Nate, the bottom layer is a unique layer. From what I can deduce from the layer, it's a dense dried bundled of vegetated layer from the local aquatic weeds that get drifted onto shore. The dried tall weeds, such as cattails and various other tall fescue, died, dried out, and fell onto of the dried aquatic weeds. Over time between late summer to winter, as the water receded, moss, creeping grass, and various other plants grew over them, intertwining. Over the winter, they all died. Spring comes, process begins, and next you know it, barks, dead plants, and continue pile of various other woody materials get embedded into the layer. Thus, this bottom layer. As the water rises, the bed floated up into under the trees. Water receded again. Leaves, grass, and various other materials get piled onto of that layer, and behold, the second layer. Over the season, the first and second layer get built up.
As Robert mentioned, coconut coir is used widely in construction industries to keep the soil from eroding, and made perfect for water impact barrier, as well as streambed embankment reinforcement to add compost onto, and under, to promote vegetation growth. Beside from coconut coir, you can use blankets of hay, or in this case, straw waddle. Cheap, but also comes with a cost of introducing unwanted grass into your pond.
