Kalmar "A horizon" ana amfani da ita a kimiyyar ƙasa kuma tana nufin saman saman ƙasa, wanda kuma aka sani da ƙasa. Wannan Layer yawanci ya fi duhu launi fiye da ƙananan yadudduka saboda tarin kwayoyin halitta da ma'adinan da aka shafe kuma sun rushe daga duwatsu na tsawon lokaci. Yawanci shi ne mafi ƙarancin ƙasa na ƙasa, tare da babban abun ciki na gina jiki da babban matakin ayyukan nazarin halittu. Kalmar "A horizon" ta fito ne daga kalmar Jamusanci "Auflagehorizont," wanda ke nufin "top Layer horizon."