Cross-bedded
sandstone; Carboniferous
This sample comes from a drill core through coal-bearing
strata. Associated with the coal beds are sandstones
deposited in river systems and deltas. This sandstone shows
the bedding structures that arise when ripples form on the
sediment surface as the sand is deposited. The bedding
planes, outlined by differing amounts of red iron oxide,
curve from ripple crest to trough, and the layers stack up
into small packages called sets. The overall structure is
called cross bedding. Because the currents that produce the
ripples erode sand as well as depositing it, later sets cut
across the ends of earlier ones at an angle. In folded
strata the cut-offs can be very useful, as they allow you to
work out which way up the rocks are.