Back in my college days, I was lucky enough to intern at the Cleveland Museum of Natural History for one summer. One aspect of my project involved studying the quarrying history of a local building stone. In the course of my research, I noticed that on many 1800s-era geologic maps, asylums were often located very near to quarries. As it turns out, this was not a coincidence – quarries often hired asylums resident as cheap labor, and in turn, the residents got a chance to leave the grounds and get some stimulation. Hence, a somewhat disturbing lesson: if older geologic maps don’t show quarries, look for asylums. Investigating the geologic record often works in the same vein.
As it turns out, the majority of geologic events in the earth’s history were never preserved in the rock record. Even a geologic event that is sufficiently significant, long-lived, and plainly lucky enough to be preserved may never be discovered – it has to be exposed, accessible, and properly observed and studied. Often, geologic events are not interpreted based on their direct effects, but by side effects and related events. Metaphorically speaking, the quarries are gone, but the asylums remain. In a prime example, in looking for evidence of an ancient transcontinental river in the
How to find evidence of a river nobody can see
Many of the archetypical rock formations of the American southwest – the towering red cliffs, psychedelically wavy hillsides, etc (pictures at left from here) – are the deposits of ancient Jurassic ergs* (sand seas). These ergs were particularly enormous – perhaps rivaling in size the Empty Quarter in the
With any erg, the obvious question is “where did all the sand come from”? (Granitic crust is only 1/3rd quartz at most, so a lot of rock has to be eroded to get a sand sea). Two easy answers presented themselves – the (Ancestral**) Rocky Mountains are practically next door, and there were plenty of voluminous sandstones across
* Deserts and ergs aren’t synonymous, but the two coincided in this instance.
** Not the same as the modern
Marzolf’s hypothesis was essentially ignored until
The long history of Canadian immigration
Recently, Dickinson et al. (2010) tested the "transcontinental river" hypothesis from another angle. I mentioned before that Jurassic rocks aren’t exposed between the Rockies and the
Marzolf, J.E., 1988. Controls on late Paleozoic and early Mesozoic eolian deposition of the western
Wilde, S.A., Valley, J.W., Peck, W.H., and Graham C.M. 2001. Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago: Nature, v. 409, p. 175-178. (pdf here)