Below is a Sandstone exposure which is part of the Externestein rock formation, this particular exposure is ~300m to the west of the main monument, it was formed as part of the regional uplift which happened ~90ma ago.
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More info at this link (German)
Here is a piece sandstone gathered from the right of the formation.
Notice the minute stratification at a,b & d and especially where it has broken along the line a.
Also worth noticing is the red colouration in the middle of the rock, this is indicative of a high iron content when the sediment was laid.
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This sandstone bed was in fact a shallow sea floor (Westfalen Bay) prior to it being uplifted, the Externsteine formation was pivoted on an axis, subsequent glaciation & weathering has led to create the distinctive exposures we see today
