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".
and from genealogist Kevin Carman:
Philip Carroll I think is how he spelled his name responded to one of your queries back in 2011. His John Pelham is telling! First, he married Susan Legg [Ligg] in Green Co., KY in 1827. Second he is living next door to Lewis & Mary in 1840. Third, Eliza J. Clarkson is living with the Pelham's in a later census. Fourth Lewis, John & Susan are all buried in the same cemetery!
So far I do not find Lewis & Mary marrying in Green - but it adjoins burned counties! Likewise with the itinerant ministers I mentioned previously in the area and a strong Presbyterian presence in the region and the family it is entirely possible there is no marriage record in the courthouse. I am, however, convinced now that Mary Legg was her maiden name - too many coincidences to be chance.