[4]A geologic formation consists of a sequence of rock layers which were deposited under essentially the same conditions, or a series of alternating conditions, and which can be easily distinguished and mapped as a unit by geologists in the field. [5]Geologic time is divided into four Eras which are designated from oldest to youngest: Precambrian, Paleozoic, Mesozoic, and Cenozoic. Each of these Eras is divided into geologic periods of time. The Devonian Period is in about the middle of the Paleozoic Era and began some 330 million years ago and ended approximately 290 million years ago (see Geologic Time Scale, Fig. 14). [6]A joint is a break in a rock mass which interrupts its physical continuity. A group of more or less parallel joints is known as a joint set. Transcriber’s Notes —Silently corrected a few typos. —Retained publication information from the printed edition: this eBook is public-domain in the country of publication. —In the text versions only, text in italics is delimited by _underscores_. End of the Project Gutenberg EBook of The Geology of Darling State Park, by Harry W. Dodge, Jr.