This is a really unique map that documents observations made by Lewis and Clark on their explorations from Pennsylvania to Pacific Ocean. The proportion or area of the circle is indicative of the width of the river bed, the color indicates water color and the position of the black dot within the triangle indicates the type of material found.

http://http//www.google.com/imgres?imgurl=http://pubs.usgs.gov/fs/fs-004-03/images/fig5lg.gif&imgrefurl=http://pubs.usgs.gov/fs/fs-004-03/&usg=__t_fsEbEa3gB_iBBLc-aL-S3p44A=&h=351&w=585&sz=30&hl=en&start=7&sig2=QSXKY66ppbnzTUP0y5ERvw&um=1&itbs=1&tbnid=GwpjEhCZiunjaM:&tbnh=81&tbnw=135&prev=/images%3Fq%3Dproportional%2Bcircle%2Bmap%26um%3D1%26hl%3Den%26rls%3Dcom.microsoft:en-us:IE-SearchBox%26rlz%3D1I7ADRA_en%26tbs%3Disch:1&ei=fgg6TMnFLIW0lQfbu-CjBA