It didn't take us long to figure out what subject this would be great for: GEOLOGY. The kids were mesmerized by all the wonderful rocks and rock formations. Geology is not a subject we have studied very much yet, so our discussions were mostly tid bits that my husband and oldest daughter knew about. A good friend and my oldest daughter's Godfather is a geologist so from trips with him they had both picked up a few things here and there.
They talked a little about sedimentary rock layers, weathering, bedding, erosion, earthquakes, and different types of rocks like gravel vs. sandstone. O.K. so maybe not just tid bits, but certainly more than I knew.
I humbly have to admit: dad can lead some nature study too ; )