Monday, February 20, 2023

Geology Outcrop Quiz: Milam Glacier Area

In the outcrop below, which direction are the strata dipping (tilting)? North is to the right. 

If your answer was towards the right (north), you would be wrong.

The apparent northward dip of the rock layers is an illusion caused by fractures intersecting the plane of the exposure. The outcrop below from a nearby spot shows the real geometry of the strata.

This is an across the bedding exposure. You can see that the strata are almost vertical and are warped into small scale folds. Look at the black arrows on the left centre of the image. They point to sub horizontal cracks (fractures) penetrating the rocks. These are axial plane fractures as they follow the plane that divides the folds symmetrically. To the top right, the red arrow points to the distinctive planar fabric that has arisen due closely spaced fractures. 

In contrast to this perspective, when the outcrop is a bedding plane exposure i.e. a slice parallel to the bedding, as it is in the first picture, you won't see the folding. Instead you can only observe a bedding surface cut by horizontal fractures that divides the rock wall into layers and gives the impression from afar of horizontal or gently tilted beds.

The photo below is a close up of an outcrop with prominent nearly vertical thin orange-brown and dark grey folded beds. Notice the white horizontal bands? These are the axial plane fractures which later were filled with quartz, as mineral saturated groundwater circulated through the rocks.  

The outdoors is such a good classroom. This final outcrop illustrates how erosion along a hillslope has sliced the rock body at different angles with respect to the bedding, exposing both, the true orientation of the strata as well as the pseudo bedding. 

To the right of the waterfall is an across the bedding cut, exposing the nearly vertical gently folded strata. To the left of the waterfall is an along the bedding exposure. Only a sheer rock wall is observed with the axial plane fractures imparting a crude layering to the rock face, giving the illusion of bedding.

All these exposures are along the road side trail near Milam village in Kumaon, Uttarakhand. You can test your observational skills when you next visit that area. 

Trekking in the Himalaya is always a treat.  And when you come across these small geology puzzles, it is with a double scoop topping.