Bovine excrement, aka a land mine, pasture puddle, bullshit, moo poo, etc.
Specimens are typically a deep brown, chocolate color when fresh, gradually darkening and hardening to a rich black. Eventually they will oxidize and turn greyish, at which point they are hard enough to safely step on/in, not that you should do this intentionally. The smell is usually robust, earthy, cow-ey, and actually rather pleasant, if you are the sort that likes "county aromas".
The typical cowflop is 1.5-2 feet in diamter, flat and disk-shaped, often 2-3 inches deep in the middle. The interior is often mushy and greenish, and smells much sharper. Specimens have been sighted up to three feet across.
Unhealthy cattle (those with bovine diarhea) produce a greenish, liquidy stuff that smells sour and looks quite unpleasant and mucky.