[Illustration: SPIRES AND PINNACLES IN THE GRAPEVINE HILLS—Erosion, like a master sculptor, has carved many weird and grotesque forms throughout the Big Bend area.] The geological story of Big Bend is vividly revealed in the rock strata, the spires, buttresses, erosive remnants, arroyos and canyons. The entire Big Bend area was submerged by an ocean millions of years ago. Sediments of sand, mud and lime deposited on the floor of the sea later hardened into rock. Common to all oceans, various types of aquatic life abounded in these waters, many of which were fossilized in the forming of the rock. Igneous action within the earth’s interior caused an uplifting of the surface and receding of the ocean waters. It was during this period that giant trees grew and later became petrified. (Evidence may be noted on the Tornillo Flats.) Swamp and shoreline vegetation provided food for the dinosaurs during this period. As the igneous action increased, molten rock was deposited in some sections and mountains were formed along the lines of greatest pressure. This newly formed rock was soft and highly susceptible to the processes of erosion, which gradually broke down the softer portions of the mountains, making deposits in the valleys and lowlands. [Illustration: ROCK FORMATIONS IN THE GRAPEVINE HILLS—Fantastic shapes have been created by erosive action in the Big Bend National Park. Here, the massive boulders form a frame for the mountains in the distance.] Evidence of the tremendous force of erosion is very pronounced throughout the Big Bend National Park. Here nature has carved a cross section out of the earth leaving the rocks of various geological eras etched in bold relief. The Big Bend country reveals to the novice a greater understanding of the geological evolution through which the earth has passed. Scientists, who have searched the arroyos, buttresses and canyons, have obtained vital information and substantiating facts which have contributed to the knowledge and advancement of the science of geology. Climatic conditions throughout the Big Bend region are mild and arid. Here again, in this land of contrasts, it is possible to experience a variation of temperatures and weather conditions. During the hottest part of the summer the high altitudes of the Chisos Mountains remain moderate and cool, while the temperatures are much higher along the lower plains, the desert and river valleys. The annual rainfall is light, varying from 8 to 20 inches. Although there is some snow and freezing weather in the mountains during the winter months, extreme temperatures normally are of short duration. In other sections of the Park the temperatures rarely drop below freezing. Throughout the winter the temperatures are mild along the Rio Grande.