What Goes Up…
I’ve been talking about mountain building events that continue for hundreds of millions of years a lot in my posts, referring to the erosion of mountains into mud, silt, and sand, carried by rivers to be deposited as broad expanses of sediment. On sufficiently long time scales, this is an accurate representation of the delicate balance between uplifting mountains and the inexorable influence of rain, ice, wind, and water to eradicate all evidence of an orogeny. For example, the collision of North America with Europe and Africa required nearly all of the Paleozoic Era, beginning with the Taconic Orogeny (550-440 Ma), reaching a crescendo during the Acadian Orogeny (375-325 Ma), and culminating in the Alleghanian Orogeny (325-260 Ma). By the way, the abbreviation Ma (mega annum) is used to indicate dates that were determined by radioactive dating, rather than the more ambiguous “my” for millions of years. There is uncertainty (error bars can never be zero), but not with respect to the general timing of geologic events.
This post is going to examine details of how uplifted rocks can be broken down into pieces that are weathered while being transported to their final resting place, whether in a river, lake, shallow bay, or the deep ocean.
Vermont (Fig. 2) was entirely covered by ice several times during the last couple million years.
The rocks at Smuggler’s Notch are the same ones we saw in the Taconic Mountains and along the White River. They are equivalent to those we encountered along the Potomac River, 500 miles to the south. What happened when rocks formed as much as 20 miles beneath the surface are exposed to low pressure and temperature?
There is a lot of missing rock from the cliff shown in Fig. 3. Where did it go?
How did these huge blocks get where we find them today?
Mechanical weathering doesn’t stop when the fallen block comes to rest. Then, water carries small grains and uses them as abrasives to grind the once-humongous blocks into gravel (Fig. 6).
All that bumping and grinding eventually produces a scene like that seen along the path of the White River (Fig. 7), with bedrock resisting the seasonal onslaught of gravel and sand carried by intermittent, torrential flows.
I hope this post connects the dots between the loftiest peaks (Fig. 1) and the lowest streams (Fig. 7).