The Ellicott City Granodiorite is a Silurian or Ordovician granitic pluton in Howard and Baltimore Counties, Maryland.

Mathews continued with a description of their historical importance: The time of opening these quarries dates back probably into the last of the 18th century, but the details are entirely wanting.

The beautiful appearance of some of the more uniformly porphyritic specimens early attracted attention, and in the earliest works which we have on this area, that by Dr. Hayden published in 1811,[6] mention is made of these quarries.

When the Cathedral was constructed during the years 1806 to 1812 and subsequently from 1815 to 1821, the material was hauled from Ellicott City to Baltimore along the old Frederick road in huge wagons drawn by nine yoke of oxen.

After furnishing the rock for this building, which must have been one of the most important stone structures in the United States at the time of its construction, the quarries evidently were worked only to meet local demands.