In the subsurface of north-central Kansas, middle Ordovician Viola limestones, named after the Oklahoma surface section, are separable, lithologically, into six member zones, three of which are cherty. Numbered downward, these zones are designated as (1) upper limestone, (2) upper cherty member, (3) middle limestone, (4) middle cherty member, (5) lower limestone, and (6) lower cherty or basal clastic member. The upper limestone is dolomitic, occurs beneath the Maquoketa-Sylvan unconformity, and in different places overlies several older zones. The upper cherty member is calcitic and limited, in central Kansas, to the northern part. The middle and lower limestones, zones 3 and 5 are calcitic in the north, becoming dolomitic toward the south. Lying between them, possibly wi h disconformity, is the middle cherty member, zone 4, characterized by black-specked dolomite-bearing chert. The lower cherty or basal clastic member, zone 6, includes cherty limestones, sandy in places and probably not everywhere coetaneous, which overlie Simpson shales and sands. Most Viola oil production comes from zones 4 and 5.

The Viola is over 250 feet thick in the north, but thins toward the south. Zone 5 is the most widespread, representing the maximum advance of Viola seas. The overlying members developed in less extensive seaways which advanced from the north. Viola beds are truncated on the flanks of the Central Kansas uplift, but formerly covered at least part of that structure. They are thinner near the Nemaha arch than on the northwest, and are missing locally between the arch and the uplift.

Lithologic comparisons with tentative correlations by Lee in northeastern Kansas indicate zones 1, 2, and 3 may be equivalent to the Stewartville (or younger), zones 4 and 5 to the Prosser, and zone 6 to the lower Prosser or Decorah formations of lowa.