Utter BS, you boob. Ask any geologist if a few dozen feet of ash, clay, and rock will ever look like sedimentary rock.
It seems to me that you may be making assumptions about what was discovered at Mt St Helens.
The fact is that layers of sediment laid down as different types of sediment which was a surprise to scientists. It did not lay down as a huge admixture as one might assume. It separated out as strata.
Here is a paragraph from one of many Internet discussions on St Helens.
Stratified layers up to 400 feet thick formed during the Mt. St. Helens eruption. A deposit more than 25 feet in thickness, and containing upwards of 100 thin layers accumulated in just one day on June 12, 1980. Naturalists have long claimed that stratified layers such as those found in the geological column have accumulated over vast periods of time, and these laminates represent long season variations or annual changes. However, the Mt. St. Helens deposits have demonstrated that catastrophic processes are able to create these geological formations in a short period of time.