A geologic formation is a type of rock formation that is created by natural processes such as volcanic activity, erosion, and sedimentation. The term 'geologic formation' can refer to a single rock layer or a group of related rock layers. In this crossword, the solutions for the term 'geologic formation' are 'VOLCANO' (7 letters) and 'ione' (4 letters). A VOLCANO is an opening in the Earth's surface through which molten rock, ash, and gases are released. Ione is a small town in California that is located near the foothills of the Sierra Nevada mountain range.