Cover. North Table Mountain, Golden, Colo., looking south. Photograph by Shawn Steigner.
Table Mountain Shoshonite Porphyry Lava Flows and Their Vents, 

Golden, Colorado

By Harald Drewes

Scientific Investigations Report 2006–5242

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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Suggested citation:

Drewes, Harald, 2008, Table Mountain shoshonite porphyry lava flows 

and their vents, Golden, Colorado: U.S. Geological Survey Scientific 

Investigations Report 2006–5242, 28 p.

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Contents iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Previous Work and Methods Used in This Study ............................................................................1

Overview of the Igneous Rocks..........................................................................................................3

Cretaceous and Tertiary Sedimentary Rocks...................................................................................3

Field Observations .........................................................................................................................................4

Ralston Intrusive Bodies......................................................................................................................4

Ralston Plug..................................................................................................................................4

Southeastern Group of Intrusive Rocks...................................................................................5

North Table Mountain Lava Flows .....................................................................................................5

Lava Flow 1....................................................................................................................................5

Lava Flow 2....................................................................................................................................5

Lava Flow 3..................................................................................................................................10

Lava Flow 4..................................................................................................................................10

Human Use of the Lava Rocks.................................................................................................10

South Table Mountain Lava Flows ...................................................................................................11

Problems in Interpreting the Lava Flows ........................................................................................13

Geophysical Data.........................................................................................................................................15

Petrographic Observations ........................................................................................................................19

Textures ................................................................................................................................................19

Minerals................................................................................................................................................20

Mineral Modes....................................................................................................................................21

Petrogenesis of the Lava Flows.................................................................................................................22

Summary of Conclusions............................................................................................................................24

Acknowledgments.......................................................................................................................................25

References Cited .........................................................................................................................................26

Glossary.........................................................................................................................................................27

Figures

1. Location of igneous rocks along western margin of Denver Basin ..................................................2

2. Geologic map of area around Ralston intrusive bodies ......................................................................6

3. Geologic map of North Table Mountain ................................................................................................7

4. Correlation and description of map units for North and South Table Mountains...........................8

5. Photograph of North Table Mountain, looking south...........................................................................9
iv Contents

6. Geologic map of South Table Mountain...............................................................................................12

7. Photograph of South Table Mountain, looking northeast, with North Table Mountain in 

distance........................................................................................................................................14

8. Map and profile of lava flows from source vents to North and South Table Mountains 

and to Green Mountain..............................................................................................................16

9. Diagrammatic cross sections across the Front Range, North and South Table Mountains, 

Green Mountain, and the Denver Basin to the Colorado-Kansas border.........................18

10. Summary histograms of six parameters (mineral modes) used in correlating lava flows 

with intrusive rocks ....................................................................................................................24

11. Amounts of the potassium-rich minerals biotite and sanidine in the intrusive and 

extrusive rocks of the shoshonite porphyry...........................................................................25

Tables

1. Mineral modes of intrusive rocks of the Golden area .......................................................................22

2. Mineral modes of lava flows of North and South Table Mountains................................................23

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Volume

cubic mile (mi3

) 4.168 cubic kilometer (km3) 

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Volume

cubic kilometer (km3

) 0.2399 cubic mile (mi3) 
Table Mountain Shoshonite Porphyry 

Lava Flows and Their Vents, Golden, 

Colorado

By Harald Drewes

Abstract

During early Paleocene time shoshonite porphyry lava was extruded from 

several plugs about 5 km north of Golden, Colo., to form lava flows intercalated in 

the upper part of the Denver Formation. These flows now form the caps of North and 

South Table Mountains. Detailed field and petrographic studies provide insights into 

magma development, linkage between vents and flows, and the history of the lava 

flows.

The magma was derived from a deep (mantle) source, was somewhat turbulent 

on its way up, paused on its way up in a shallow granite-hosted chamber, and near 

the surface followed the steep Golden fault and the thick, weak, steeply dipping 

Upper Cretaceous Pierre Shale. At the surface the lava flowed out of several plug and 

dike vents in a nonexplosive manner, four times during a span of about 1 m.y. 

