Muddy hillside spring

Down in a gully I found there was water trickling out of the hillside. Maybe 10 feet below grade and 3 feet above the bottom of the gully. There’s always been a big wet sticky mud pile there with some clear water trickling over it. So I put on the muck boots and started digging. More like slopping this stuff down the hill.

What I found was two clearly defined layers- orange sand/gravel on the bottom, gray clay on top. The water seems to be percolating up through the sand/gravel layer and running out.

This is odd to me since my understanding was that water flows down until it hits something impervious like clay, but here the impervious clay layer is on top.

The more I dug into the hillside, the more clay clumps fell down. The next day the hole had collapsed entirely, but still had a clear trickle of water (maybe a bit more concentrated into one stream now.)

My intention is to head it and measure the flow, maybe someday going into a spring box and whatnot for collection. Since I clearly cannot dig very far back into this hill (since it’s a 10 foot wall of clay and mud above) Should I maybe stack some stone to stabilize the wall and put my spring head in front of that? Maybe cut a plastic drum in half lengthwise to make an arch to stick into the hillside and support the dig?

I just had an idea that may or may not be useful to you.....

It looks like you have clay laden water; if you funnel that clay laden water to a broad, shallow settling tank of stone or hard ground and make a spill over lip on the slightly downhill side, you will end up with the highest grade clay available on your land.

You should be able to make pottery from this clay if you can keep other things from blowing into your settling tank.

An idea that might be useful to you is to get skinny flat rock that is 4" or more wide.
Then make an exit grill that looks like this out of the rock:

Sometimes the water is below the clay.

Why not?  It sounds like a minimum investment of both time and money.