11/10/2012 10:22 am ET Updated Nov 10, 2012

Valerie Simpson on 'Ain't No Mountain High Enough' (VIDEO)

The iconic '60s anthem "Ain't No Mountain High Enough" has been a part of many a playlist for decades, but how did the hit song come together?

For almost 40 years, Valerie Simpson and her late husband, Nick Ashford, created chart-topping R&B soul hits as the songwriting duo Ashford & Simpson. Together they penned songs for music legends such as Diana Ross, Ray Charles and Chaka Khan, including "Reach Out and Touch (Somebody's Hand)" and "I'm Every Woman."

In this sneak peek, watch as Valerie tells Oprah the inspiration behind one of the duo's most unforgettable hits, "Ain't No Mountain High Enough" -- and reveals why she and Nick didn't record it themselves. "I couldn't do -- Nick couldn't do -- what Diana Ross could do, what Marvin Gaye and Tammi Terrell [could do]," she tells Oprah.

Valerie opens up about the loss of her husband of 37 years, and about their musical legacy, on "Oprah's Next Chapter," Sunday, November 11, 2012, at 9 p.m./8C on OWN.