The Marshall aquifer underlies much of the Lower Peninsula of Michigan and has a maximum thickness of 493 feet (Lampe, 2009). The aquifer consists mainly of medium-grained sandstone and is overlain by Pennsylvanian-age rocks and glacial deposits and underlain by the Devonian-Mississippian-age confining unit. The Marshall aquifer is one of the most productive aquifers in the state where unconfined conditions occur (HA 730-J). This product provides source data for the Marshall aquifer framework, including: Extent shapefiles: 1. p_32MRSHLL.shp: Polygon shapefile containing the areal extent of the Marshall aquifer (32MRSHLL_AqExtent). The extent file contains no aquifer subunits. Point shapefiles: 1. po_32MRSHLL_top.shp: Point dataset containing altitude values, in feet reference to North American Vertical Datum of 1988 (NAVD88), across the top of the Marshall aquifer (Lampe, 2009). These data were used to create the ra_32MRSHLL_top.tif raster dataset. 2. po_32MRSHLL_bot.shp: Point dataset containing altitude values, in feet reference to NAVD88, across the bottom of the Marshall aquifer (Lampe, 2009). These data were used to create the ra_32MRSHLL_bot.tif raster dataset. Altitude raster files: 1. ra_32MRSHLL_top.tif : Altitude raster dataset of the top of the Marshall aquifer. The altitude values are in meters reference to NAVD88 vertical datum. This raster was interpolated from the po_32MRSHLL_top.shp point dataset. 2. ra_32MRSHLL_bot.tif: Altitude raster dataset of the bottom of the Marshall aquifer. The altitude values are in meters reference to NAVD88 vertical datum. This raster was interpolated from the po_32MRSHLL_bot.shp point dataset. Depth raster files: 1. rd_32MRSHLL_top.tif: Depth raster dataset of the top of the Marshall aquifer. The depth values are in meters below land-surface (NED, 100-meter). 2. rd_32MRSHLL_bot.tif: Depth raster dataset of the bottom of the Marshall aquifer. The depth values are in meters below land-surface (NED, 100-meter).