OK, u know how on Doc's tombstone it says his "beloved Clara". Well if Doc died on September 7th and say Marty never went back to get him, wouldn't Clara have fallen into the Ravine before Doc ever met her? So how is it possible that he met her if time hadn't been altered for Clara?
No, because if Marty hadn't gone back, Doc wouldn't have known that he was going to fall in love with Clara ("Who is this Clara? I don't know anyone named Clara!"), so he would've picked her up from the train station, as he told the Mayor he would.
Here's a better explained answer, from the BTTF FAQ:
Q: How could Clara have erected the tombstone for Doc after September 7, 1885 if she was supposed to have gone over the cliff on September 4th? At the beginning of Back to the Future Part III, would the name of the ravine be "Clayton," "Shonash" or "Eastwood?"
A:
Version #1 -- "Original History"
The "Original History" occurred before Doc Brown was ever born or invented the time machine. This is how things would have been written in the history books in Back to the Future, and in most of Back to the Future Part II.
* August 29, 1885: Hill Valley Town Meeting. No one volunteers to meet the new school teacher at the station.
* September 4, 1885: Clara arrives at the train station. Since no one is there to meet her, she rents a buckboard. While heading out to the school house, a snake spooks the horses, they run wild, the buckboard goes out of control, and over the edge of Shonash Ravine. Clara is killed.
* September 9, 1885: After a memorial service for Clara Clayton, the city fathers decide to name the ravine in her memory. Thus, "Shonash Ravine" becomes "Clayton Ravine."
Again, Version #1 is the history of Hill Valley that happened BEFORE the beginning of Back to the Future.
At the conclusion of Back to the Future Part II, Doc is zapped back to January 1, 1885. He settles in Hill Valley as a blacksmith, and the above events are altered because of his presence, as follows:
Version #2 -- Doc in 1885, without Marty.
* August 29, 1885: Hill Valley Town Meeting. Doc Brown volunteers to meet the school teacher at the train station.
* September 4, 1885: Doc meets Clara at the train station and they fall in love at first sight.
* September 5, 1885: Doc takes Clara to the festival. Buford shows up and shoots Doc in the back with the derringer. Despite Clara's efforts at nursing him, Doc dies two days later from internal bleeding as a result of the gunshot wound.
* September 9, 1885: Clara dedicates Doc's tombstone, "In loving memory from his beloved Clara."
In this sequence, the name of the ravine remains "Shonash Ravine." This history ripples into the future AFTER Doc is struck by lightning at the end of Back to the Future Part II. Marty, however, retains his knowledge and memory of the original history because he has come from a point in the space-time continuum in which the original history applied. If Marty were to go to the ravine in 1955 at the beginning of Back to the Future Part III (on his way to the Pohatchee Drive-In, for example), he would discover that the ravine is called "Shonash Ravine."
In Back to the Future Part III, Marty's trip to September 2, 1885 alters Version #2 as follows:
Version #3 -- Doc and Marty both in 1885
* August 29, 1885: Exactly the same as in version #2: Doc volunteers to meet the school teacher.
* September 3, 1885: As seen in Back to the Future Part III, Marty shows Doc the photo of the Tombstone. Doc decides NOT to meet Clara at the station.
* September 4, 1885: Clara arrives at the station. No one is there to meet her, so she rents a buckboard, as in Version #1. Similarly, on her journey to the schoolhouse, the snake spooks the horses and they run wild toward the ravine. As seen in the film, Doc rescues her from going over into the ravine. They meet and fall in love at first sight.
* September 5, 1885: At the festival, Doc's behavior is now different due to his knowledge that Buford is going to shoot him in the back (which is why Doc keeps facing front to Buford). Because Buford never does shoot him at the festival, and due to Marty's interference, the name on the tombstone photo vanishes.
* September 7, 1885: "Clint Eastwood" is apparently killed when the runaway locomotive plunges into the ravine. In honor of his heroic action against Buford Tannen, the city fathers decide to name the ravine after him.
(Incidentally, there is an alternative scenario that may have occurred in Version #2: On September 15, 1885, Clara, distraught over Doc's death, commits suicide by jumping into the ravine. As a gesture of sympathy, the people of Hill Valley decide to name the ravine in her in memory, thus putting the space-time continuum back into a similar situation as in Version #1. We will remain ambiguous about whether this suicide incident actually happened in Version #2 so that the viewer may choose whichever scenario fits into his own theories about time travel.)
oh this makes much more sense now. BTW where can I find the BTTF faq?
thanks alot, although I actually found it after I asked, should have said that, but thanks anyways