Farther down beach, sand gave way to thick slabs of gray clay, slippery. Whole veins of it where waves took away sand covering. Geologists say last glacier pushed in clay (and gravel and dirt), creating Valparaiso Moraine: kind of ridge here at southern edge of lake. Glacier = bulldozer, bulldozing moraine, which then acts as dam or divide. North, the great lake; south, water drains into Kankakee floodplain. M. and I stand at ancient, slippery edge.