Episode 5 Recycled rock

There are places in Santa Fe River where the channel has cut down into older deposits of alluvial sediment like the ones in the photograph below:

As you can see in these close ups, the coarse sand and gravel that makes up these beds is identical to the loose sediment in the river bed. 

Cemented sand making up the ledge

Loose sand in the river bed

But there is a big difference. These clastic sediments are no longer loose. They've been consolidated, glued together by tiny films of mineral matter between the grains, and literally turned to stone. Or, as geologists would say, undergone lithification.

This is no longer sediment, but rather sedimentary rock.

In a sense, this is the end of a journey, from solid rock, into sediment, back to solid rock.  Or is it? Look again at this sandstone. It is weathering itself, disintegrating back into sand, and contributing new sediment to the river.

We could summarize this repeating path by drawing the sedimentary rock cycle:

Many sediments, particularly durable quartz sand, show evidence of multiple passes through this cycle.