You are at the beach, by the river, in the garden; you walk through soft sand, squish through mud, pull weeds and sow seed in soil. They’re all soft, squishable, digable. But throughout the 3400 million years of our Earth’s known sedimentary record, countless millions of times, these same deposits have hardened to rock. Continue reading
Imagine the squelch!
Footprints of people, birds, and dogs at the beach. Most of us have probably seen this kind of thing. Look a little closer and you will see other markings in the sand or mud – skinny trails meandering across the surface, or burrows. 
Sometimes you will see the critter that made these. We can also observe structures like these preserved in many kinds of sedimentary rock; we call them Trace Fossils. The study of trace fossils is also called Ichnology (a branch of Paleontology), but we’ll stick to the more intuitive name trace fossil. Continue reading
You are confronted by a rocky cliff and your geologist friend tells you that these rocks formed originally in rivers that flowed through a wooded valley. How did your buddy come to this conclusion?
Nearly all sedimentary rocks contain structures – fabrics, planes, contortions. If properly identified these sedimentary structures provide important clues to how the original sediments were deposited.
There are many different kinds of sedimentary structures formed by layers of sediment oriented at different angles, or layers that have been contorted and squished, structures formed by wetting and drying of sediment, structures formed by slip and slide, and by animals leaving tracks and traces as evidence of their activity.
All of these structures can be thought of as contributing to the architecture of sediments and sedimentary rocks.
We are going to examine two of the more common kinds of sedimentary structure – Ripples, and Mud Cracks (sometimes called Desiccation Cracks).
You are confronted with rock formations that might look something like this…
The local geologist tells you that the rocks you see here originally were deposited as sands and muds in shallow seas, where beaches and broad coastal tidal flats passed seawards to deeper waters, and landwards to marshes and scrubby coastal plains across which rivers and streams coursed. How did our geologist figure this out? What is it that geologists see in the rocks that help them paint this picture of an idyllic world that existed so many millions of years ago – a world beyond memory, where, in a different eon, a summer cottage would have been rather nice. Continue reading