Potassium-rich material acquired in the shallow chamber produced distinctive textures and mineral associations in the igneous rocks.

Lava flows 1 (the lowest) and 2 are channel deposits derived from the southeastern group of intrusions, and flow 1 (a composite, multiple-tongued flow) lies about 

50 m below the capping flows. Provisionally, the unit termed flow 1 is considered to 

include older, felty-textured flows that are distinguished from a blocky-textured unit, 

flow 1a. Flow 2, newly recognized in this study, lies immediately beneath the capping flows. Lava flows 3 and 4, more voluminous than the earlier ones, were derived 

from a plug vent 1–2 km farther north-northwest and flowed south-southeast across a 

broad alluvial plain. This plug is a composite body; the rim phase fed flow 3, and the 

core phase was the source of flow 4. During the time between the effusion of the four 

flows, the composition of the shoshonite porphyry magma changed subtly; the later 

flows contain more alkali, as shown by higher proportions of sanidine.

On North Table Mountain, lava flows 3 and 4 form an elongate tumulus above 

a stream channel that carried water at the time of their eruption. On South Table 

Mountain, lava flow 3 forms a low, broad dome that forced flow 4 into channels now 

restricted to the west and northeast flanks of that mesa.

Mesa-capping lava flows 3 and 4 are broken by many small normal faults and 

are warped into open synclines, probably in response to local stresses associated with 

the settling of piedmont deposits into the Denver Basin. Mid-Tertiary deposits are 

inferred to have covered the upper part of the Denver Formation and its lavas; these 

deposits could thus have been instrumental in changing the stream flow direction to 

the east before the onset of Neogene uplift and consequent canyon cutting across the 

flows. Other younger deposits may also have covered the area, to be linked to this 

consequent canyon cutting.

Introduction

During the early Paleocene the area of Golden, Colo., lay at the foot of the 

Laramide Rocky Mountains, much as the town now is at the front of the present 

Rocky Mountains. However, some geologic conditions changed between that time 

and the present. During the earlier time of mountain building, uplift was accompanied by tilting and faulting of the sedimentary sequence. Not only were there 

abundant andesitic eruptions to the west, but a shoshonite porphyry volcanic system 

developed locally. The present uplift has been unaccompanied by volcanism, and 

reactivation of the Golden fault is suggested by only very limited evidence. 

The present study focuses on the volcanic rocks and their vents (fig. 1); the 

lava flows are dated as 64–63 Ma (mega-annum, or millions of years). The magma 

reached the surface along the Golden fault and flowed south-southeastward along 

the paleo-drainage system. Although a considerable body of knowledge about these 

rocks has already accumulated, most past studies were aimed at stratigraphy, surficial 

geology, and zeolite mineralogy. Thus, the present study—based on more detailed 

mapping, structural geology emphasis, and petrographic considerations—permits 

new interpretations and raises new questions.

Previous Work and Methods Used in This Study

Earlier studies, such as the work of Johnson (1925), explained aspects of the 

structure of the Golden area, and the regional concepts of Drewes (1991a,b) suggest 
2 Table Mountain Lava Flows, Colorado

× 

40

6

70

70

74

26

C-470

93

72

6

70

70

Morrison

Lakewood

Alameda Pkwy 

Colfax Ave To Denver

1.5 mi

Wadsworth Ave 

Kipling Ave 

Wheat Ridge

Arvada

Rocky Flats

Dakota

Ridge

(Hogback)

W. 58th St 

Morrison Rd

6855

7881

Mount Morrison

Coal Creek

Ralston Creek

Leyden 

Creek

Van

Bibber Creek

Bear Creek

Mount Vernon Creek

ApexGulch

Dakota (Dinosaur)

Ridge Red Rocks

Park

Lena Gulch

Golden

Clear Creek Canyon

Go dl en

Gate

Canyon

(FLOW)

