Re: C or R horizon?

The two horizons with question marks would best be labeled a “2C” and “4C” based both upon the generalized description of the materials of which they consist, and the horizon designations of the layers above and below them. It appears the layers in question are fragmental in nature. That is, consisting mostly (>90%) of unconsolidated rock fragments with little or no sand, silt, and clay, referred to as ‘fine-earth’ (particles of <2mm). The general description provides no indication of cementation or lithification of the pebble, cobble and shale materials, so a reasonable assumption is these are unconsolidated. A C-horizon can consist of unconsolidated fragments with little or no fine earth materials.

A lithologic discontinuity is not used to indicate presence of bedrock or an otherwise cemented or lithified layers such bedrock (R horizon). A lithologic discontinuity is used to note a significant change in particle-size, or a difference in the mineralogy of the soil constituents, from one horizon to the next, or can even be used to indicate significant differences in the age of the materials in the profile.