Referring to the family name Ebbett, “Ebbett's Pass” is a correctly written possessive phrase. But why do we find the form “Ebbetts Pass” in so many documents?
Apparently, authors didn't simply got tired of including the possessive-indicating apostrophe. Instead, they refer to “Captain” John Ebbetts :
It wasn't until 1850 when John Ebbetts—Captain of the Knickerbocker Exploring Party of New York—crossed this pass with a large train of mules, guiding a party of miners into the then gold-frenzied California.A few years later, John Ebbett, let's call him John Ebbetts from now on, led a survey party for the Atlantic and Pacific Railroad Company to this high mountain pass in search of a possible route for the Transcontinental Railroad. His friend and lead surveyor George H. Goddard eventually named the pass Ebbetts Pass in honor of the “Knickerbocker pioneer” [2,3].
Referring to the surname Ebbetts, the genitive case seems to be dismissed: I haven't seen the writing “Ebbetts' Pass” or, worse, “Ebbetts's Pass.”
Keywords: grammar, spelling, writing, name places, history.
References and more to explore
 Wikipedia: Historical Landmark: Ebbett's Pass.JPG [http://en.wikipedia.org/wiki/File:R39287918565_Sign_-_Historical_Landmark%EF%BC%9A_Ebbett%27s_Pass.JPG].
 Sierra Nevada Geotourism MapGuide: Ebbetts Pass National Byway [www.sierranevadageotourism.org/content_detail.php?uid=sie6DA429BAE628F80E3].
 Judith Marvin: Ebbetts Pass History [www.calaverashistory.org/overview/ebbetts-pass-history].