Moderate to large mushroom, 4 to 12 inches across or even lager, woody, semicircular, and emerging directly from wood, without a stalk.  Top of cap is brown, dull, and bumpy.  Lower surface is shiny white, with tiny pores from which spores are released.  Grows on stumps and fallen logs and helps to decay them, but sometimes parasitic on living trees.  A perennial mushroom, it produces a new layer of pores each year and thus gets gradually larger and thicker.

Common and widespread throughout North America.  Occasional in the Park.  The one pictured was found on fallen tree in the floodplain of Connelly's Run, just north of the South Bridge.

Cracked Cap Polypore (Phellinus rimosus) is similar, but has a darker, cracked upper sufrace, and a brownish pore surface.  It seems to be more common in the Park.

The common name comes from the fact that the white pore surface stains permanently brown when bruised.  Therefore, artists would harvest them, draw on them with a pointed object, and sell the resulting craft.  "Conk" is an old word for any woody, stalkless mushroom growing out of a stump, log or tree.