Unproved information on the Internet states that Mary's surname was Legg. It is possible she was related to Susan Pelham of Ozark Co Mo, since it is also claimed that Susan's maiden name was Legg (Ligg.)
In any case, the Clarkston and Pelham families were closely connected since James Clarkston moved to Texas with the Pelham family circa 1860 before they moved back to MO during the 1860s and Lewis' daughter Eliza Jane was living with the Pelhams in the 1860 census.
Kay Silkey writes:
I know where the Legg name came from. The Internet. Here's what I found:
Levi Legg married Sarah J. May 5 Sept 1837 in Sangamon County, IL. (Illinois Marriage Database) They had at least two children: Lewis Clarkston Legg born 1837 Sangamon County died 1888 Humboldt, Richardson County, Nebraska and William Thomas Legg born 1835 Sangamon County, IL and died 1913 Humboldt, Richardson County, Nebraska. When Lewis purchased his land from the US Govt in Illinois he stated that he was "of Sangamon County".