In the early eighteen hundreds, geology was still little developed, especially in America, and neither Meriwether Lewis nor William Clark had formal training in geology, mining, or mineralogy. Much of the expedition's understanding reflected common knowledge and misconceptions of the time, yet their journals show a broad geologic vocabulary and unusually strong powers of observation. They named many kinds of rocks and minerals, sometimes correctly and often incorrectly, using terms such as chalk, sandstone, limestone, granite, flint, slate, marble, lava, pumicestone, alum, arsenic, cobalt, pyrite, quartz, and salt. Lewis in particular also used more specialized terms such as argillaceous, calcareous, fossil, incrustation, interstice, and strata. Their descriptions of rocks were sometimes confused by changing light, shadow, vegetation, or contemporary terminology, as in Lewis's July 19, 1805, description of the Gates of the Rocky Mountains, but careful study of Clark's maps and the expedition's original river survey places that observation at the site now known by that name. Despite limited scientific training, both Lewis and Clark showed a strong command of geographic terms and a keen understanding of landforms and river processes, and their geologic observations ranked among the finest in America at the time. One example came on Monday 15th, 1805, when Lewis described Fort Mountain, now known as Square Butte, as a level-topped height rising from the plain with steep sides of yellow clay, though later geologic interpretation identified it instead as a laccolith capped by basalt above sandstone and shale, standing more than 1,000 feet above the surrounding plains.