Epichnia are found on the tops of the strata of origin, being those ridges and grooves that were formed by benthic organisms or infaunal burrows that have been exposed by erosion. Exichnia are traces that are made of material that is different from the surrounding medium, having either been actively filled by an organism or eroded out and re-covered by an alien sediment. Hypichnia are ridges and grooves found on the soles of the beds of origin at their interfaces with other strata, representing the opposite of epichnia.