— = 
Geology of Castle Rock.—Lee. 101 
some of the overlying Monument Creek beds. In some cases 
the two cannot be satisfactcrily distinguished unless the dip 
can be determined. The Arapzhoe stands nearly vertical, 
while the highest incknation noted in the undoubted Monument 
Creek beds is 45 degrees. In some cases st ata of intermediate 
dip could not be referred definitely to either formation. The 
thickness of the Arapahoe conglomerate is greater in the Castle 
Rock region than in the Denver bas’n. Near the northern bor- 
der a thickness of 200 feet is exposed. How much thicker it is 
at this point could not be determined. South of Indian creek a 
vertical series which seems to belong to the Arapahoe, is some- 
thing like 500 feet thick. The exposures are numerous enough 
to warrant the supposition that the formation extended unin- 
terruptedly across the Castle Rock region. The only points 
of serious doubt are, first, across the Platte-Arkansas divide, 
near the southern border of the area mapped, where the Monu- 
ment Creek beds cover the older formations, and second, a few 
miles north of Perry park where the Arap:hoe approaches the 
mountains so closely as to be covered, if present, by the crys- 
talline debris. 
13,—Monument Creek. The Monument Creek formation 
was originally described and named by Hayden. It has been 
referred by Emmons and others to the Miocene. It forms one 
of the so-called Tertiary lake deposits. It is composed of con- 
glomerates, breccias, sands and clays which alternate and in- 
termingle and grade into each other in the most lawless man- 
ner. There are beds of coarse conglomerate and breccia with 
no clay ; beds of the same with clay filling the interstices ; beds 
of sand and clay with a few large fragments; beds of pure 
sand and beds of pure clay.. Much of the clay is colored, the 
dull shades predominating. Fire clay occurs in several places. 
Many of the beds have a strikingly massive appearance. Ver- 
tical sections of twenty and thirty feet are common in which 
little evidence of bedding is seen. In many places the beds 
bear evidence of tumultuous deposit. The materials show lit- 
tle evidence of sorting. Coarse and fine; angular and rounded 
are all thrown together in confusion. Cross-bedd’ng is fre- 
quent and the cross-bedded layers themselves truncated and 
crossed. The induration of the beds is as unequal as the distri- 
bution of the materials composing them. In general, the sands 
