The simplest but most important idea here is that for the most part, geology and geologic time is a bottom-up affair. In a given setting (like Ottawa, shown here) the oldest rocks are on the bottom and newer rock forms in layers ("strata") on top of the old.* So the right hand image is telling us that the rock coloured in shades of pink is older than the yellow stuff on top of it and blue stuff is younger still. And no, the rocks aren't necessarily those colours. Although sometimes they are.
The column on the left breaks geologic time into sections with the oldest time periods at the bottom and, once again, the newer stuff above it. Geologic time is broken down into a system whereby Eons beget Eras beget Periods beget Epochs beget Ages, all with colourfully weird names, none of which you really need to learn unless you're geologist, in which case, be ready to recite all of them in your sleep, along with their associated dates. The rest of us can just Google them if we get stuck.
One of the rather gob-smacking take-aways from the left column is just how old all of this is. The figures running down the left side of the left column are millions of years. So where part of the brown section is labeled "Jurassic" — a name that should ring a bell — the graphic is saying that the Jurassic Period began some 208 million years ago (MYA) and lasted until about 140 MYA (some sources specify 144 MYA, but what's 4 million years between friends). So at the bottom of the chart, Archean rocks date back to at least 4,500 million years (I know, my head hurts too), while at the top of the column, the Holocene is dated in thousands of years and includes woolly mammoths, saber-tooth tigers and Fred Flintstone.
But here's the weird thing, and this is where all those arrows come in. If you go boring into Ottawa bedrock, you aren't going to find all those different layers from all those different time periods. In fact, you're only going rock from two specific time frames. The bottom stuff (in pink) comes from the Proterozoic Eon, the more recent part of the Precambrian Supereon**, back when life on Earth, in the spots where life did exist, took the form of slime, gunk and blobs. Rocks from this time period have since been heated and crushed and smooshed to become "metamorphic" — stuff like granite and marble (and yes, gneiss). We don't encounter much of this kind of rock in our area unless we dig really deep, or visit the Gatineau Hills, where it literally sticks out of the ground. Hence hills.
After this, the chart indicates a gap where the upper parts of the Proterozoic, and most of the Cambrian stuff above it (in our area at least) got worn down and washed away by erosion. Ottawa's upper layer, the "yellow" stuff, was laid down during the Ordovician Period. Sedimentary rocks from the Ordovician are younger than the stuff beneath them, but they're still pretty old. During the Ordovician we didn't even have land animals yet, let alone dinosaurs. Ordovician animals were things like trilobites and corals and sea-lilies and brachiopods (sort of like clams) and nautiloids (like this guy...)
...and then it got cold and a lot of stuff died. Thus ended the Ordovician.
Then the chart shows another really big gap until you get to the blue-coloured Holocene layer on top. This means that the blue stuff is a lot younger than limestones and shales and sandstone. In fact, it's so young it hasn't even turned into stone yet — geologists say it's "unconsolidated", which has twice as many syllables as "not rock yet". It's a bit hard to read on the drawing but this top layer consists of clay (from when we were at the bottom of the Champlain Sea) and "till", which is stuff dumped here by glaciers during the last Ice Age. In some places there's also sand, where it was pushed into sandbars by the proto-Ottawa River, when it was still a lot deeper than it is today.
One more thing. On the right-hand side there's line where it says "Fault". Now I don't know whose fault it is and maybe it's no-one's fault in particular. But it does indicate that cracks sometimes form in the bedrock and layers shift so they don't line up properly any more. It doesn't mean you were bad, it just means that daddy's going to be living at motel for a while and like everything else in life, it's something we can all learn from.
*Unless they get tilted or flipped. Learn how to carry a tray.
**Internet Coupons valid through December 31 2018.
