Hello, Robert,
Please excuse my delay in responding to your submission. he University I work with has just resumed classes this week.

Greywacke Sandstone Slab
Late Mesozoic Era (252 to 66 million years ago)
Approximate measurements: 48" x 48" x (no depth given); unknown weight.

Greywacke is a weathering feature common in coastal sandstones. The formation of salt crystallization interacting with cyclical weathering pattern and possibly with microbiological assistance, create a characteristic pattern of closely spaced, rounded, concave cavities with very thin walls between them. Grewacke is common along the Northern California coast in surf-facing exposures of sedimentary units of the late Mesozoic Franciscan Formation.

While greywacke stone does not have a valued auction or collectables market; it does have many functional industrial uses. In architecture it is used as: decorative aggregate, building stone, facing stone, garden decoration, and paving stone. Depending on the strength of the example it can also be used in construction aggregate, paving rock for sea walls, and tombstones. A finished/surfaced treatment to the raw example would increase the FMV estimate.