The Cretaceous Seafloor

Creatures of the Western Interior Seaway.
Whenever we take tours out to the field, one thing they always find a ton of is shale. Kids and adults alike can get sick of it pretty quickly and we often think of shale as being in the way since we're usually only interested in finding the fossils in it. However, shale is actually pretty neat in and of itself, and is only considered to be "boring" because it's so common. Many people don't realize that shale, although it's dry and dusty now, is former seabed from the Western Interior Seaway, which is where the animals that we currently find as fossils used to swim (sometimes even we field techs forget that!). Think about it: above the sea floor swam sharks, squid, and marine reptiles like mosasaurs and plesiosaurs, while closer to shore were numerous birds like Hesperornis and Icthyornis. Near the underlying seabed were all kinds of corals and shellfish, while tiny invertebrates burrowed through the murky bottom. The sea floor and the area above it were teeming with life; now, all we have are the rock remnants that they left behind, but there's still a multitude of things we can learn from them.
Shale layers; we mostly excavate
 in Boyne through Millwood.

There are several different shale layers that we find fossils in around Morden, each representing a period of time. These layers are usually quite different from each other, although certain layers are sometimes very similar (like the Millwood and the Upper Gammon that I've written about before). Changing composition of shale means that the conditions in the Seaway were changing as well. For example, a shale formation with high levels of bentonite indicate a lot of volcanic activity (bentonite is a common mineral formed from compressed volcanic ash). As well, fossils can have major differences in appearance depending on which layer they were formed in; fossils from the Pembina shale tend to be reddish-grey with fairly poor preservation, while Boyne fossils are black and shine gold in the sun. The Gammon Ferruginous can be extremely hard to dig through, but its fossils are likewise very well preserved.

Attempting to sift through Millwood.
Whenever we go to the field, we never refer to the rock we dig in as simply 'shale'; rather, we use names like 'Pembina' or 'Millwood' to describe the kind of shale we're excavating in, since the differences between layers are so pronounced. Odanah shale excels at staying dry when it rains, for example (making it popular for use on country roads), while Millwood and Upper Gammon are the only local shales that turn into sludge when they become wet. Last week, we tried sifting for shark teeth in some wet Millwood, but the stuff just clumps together and refuses to separate, so we didn't have much success. We might try again next week, but until then, we'll have to find fossils somewhere else.

Matt Remple
Field Tech