Below is a picture of a cobble from a delightful book, Dance of the Continents, by John W. Harrington, 1983. This cobble shows something that is difficult to explain in young-earth creationism. The cobble is from the Old Red Sandstone of Ireland. This cobble shows 4 generations of conglomerate. The oldest is the smallest. Under the arrow you see a dark spot in the whitish part of the rock. That dark area is a conglomerate itself. It contains a bit of sandstone. Inside that dark spot you see a small light colored area. That is an area of sandstone. Those sand grains were eroded and deposited in the darkish sediments when the dark sediment was still soft. Thus we have the erosion of a sandstone as step 1. See 1; red arrow.
The darkish sediment was lithified and eroded and placed into another sedimentary conglomerate. This is step 2 the darkish pebble is placed inside the 3rd conglomerate pebble which is the green arrow.
The sediment marked by the green arrow was then lithified and eroded. That pebble was then deposited in the Devonian Old Red Sandstone, 360 million years ago.
There it sat where it was lithified long ago but eroded sometime within the past 2 million years during the glacial age. That cobble then was deposited in a new conglomerate being formed today along the shores of Ireland (Brandon Bay). Harrington picked the cobble up, saving it from a further cycle of incorporation into a conglomerate. But assuredly there are other cobbles just like it being buried today. Here is the pebble.
This sequence is better than the sequence seen in an Old Red Sandstone cobble I picked up on the North Sea coast at Crawton, Scotland. But my cobble shows time sequence in a way that Harrington's pebble doesn't. Here is my cobble.
One can clearly see the rounded pebbles contained inside my cobble. These rocks are all volcanic and are very very hard. But the rounding shows that these pebbles spent considerable time being bounced along a river bed, being chipped until they were round in shape. My pebble shows the following sequence. 1. various volcanic rocks were deposited somewhere in the river that fed this area. The mineralogies of the various pebbles show that they came from different areas or flows. These rocks were eroded and were bounced along a river for a while. Then they were deposited in the Old Red Sandstone 360 million years ago where they were incorporated into the Old Red. Then sometime (most likely again within the past 2 million years) this pebble was eroded out of the Old Red Sandstone and bounced along a modern river which rounded the conglomeratic cobble. This was then deposited in a modern day conglomeratic deposit in the making.
Harrington's cobble contains pebbles which are angular. That shows that his pebbles spent less time being bounced along a river bottom than the pebbles in my cobble. But both show incredible amounts of time are required to explain the geology seen in simple pebbles.