, the extraction and processing of the fuel are harmful to the environment. The water used in the mines is acidified and enriched in metals. Large emissions of sulphur-containing gases and particles cause substantial atmospheric pollution. Furthermore, the production of oil from shale produces a larger amount of CO₂ than the traditional petrochemical industry."