Magnetite collected from Centennial Brook perched dramatically on a ceramic magnet
Geology can be a tricky thing to grasp, given that it deals with incomprehensible timescales and with process that are often very difficult to observe (because they happen slowly or because they happened in the past). We’re left with trying to construct compelling narratives from patchy evidence. National Geographic and others have created some incredible films that give visual power to the geologic story (my personal favorite: link). Telling a more nuanced story rooted in place puts us back at difficult.

A few things can be helpful. Number one is the idea of uniformitarianism. Uniformitarianism – in a simplified view of the term – is the assumption that the laws of nature operated in the past in the same way as they operate in the present (for a pretty great debate centered around opposing views, check out Bill Nye debate young earth creationist Ken Ham). Charles Lyell formulated this idea to make sense of the frozen stories embedded in rock that seemed to mirror things he saw occurring in the present (present for him was early 1800s). So if you saw asymetric ripples lithified in a layer of Ordovician sandstone, you could infer that around 450mya there was a river flowing there, since that’s where you would find that feature being shaped today.

Delta sediments from Lee River in Jericho, flowing left to right
Monkton quartzite exposure at Salmon Hole in Winooski
showing same asymmetrical pattern in old stream bed

Erosion of big landscapes can be difficult to imagine. With my college classes, for our study of surficial geology we’ve been going into brooks in Burlington with magnets and pulling out whatever we can find. In my exploration with my Natural History of Vermont class at CCV earlier this semester we found a brook that was pretty darn loaded with magnetite (located just east of SD Ireland). It’s being eroded out of the soft sediments from the Winooski River delta when it dumped into the Champlain Sea about 10,000 years ago. So all that sediment would have been washed down into the valley from upstream, in the case the Green Mountains. The rock below is from Smuggler’s Notch, and has somewhat large chunks of magnetite embedded within it.

Magnetite is a common mineral in many rock types (typically igneous and metamorphic), though often occurs in quantities that are too small to be detected with a hand-held magnet.