A mesa is an isolated, flat-topped elevation, ridge or hill, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain. Mesas characteristically consist of flat-lying soft sedimentary rocks capped by a more resistant layer or layers of harder rock, e.g. shales overlain by sandstones. The resistant layer acts as a caprock that forms the flat summit of a mesa. The caprock can consist of either sedimentary rocks such as sandstone and limestone; dissected lava flows; or a deeply eroded duricrust. Unlike plateau, whose usage does not imply horizontal layers of bedrock, e.g. Tibetan Plateau, the term mesa applies exclusively to the landforms built of flat-lying strata. Instead, flat-topped plateaus are specifically known as tablelands.
When he was young, he was sick, deaf in both ears and blind in both eyes. At the age of seven, Anne Sullivan became her tutor and became her mentor and mentor for 50 years. With the help of Sullivan, he entered the university and graduated with honors. When she was in college, she wrote "the Story of my Life", which tells how she overcame the disabled and inspired thousands of disabled and normal people. The book has been translated into 50 languages and spread all over the world. Since then, many words and several autobiographical novels have been written to show that darkness and silence do not exist. Later, Keller became an outstanding social reformer, making speeches all over the United States, Europe and Asia, raising money for the blind and deaf-mute. During World War II, she visited a number of hospitals to comfort blind soldiers, and her spirit was respected by people. 1964