"In the rocky mountains, from Canada down to the state of Montana, there is large geological overshoot where the bottom layer is dated a hundred million years, and the top layer a billion years. The area is about 30000 square kilometres."

In article <C322KA.M3p@cs.umu.se> jan@cs.umu.se (Jan T}ngring) writes:
>Is this correct?

Almost.

You refer to the Lewis Thrust, a region where a kilometers-thick slice of Paleozoic sediments lies unconformably atop more recent strata. The geology of the region has been beautifully exposed by glacial action in Glacier National Park, in some of the most magnificent scenery in the US. The Paleozoic strata are tinted in striking pinks, reds, and greens, with white and black intruding dikes and contact metamorphism.

>How did the younger layer get on top of the older?

Teemu Makinen writes:

[ explanation by overturning plate segment deleted. nice ASCII graphics, tho :-) ]

Well, overturning has occurred in various places around the world, but it doesn't account for the Lewis Thrust.

The error in Jan T's statement of the Lewis stratigraphy is in the implication that the ages of the strata look like this, (with ages in Millions of years):

---------------
1000
---------------
750
--------------
500
--------------
250
--------------
100
--------------
undateable basement

which would be upside-down ordering, oldest on top and successively younger layers downward; a situation that could be the result of the overturn diagrammed by Teemu.

In fact, the Lewis stratigraphy looks more like this:

----------------
1000
----------------
1050
----------------
1100
===============   thrust fault
100
--------------
200
--------------
300
--------------
... many more layers

That is, the top layer is around a billion years old, and it is above a 100-million-year-old layer, but the 100My layer is definitely not the "bottom."

What in fact we have is a big slab of normally-ordered old sediments laying right-side-up on top of a big slab of normally-ordered newer sediments.

The plate-tectonic explanation goes like this:

Once upon a time, there was a passive continental margin that collected sediments in deep layers:

west                                      Montana           east
-------------------------------------------------------------
300
-------------------------------------------------------------
500
--------------------------------------------------------------
750
--------------------------------------------------------------
1000
-------------------------------------------------------------
1050
...
etc

Then subduction started near the continental shelf, and other landmasses began to collide with our continent, riding in from the west on the moving ocean crust. The collision, which occurs offstage to the left in my pictures, produced in its earliest stages some high-angle normal faulting and uplift to the west of present-day Montana, thus:

                Faulting
west                                      Montana           east
---------------------------|---------------------------------
200                        |
---------------------------|---------------------------------
300                       /
-------------------------/-----------------------------------
500                     /
-----------------------/--------------------------------------
750                   /
---------------------/----------------------------------------
1000                /
-------------------/-----------------------------------------
1050
...
etc

Uplift and Erosion in the west, sedimentation in the east:

west                                      Montana           east
-------------------
300                \
---------------------
500                   \
------------------------
750                      \
-------------------------/-----------------------------------
1000                    /    100
-----------------------/--------------------------------------
1050                  /      200
---------------------/----------------------------------------
1100                /        300
-------------------/-----------------------------------------
1200              /          500
...
etc

Then, as the tectonic collision to the west intensified, further uplift was coupled with crustal folding, tilting the uplifted block, and accelerating the erosive removal of the more recent sediments from the tops of the high young mountains.

Further uplift, crustal shortening, block tilts, further erosion:

west                                      Montana           east

 /
/  \  / \
    \/   \  / \  / \
\    \    \/   \/   \
 \ 10 \ 1K \750 \ 500 \
  \ 50 \    \    \     \
   \    \    \    \      \
11  \    \    \    \       \
\ 00 \    \    \    \        \
 \    \    \    \    \ /--------------------------------------
  \    \    \    \    /      100
   \    \    \    \  /----------------------------------------
\   \    \    \   \ /        200
 \   \    \    \   /-----------------------------------------
  \   \    \    \ /          300

Then, as the impacting plate smashed everything before it, the compressive force of the collision detached an immense block from the eastern face of the uplifted mountains, which slid to the east on the Lewis Thrust fault.

Overthrust, big block slides to east:

west                          Montana           east

 /
/  \  / \                -----------
    \/   \  / \  / \  /       750    \
\    \    \/   \/   \/   -------------------
 \ Intensely crushed  \             1000     \
  \and folded, too     \  ----------------------
/  hard to draw well     \          1050         \
    \    \    \    \     /----------------------------
\ /  \  / \    \    \   /           1100
 \    \    \    \    \ /==================================  thrust fault
/ \  / \    \    \    /      100
   \    \    \    \  /----------------------------------------
\   \    \    \   \ /        200
 \   \    \    \   /-----------------------------------------
  \   \    \    \ /          300
...
etc

Igneous dikes then intrude along faults, and between some of the sedimentary layers. After further erosion removes the top of the whole picture, and glaciers carve big U-shaped valleys in the block that slid; hey presto! Glacier National Park.

I was there three years ago - what a wonderful place to look at rock! Deep time is spread out all over the landscape.