They're are not really riding sidesaddle - there are no saddles! They're sitting sideways on blankets. The bridles on the 3 horses from left are intended for driving (note the blinkers, or blinders), not riding, so I'm guessing these are harness horses, unhitched. The women are not wearing riding skirts, either, so they probably just jumped on for the photo.

That sidesaddle business must have been tough! I thought about that as I was posting this. And I think this may have a high school class outing since I have another photo of the same ladies in the same dresses (but no horses) with some boys. Perhaps it was their "senior trip"!

This is a wonderful picture of the young lady who became your grandmother, and her friends, enjoying themselves! They do look a bit precarious, though. They look elegant in the side saddles, but if any of those horses decided to be ornery (like several of the horses I've ridden), I'm afraid they would have had a hard time staying on!

I can't imagine what it must be like to ride sidesaddle over rough terrain.