Jefferson Co. Fairgrounds

Indiana St 

W. 64th Ave 

Bear Creek Canyon

Green

Mountain

Conglomerate

NORTH TABLEMOUNTAIN(FLOWS)

SOUTH TABLEMOUNTAIN(FLOWS)

Precambrianquartzite

RALSTONPLUG

Mountain front

0

0

3

4

MILES

KILOMETERS

Figure 1. Location of igneous rocks (patterned areas) along western margin of Denver Basin near Golden, Colo. 
that the effects of the Laramide orogeny might be viewed as a young foreland phase 

of a more regional Cordilleran orogeny. A more regional interpretation emphasizes 

an east-west–oriented compressional stress field in which the local steep structures, 

such as the Golden fault, may be shallow-level “sled-runner” structures or even 

backthrusts. 

The initial effort of the present study was the preparation of 1:6,000 to 1:12,000 

geologic maps of key areas. Base maps were prepared from the Golden and Morrison 

71⁄2-minute U.S. Geological Survey topographic maps of 1965, which were also the 

base maps used by Van Horn (1972) and Scott (1976). Their geologic maps provided 

a valuable foundation to my initial effort. Rock samples were collected in the mapping effort of 2001–03.

Petrographic study followed the mapping and was based on 48 thin sections of 

selected rocks. These sections were cut and some also stained for potassium feldspar by the Western Petrographic Company of Provo, Utah. The thin sections were 

studied during 2003–04.

Structural features encountered in the Cretaceous and Tertiary sedimentary 

formations were recorded during the present study. The stratigraphic units associated 

with the lava flows are the Cretaceous Pierre Shale and the Cretaceous and Tertiary 

Denver Formation. The stratigraphy and sedimentology of these units were not studied for this report; however, brief descriptions of them are included. For further data 

on stratigraphy see Van Horn (1957, 1972), Scott (1976), and Weimer (1996), and for 

details on the Paleocene Green Mountain Conglomerate see Drewes and Townrow 

(1999). Older works of note include Johnson (1925), Van Tuyl and others (1938), and 

Waage (1953), and those of historic interest include LeConte (1868), Hayden (1873), 

and Emmons and others (1896).

Overview of the Igneous Rocks

The igneous rocks are unquestionably closely related; intrusive and extrusive 

rocks alike are shoshonite porphyry (a potassium-rich basalt) having small phenocrysts of plagioclase, pyroxene, and olivine. The lava flows are dated as 64.2±1.1 

to 63±1.7 Ma, as reported by Obradovich (2002). Intrusive bodies north-northwest 

of the Table Mountain lava flows consist of one large composite plug, now being 

quarried, and a cluster of smaller dikes, sills, and another plug. Known as the Ralston 

intrusive bodies or the Ralston dike or plug, these are hosted in the Pierre Shale.

About 2 km south-southeast of the nearest small intrusive body (a small plug) is 

North Table Mountain, on which four levels of lava flows are recognized. The flows 

are numbered sequentially upward. Small flows at the same stratigraphic level are 

given one number, 1, except for one with a unique texture here designated 1a. Flow 

1 is considered in this study to be a composite, multiple-tongued flow, and where the 

tongues can be distinguished, they have been mapped as flow 1 or flow 1a. Lava flow 

2 is newly recognized as a distinct flow in this study; previously it had been assigned 

to flow 3 (or to unit Tv2 of Van Horn, 1972). Flows 1 and 2 are channel flows; flow 1 

occupied three or four channels and flow 2 a single channel. Flow 1 lies about 50 m 

below flow 3 (with Denver Formation intervening), whereas flow 2 has little or no 

sedimentary material separating it from flow 3. The mesa-capping flows are 3 and 4. 

On North Table Mountain these flows are separated from each other by small lenses 

of conglomerate and sandstone.

The kilometer-wide Clear Creek Canyon separates North from South Table 

Mountain. Only lava flows 3 and 4 reached South Table Mountain. No intervening 

sedimentary lenses have been found between these flows on South Table Mountain. 

