Before revision: However, is not necessary as any two solid materials that repeatedly rub against each other will tend to wear each other away; examples include, softer shoe soles wearing away wooden or stone steps over decades or centuries or glaciers abrading stone valleys.
After revision: itHowever, is not necessary as any two solid materials that repeatedly rub against each other will tend to wear each other away; examples include, softer shoe soles wearing away wooden or stone steps over decades or centuries or glaciers abrading stone valleys.