1) As we know in any sedimentary sequence, the rock beds exist in order, from the bottom to the top of the column. The bottom most bed is naturally the oldest one while the top most is the youngest.

2) But where the sedimentary columns have undergone very severe diastrophism, it is difficult to determine the order of superposition. The following criteria may be used for determining the order of superposition in the areas, which have been tectonically deformed.

*Primary Sedimentary Structures*

(a) Ripple marks

Only oscillation or symmetrical ripple marks which have rounded troughs and pointed crests are mostly used for the said purpose. In case of inverted sequence the tapering crest points downward and rounded troughs point upwards.

(b) Current bending

Since these beds are usually slightly oblique to the major bedding planes, these are conveniently used for determining the top and bottom of beds.

(c) Graded bedding

As it shows a gradation in the size of grains from coarse below to fine above, is used as an important criteria.

(d) Mud-cracks

These are usually wide at the top and taper downwards but in inverted sequence the tapering end faces the upward direction.

(e) Local unconformities:

It also gives conclusive evidence about the order of superposition of strata. Similarly a channel cut into the underlying beds also indicates the top and bottom of original beds.

(f) Orientation of fossil-shells:

Certain types of shells tend to lie on the sea-floor in their most stable orientation