PATUXENT FORMATION 193 



limestone of Algonkian age, lie 100 feet lower than the base of" the Patux- 

 ent at Towson, immediately southeastward. Moreover, they show a 

 slight northwestward dip. A similar case occurs in the Conshohocken- 

 Rubicam valley, some 12 miles north-northwest of Philadelphia, where 

 the Patuxent beds, which also rest in part on marble, lie but 200 feet 

 above tide. These sands and clays exhibit unmistakable evidence of 

 local disturbance, due possibly to solution of their calcareous substrata. 

 Whether these two cases are to be explained on the basis of sedimenta- 

 tion on an uneven surface, or to the downthrow westward or some other 

 cause, remains to be determined. 



The thickness of the Patuxent formation in central Maryland at the 

 point' where its summit passes beneath tide is about 100 feet. The 

 greatest thickness of the exposed beds in a single section of the Man- 

 kind area occurs at Schoolhouse hill, Baltimore county, where they 

 reach some 60 feet. At this station there is a heavy bed of Patuxent 

 materials beneath, sufficient probably to raise the total thickness to at 

 least 100 feet. Deep well borings to the southward suggest a well marked 

 .thickening of the deposits in that direction, as a glance at the section 

 on the map will show. The well at Saint Elizabeth's Insane Asylum 

 penetrated the Patuxent to a depth of 340 feet, and that of Indian head 

 to a depth of about 353 feet without reaching the base. Allowance must, 

 however, be made for the fact that these borings are at some distance 

 from the margin of the Coastal plain. 



Areal distribution and boundaries. — The Patuxent as the basal formation 

 of the Potomac group occupies a position along the landward margin 

 of the Coastal plain. In Maryland its outcrop begins near Indian head 

 on the Potomac river and follows the river shore, generally beneath 

 Potomac deposits of higher horizons to Anacostia, where it passes be- 

 neath tide. It extends to the westward beneath the city of Washington 

 and continues northeastward in a deeply dissected and often interrupted 

 belt through Laurel, Relay, Baltimore, Havre de Grace, Northeast, and 

 Elkton to the Delaware line. 



It incloses two sorts of Algonkian inliers. One of them, a depressed 

 erosional inlier, due to perforation of the Patuxent terrane by stream 

 erosion, is exposed at many points along the Fall Line zone, notably in 

 the Gunpowder and Laurel quadrangles. The other, a raised original 

 inlier, is exhibited at Grays hill, Mainland, and at Chestnut and Iron 

 hills, Delaware, in the Elkton quadrangle. Though the Patuxtent beds 

 surround these hills, there is no evidence that either this or the superior 

 members of the Potomac ever covered them. 



The Algonkian- Patuxent boundary. — This boundary is on the whole the 

 least difficult of the Potomac lines to trace. The chief difficulties are 



