When is soil too dirty, and what is this?

So I dug down about 3' at my sight and tried another soil shake test...I still can't find a line differentiating the silt from clay (see my jar picture).
Playing with it to make balls and "worms" I noticed a couple of things. No matter how deep I go, I'm going to have a lot of fine roots in the soil mix--I can pick out big stuff, but will little 1/16th inch diameter pieces broken roots matter if left in the cob mix? Straw is organic, right? The next thing I noticed is that this soil is sticky only when there's enough moisture to give it the consistency of fresh play-dough. If it's drier, it will still mold into solid shapes, but it rubs off my hands fairly easily. In the play-dough consistency, it holds finger prints somewhat, and will roll out in a pencil-thin or smaller worm, and usually hang over my finger in a 4-5" strand. I say "usually" because when it does break off, I find that it does so where there's organic or small gravel. The pinch-a-ribbon test is more challenging--I can occassionally get the ribbon to hang almost 3", but more often it breaks, again where there's root and tiny gravel/large sand particles. If I sieve this soil to clean out the trash, will I lose the real-soil condition in testing?

I also packed a couple of 1.5" cylindars (PVC pipe) so I can measure shrinkage. The photos were after the samples sat overnight. The very thin layer on top of the shake test sample is liquid, not a clay layer. My county's soil survey calls this soil "silty loam." Shouldn't I see a line between the silt and clay? Can the silt to clay transition be so evenly graded that one can't see a difference?

Frank;  What part of the country do you live in ?  Clay is usually easily found near / in rivers and creeks / fishing access dirt roads,  many places. Ask your county road crew where a clay bank is, I promise they will know of at least one.  Building contractors excavating foundations will know what part of your county has a good clay layer. If they are working they might just fill your truck up for free!
What are you making cob for ?  If it is a house , then you need to get your mix just right. If it is for the mass on a RMH then not as perfect will do.
I can tell you that with the clay I got (very clean, from a basement excavation) I needed 3x as much sand as I did clay. One pickup load of clay versus 3 loads of sand ! I had no idea when I started. I did learn that if you don't have enough sand where it gets hot it WILL crack.  Straw is added only when you want your cob to be insulated or as a binder on a top coat to help hold it together.

If your hand digging in your yard I HIGHLY recommend finding an easier location...  hopefully with an excavator or backhoe  on site!

I want to build a two-story shop, preferably from materials found on the land. Right now, I'm considering a little 5x5x6 well house just to get experience.

If I give the well house a tall footing and a broad roof, will a lot of moisture enclosed in the small space cause any issues? Being filled with cold water, the pressure tank and pipes do sweat profusely in Missouri humidity, and a well house usually gets closed up and forgotten. It will have a gravel floor to the depth of the rubble trench, so drips won't create puddles. I'd like to also experiment with clay-slip as insulation on the inside, and a lime plaster on the outside.

Hi Frank;
All my experience with cob has been for RMH's  I have not built a building out of cob.
I will say from your description, it sounds like your well house would be fine. But, possibly if the moisture is a problem then adding a dehumidifier would help.  

You will need more than a few days to test the soil.
Watch the square block and see if it cracks a lot as it dries, it it does it is 'expansive clay and it not suitable since it will always crack.
Read up on adobe the issue of soils is covered very well.

time is the key here.
how long does it take for the water to clear? less than an hour= little to no clay.
our tests have shown that after 20 minutes most-all of your sand and silts will have fallen out of suspension. everything after that should be clay.