However, there may be remnants of downfaulted sedimentary rocks overlying flow 

4 at two sites, described subsequently. Furthermore, only a small volume of flow 4 

reached South Table Mountain, where it filled two local channels flanking a domal 

core on flow 3.

Lava flow 3 reached the north flank of Green Mountain about 2 km south of 

South Table Mountain. The intermittent stream in Lena Gulch now flows through the 

broad, shallow gap. Until about 1990 there was a single outcrop of flow 3 accessible 

just above (south of) the Jefferson County Fairgrounds, but that site was then “suburbanized” (that is, scraped off and covered).

Cretaceous and Tertiary Sedimentary Rocks

Among the host rocks pertinent to this investigation are the Upper Cretaceous 

Pierre Shale and the Cretaceous and Tertiary Denver Formation. The Pierre Shale—a 

medium-gray clay shale or mudstone and some silty shale—is about 1,880 m thick. It 

generally does not weather out, even as flakes. Where the Pierre is naturally exposed, 

it generally appears as layered mud, but it typically is thickly covered by colluvium 

or by terrace deposits. Inevitably, exposures of Pierre Shale in this area are the result 

of excavations. In the area of the Ralston intrusive bodies, which is also disturbed 

by the north-trending Golden fault, the vertical to steep eastward dips of the Pierre 

reflect the up-arching of the whole sedimentary sequence over the Rocky Mountains. 

Locally, however, the shale dips anomalously westward. Fossils of pelecypods and 

orthocerids (cephalopods) were found at only two real outcrops; yet Van Horn (1972) 

reported fossils in sufficient abundance and sequential order so as to infer structure 

from them, although the fossils come mainly from colluvial areas that are heavily 

grass covered. No effort was made to verify Van Horn’s recorded observations; his 

structural interpretations therefore are simply accepted in the present study.

The upper part of the Denver Formation, in which the lava flows are intercalated, is poorly consolidated, pale-yellowish-brown to light-yellowish-gray or, rarely, 

Introduction 3
4 Table Mountain Lava Flows, Colorado

nearly white siltstone, sandstone, and some conglomerate. Clast sorting is poor to 

moderate, and clasts are subrounded. Stream-type crossbedding, channel deposits, 

and mudflow beds are common. Most clasts are of andesitic rock; these include 

various porphyries, aphanitic rock, and some vesicular rock. At the southeast end of 

South Table Mountain, andesitic scoria is present in a bed about 50 m below flow 

3. Conspicuous by their absence are shoshonite ejecta. Biotite from these clasts was 

dated by the K-Ar method, using modern decay constants, as 65.8±1.4 Ma 

(Evernden and others, 1964; Obradovich and Cobban, 1975). Although the cited 

authors discussed the relationship of this bed to the nearby Cretaceous-Tertiary 

boundary, I suspect their date simply supplies the age of the volcanic source rocks of 

the Denver Formation, a source that lay perhaps a few tens of kilometers to the northwest. The date thus has only an indirect bearing on the age of the overlying lava flow. 

The dark crystals within the clasts of the Denver Formation are amphibole 

(those in the dated scoria were biotite, however). In only a few places were clasts of 

shoshonite porphyry found; either these places were at the level of, and downstream 

from, the distal end of a part of flow 1, or they were near lava flows 3 or 4.

Fossil wood fragments are common in the Denver Formation, but most are surface finds and possibly lag concentrates. Most of this wood is medium gray to nearly 

white and not very siliceous. Charcoal chips are abundant in a 20-cm-thick mudflow 

bed (within the Denver Formation at site P in fig. 6) that is 3 m below flow 3 on the 

west flank of South Table Mountain.

