In the lowest part of the deposit it becomes more clayey, and passes gradually to sandy clay, and eventually to the stiff blue clay called London Clay. Many well-preserved fossils are found in the sandy clay, which proves that the deposit was formed 50 fathoms below the sea-level; while the fossils of the London Clay indicate a much deeper sea.[224] The lowest portion of the sandy clay is known by the appearance of swampy ground, and by the oozing out of the springs, as in Well Walk, in Conduit Fields, and at North End. It is the property of clay to hold up water, and the lower part of the sand, through which it percolates, lying horizontally on the clay, and becoming very full, the water comes out at the edges of the hill, especially at the places indicated. The sandy clay leading down to the London Clay is about 50 feet thick, and from that at Child’s Hill beautiful marine shells, quite perfect, showing that they had neither been rolled nor drifted, were found at a depth of 30 feet in an excavation for drainage in the Finchley Road (Child’s Hill, 1872). Below this comes the stiff London Clay, about 350 feet thick. The chalk at Hampstead Hill is another proof of its submarine formation. This is many feet thick, and is pure carbonate of lime, composed of minute sea-shells, and must have taken an immense period of time to form. There have been found in it hard portions of animals similar to those which now dwell in the sea. So many evidences exist around the British Islands of change of levels, both by elevation and depression, that there is no improbability in supposing that Hampstead Hill has through past ages been gradually raised from below the level of the sea, and at times has been again depressed, which change geologists believe to have taken place more than once, the hill not taking its present form till after several upheavals.