In the study of fossils there are two important field aids usually available. For any locality there should be a geological map and a section showing the sequence and character of the strata. On a small-scale map many of the local details have to be omitted, but the position of the larger exposures is indicated and, with this information at hand, the fossil-bearing strata may be located with the help of a geological section. The latter is frequently obtained from technical reports published by State and National Geological Surveys. Frequently, however, it is possible to obtain only a general plan for a given locality, and a great deal of literature may have to be scanned in order to get that. Excellent geological maps of Colorado have been published by the Colorado Geological Survey and the United States Geological Survey. It often happens that a formation is not where we expect to find it, this being due to several possible factors. The sediments may not have been deposited there, or they may have been removed by erosion. Where the structure has been disturbed by folding and faulting, a multitude of complications is introduced. The expected sequence is sometimes inverted and repeated through a series of folds. Formations also may be moved miles out of place by faulting. Both thickness and character of sediments may vary considerably within a formation. In some regions the geology is very simple, in others extremely difficult to understand. [Illustration: FORMATIONS of the DENVER-FOOTHILLS REGION A GENERALIZED SECTION SHOWING SOME OF THE SURFACE FEATURES RED BEDS HOGBACKS TABLE MOUNTAIN DENVER DISTRICT REGION OF MOUNTAIN-MAKING UPLIFT Formations bordering the mountains have been bent into upright positions.] PERIODS RECENT PLEISTOCENE PLIOCENE MIOCENE OLIGOCENE EOCENE CRETACEOUS THICKNESS SOFT SANDSTONES GRITS & CLAYS DENVER & ARAPAHOE 2000 ft. SANDSTONES, SHALES & LIGNITE LARAMIE 1000 ft. YELLOWISH SANDS & SHALES FOX HILLS 1000 ft. SOFT DARK GRAY OR RUSTY SHALE PIERRE 5000 ft. LIMESTONES & SHALES NIOBRARA 500 ft. DARK SHALES & LIME BENTON 400 ft. GRAY OR BUFF SANDSTONES & CLAYS DAKOTA 300 ft. SHALES, SANDSTONE & LIME MORRISON 200 ft. JURASSIC TRIASSIC PERMIAN DEEP-RED SANDY SHALES, LIME, GYPSUM LYKINS 700 ft. CARBONIFEROUS MASSIVE PINK OR WHITE SANDSTONE LYONS 200 ft. RED OR BROWN SANDSTONE & FOUNTAIN 1500 ft. CONGLOMERATE DEVONIAN SILURIAN ORDOVICIAN CAMBRIAN PRE-CAMBRIAN METAMORPHIC & INTRUDED ROCKS IDAHO SPRINGS SCHIST, GNEISS, QUARTZITE (PART) BASEMENT ROCKS of IGNEOUS ORIGIN