All signs indicate that the Pierre Shale was a marine deposit and that the Denver 

Formation was a fluvial channel deposit. To judge by the thickness of the mud deposited by the Pierre sea, its floor must have been dropping slowly but steadily. At the 

end of Pierre time, at about 80 Ma, the region started to rise, and by Denver Formation time a moderately high area lay to the west. Streams from this upland drained 

east-southeastward as seen on Green Mountain (Drewes and Townrow, 1999) and as 

will be shown in this study.

Field Observations

Ralston Intrusive Bodies

The Ralston shoshonite porphyry intrusive bodies comprise one large composite 

plug to the northwest and a small plug plus many small dikes and sills to the southeast. The large plug has previously been referred to as the “Ralston dike,” but its 

oval shape (2 km long and 1 km wide) suggests the term “plug.” Perhaps the poorly 

exposed center of the intrusive body gave the impression that only the narrow eastern 

and western ridges were dikes (fig. 2). In this plug a core phase and a rim phase are 

recognized.

Ralston Plug

At the time of this study quarrying was in progress in the rim-phase rock of the 

western higher ridge, in which seven levels have been cut. Although this quarry provided excellent fresh outcrops, the sample sites have already vanished or will soon. 

Smaller inactive quarries and tunnel entrances offer other sites of fresh rock. On the 

other hand, artificial fill covers some parts of the plug.

The topography of the northwestern plug is instructive; its edges are high, 

whereas its elongate core forms a basin. With the aid of some small earthen dams, 

much of this core area has been turned into a reservoir, Upper Long Lake. The 

remainder of the basin is largely covered by colluvium. Tunnels through the eastern 

and western ridges bring water into the lake and then out of it to a lower reservoir, 

Lower Long Lake.

In projecting the Golden fault along the north-northwest–trending axis of the 

Ralston plug, I concur with the mapping of Van Horn (1972). Other branches of that 

fault are based on fossil distribution (Van Horn) and on structural anomalies (this 

study).

The rim phase is very hard, medium-gray to dark-gray rock that is cut by widely 

spaced, steep, east-northeast–trending joints. This trend is approximately normal to 

the axis of the plug. Its outer contact with Pierre Shale dips inward, more steeply 

west of the western ridge and more gently east of the eastern ridge. The net effect is 

that the plug has the shape of an upright, elongate, eastward-tilted funnel. Of course, 

magma did not go down the funnel, but rather as it rose the confining pressure 

decreased more rapidly on the east side than the west side of the plug. Trace amounts 

of pyrite in veinlets and tiny pockets were seen in two places near the outer contact 

on the west and north sides of the plug.

The core phase resembles that of the rim phase in many ways; yet it is subtly 

different. It weathers browner and is cut by closely spaced joints that mostly dip 

gently inward. This difference in jointing and weathering intensity explains the 

contrasting topographic habits of plug rim and core. The contact between rim and 

core rocks is chiefly covered but in the quarry is gradational across tens of meters. 

This observation suggests that the rim-phase rocks were still hot when the core-phase 

magma came through; the arrivals of the two magma phases were separated only by 

a small span of time.

The Ralston plug is surrounded by an altered, or “baked,” zone of Pierre Shale. 

Along the western ridge, where the baked zone is well exposed by quarry operations, 

it is 10–15 m wide. This width extends around the north end of the plug, but it thins 

slightly along the eastern ridge and is concealed to the south. In the baked zone the 

mudstone is sufficiently hardened to weather into flakes or even plates. Its color is 

also changed from gray to pale yellowish brown. Although the core-phase rock near 

the outer contact becomes finer grained, it is not vitreous.
Several small dikes crop out northwest of the Ralston plug (Van Horn, 1972) but 

are now beneath the water of the Ralston Reservoir or beyond the area of figure 2. 

They appear to lie closer to the projected western branch of the Golden fault than to 

the main fault.

