Fly Ash Use as a Time Marker in Sedimentation Studies

Fly ash from coal burning often takes the form of magnetic spherules, making it easy to separate and identify. Illinois soils near to and remote from known sources of fly ash were studied to evaluate ash incidence and accumulation of ash‐bearing sediments in the profile. Soils from both types of areas contained magnetic fly‐ash spherules. Concentrations >75% occurred in the magnetic fraction of the surface horizon (mostly in the top 12.5 cm) of a soil on an uncultivated, stable ridgetop near Springfield, IL. On the nearby Sangamon River floodplain, fly ash occurred in large numbers to the 25‐cm depth, and in lesser numbers to 45 cm, the top of a buried, organic‐C‐rich layer. This buried layer may represent the surface in the 1850s, when railroads and perhaps also industrial boilers were introduced. In depositional horizons of soils from remote sites, fly ash occurred at levels of <1%; it probably came from steam locomotives and, perhaps, steam‐powered farm machinery. From the 1850s to the late 1920s, railroads in Illinois increased to about 16 000 km of track; consequently, fly ash is ubiquitous in surface soils and is easily assayed. Steam‐powered farm machinery (although not always fueled by coal) was common from 1880 until about 1920. At depositional sites, the buried A horizon contained fly ash, but subjacent horizons did not. This pattern indicates that fly ash occurrence is useful in distinguishing the presettlement surface, thereby making possible the determination of rates of sedimentation at a particular site.