where in the colon is stool shaped finally?

I think mine was like this: I had a fold about 1-2 inches up into the rectum, that caused a flat, ribbon when the stool was soft. then the anus rounds out the edges so there is a "groove", but it isn't, it is the middle of a fold or rounding out.
This is like a garden hose folded over to stop the water flowing. Warning, there is no evidence of this with an empty colon (ie, colonoscopy) and no hard place to feel with a finger, nor anything except maybe a flat x-ray plate when you have some barium mixed in the stool. Ha. No one listens! As a result I eventually lost the entire lg bowel. No one would even let a radiologist even talk to me. This is basically a blockage which is invisible. This is worse with pushing, and not blocked with a moderately formed stool that pushes its way through the fold. Good luck. I believe a very experienced surgeon could stiffen up the rectum and fix this. Oh, yes, little enemas do open up the fold from the far end and allow stool to pass.

Yes, no tumor, I wondered, too, but no tumor in my case. This disappearing folding over did a huge amount of damage by intermittant blockage over the years. The shape is caused by two things, first the flat ribbon then it is rolled into a tube, (almost), by the passage through the anus. The stool has to be shapeable , not too hard, not too soft. You probably should follow up on this.

Page two (2) of this www.med.umich.edu/bowelcontrol/patient/teaching/Bowel%20Function.pdf will tell exactly how the stool is formed and moved through the lower end of your GI tract. You have internal hemorrhoids correct? That could be making the groove.

You have asked this question repeatedly over the last couple of months. We are not doctors here and cannot tell you why this is happening. If you are dissatisfied with your doctor's explanation, I suggest getting a second opinion. Please do not start anymore threads asking this question. If the one falls off the front page and you worry someone will not see it, then bump it up.