My friend Malcolm runs a field trip to Baculite Mesa for the Western Interior Paleontological Society almost every spring. My mom and I had gone a few years ago, and found the site so prolific with the shells of these uncoiled ammonites that we came home with rock bags heavy with over 200 specimens after only a day of collecting. Most of what you find are casts, but occasionally you find one with some of the original pearly shell preserved, including the beautiful dendritic patterns of the complex sutures. Now that Mattie is almost 5 and Allison is 8, I decided that a trip to Baculite Mesa would be an ideal collecting opportunity for the kids. Allison is already a real dinosaur aficionado, and has a beautiful fossil leaf she found at Florissant three years ago. Mattie is still too young to really be into fossils, but loves to collect things, and they both had a good time hunting trilobites two summers ago. I knew that a place with a lot of specimens and not too much hard walking or digging involved would make a great trip for the three of us.
Baculite Mesa is only about 5 miles east of the north end of Pueblo. The drive from Boulder takes about 3 hours, and none of us were the least bit thrilled with the idea of getting up early to be down there by 9, so we drove down Friday night and got a motel. (Getting the chance to stay at a motel, one with a swimming pool no less, excited the kids just as much as the prospect of finding fossils.) The site is on private land, so we had to have permission from the landowners to collect. We assembled on top of the mesa Saturday morning, and Malcolm gave us an orientation as to the geology and paleontology of the region before we set out collecting.
Baculite Mesa exposes the Cretaceous Pierre (pronounced "peer") Shale, a marine unit between 82 and 67 million years old. At that time, a great shallow epeiric seaway cut the continent of North America in half, connecting the Arctic Ocean with the Gulf of Mexico. The seaway was inhabited by mosasaurs, ammonites, and large fish, many of which are found in the chalk beds of Kansas laid down at the same time. In most places, the Pierre is not a very productive unit for fossils. It is a grey shale thought to represent sedimentation in nearshore anoxic bottom waters, and thus does not preserve an extensive benthic fauna such as might be representative of more oxygenated conditions. Inoceramid clams are the exception, however, and are found throughout the Pierre. Baculite Mesa is a unique locality within the Pierre that is highly productive of the nektic, uncoiled ammonites called baculites.
Malcolm has been collecting the Pierre in the Baculite Mesa area for years, and showed us his nice collection of ammonites, baculites, clams, and snails, many with the pearly nacre and sutures preserved. He was careful to tell us, however, not to expect to find many such prime specimens in only a few hours, but he wanted to show us that the potential for some really nice finds was there. I hadn't seen anything like that the last time I was there, and I really felt that the kids would be more interested in finding a lot of stuff even if it wasn't so great, so we opted to collect on the flats near the south end of the mesa where we were assured that we would find lots of baculites.
Instead of having to scramble down the steep sides of the mesa like my mom and I had done, we now had the option of driving around the south end and parking on the flats, then approaching the mesa slopes from the south. That was great, because it made walking a lot easier especially for the kids. Mattie had insisted on wearing her black patent-leather shoes, which are not exactly the best choice for tromping around between prickly pears, so we had to be careful. She was also wearing shorts, appropriate for the sunny, mid-70°s weather, but which offered little protection against the cactus thorns and yuccas. But we started finding baculites almost right away. It only took a few minutes for me to show them what the baculites looked like, and then they began spotting them everywhere. The best places to look were where there wasn't too much ground cover, near the summits of the small, rolling hills. Our rock bags filled quickly, and soon I was the pack mule carrying three heavy rock bags, as the kids dumped more and more baculites into them.
Somehow or other the water bottle I had in my rock bag got left in the truck, and soon we were all getting thirsty. I was also getting tired of carrying those three heavy rock bags. So about lunchtime we decided to call it quits and go back to the motel for a swim and a late lunch. We all had a really great time, and both girls were really proud of all their finds. Allison found several sections which had broken at a suture, revealing the interfingering surface. Mattie was most excited when she found a "tail", or rounded end section. Both of them took their fossils to school for show and tell after we got home, and the three of us are looking forward to our next fossil-hunting trip together.