Southeastern Group of Intrusive Rocks

The southeastern group of intrusive rocks lies mainly between the quarry access 

road to the north and Van Bibber Creek to the south (fig. 2). Most of these elongate 

bodies are a few tens to a few hundreds of meters in length and seem randomly 

scattered. Several western bodies are roughly aligned, although in detail they are en 

echelon and are steeply inclined, perhaps near a fault. Certain features of the eastern 

intrusions suggest that they have gently inclined contacts; the small bodies seem to 

curve around the topography like sills, whereas the small plug just north of the creek 

has an exposed contact dipping 5°–30° W. This small plug is about 0.5 km in diameter and is generally subcircular in plan but with some prongs heading northward. 

The gently westward dipping features along these eastern bodies again suggest that 

the rising magma expanded more freely eastward.

The recurrence of this inferred eastward expansion of rising magma at both 

plugs may support Van Horn’s (1972) interpretation that the mass of Pierre Shale east 

of Highway 93 is a thrust flap in the area of the intrusive rocks, rather than a Quaternary landslide. Such eastward expansion at least permits Van Horn’s thrust-flap 

interpretation, whereas the preservation of faunal zones requires it.

Probably few of the southeastern group intrusive bodies reached the Paleocene 

surface. Those most likely to have done so are the small plug and maybe one of the 

western dikes, on the basis of their size. It is noteworthy that none of these bodies 

has surrounding baked zones. Apparently, insufficient magma passed up these vents 

to provide the necessary heat to bake the wall rock.

North Table Mountain Lava Flows

Shoshonite porphyry lava flows crop out at four levels on North Table Mountain. The distribution of the flows on North Table Mountain is shown in figure 3. The 

lower two levels are channel flows; the upper two flows spread across a broad southsoutheast–sloping valley plain. Lava flow 2 (site A in fig. 3) is newly identified in 

this study; previously it had been mapped as part of flow 3 (or of Tv2 of Van Horn, 

1972), the lower of the mesa-capping flows. Many of these features are shown in the 

photograph of North Table Mountain (fig. 5).

To judge by the relative thickness of sedimentary beds between the flows, 

considerable time elapsed between extrusion of flows 1 and 2 but only a little time 

separated extrusion of the other flows. The dates on flows 1 and 4 are only about 

1 m.y. apart; thus the time between extrusion of flows 1 and 2 may have been as 

much as 0.95 m.y., and the spread between the extrusion of the others would then 

be measurable in tens of thousands of years at most. This statement is speculative, 

however, because the margin of error for the dates of the flows is such that their date 

ranges overlap and are therefore statistically identical.

The lava-flow foliation on North Table Mountain generally dips 3°–5° SE. 

However, at the highest part of the mesa along its northeastern edge, dips are 

5°–7° SW., thereby suggesting an open, southeast-plunging syncline. Additionally, 

flows 3 and 4 are cut by about 25 small normal faults, some of which control the 

predominant, southeast-flowing drainage on the mesa top. Offsets range from 1 m 

to a few tens of meters. Likely, both the fold and faults were formed as the forelandbasin deposits settled in the Denver Basin, thereby generating local stresses.

Some modifications have been made to the North Table Mountain mapping 

of Van Horn (1972). The small lens of lava flow 1 on the northwest flank of the 

mesa (fig. 3, near site B) has been extended southwestward to the gully just east of 

flow 2 (fig. 3, near site C). Also, only three, not four, lenses of flow 1 crop out on 

the south flank of North Table Mountain; the westernmost one as previously shown 

(fig. 3, site D) is thought to be a patch of slump material derived from overlying 

flows. Additionally, the contact between flows 3 and 4 is considered to have been 

eroded back from the mesa edge between the northern end of the mesa and the 

communications building (fig. 3, site E). This interpretation is based on tracing the 

top of the red, oxidized, amygdaloidal upper zone of flow 3 along the base of a low 

cuesta. It is this mapping that led to the recognition of flow 2 (unit Tf2) at site A 

(fig. 3).

Lava Flow 1

Lava flows 1 and 1a form several small lenses around the flanks of North Table 

Mountain, where they occupy three or four shallow channels about 50 m beneath the 

mesa-capping flows. These flows are at most 15 m thick and are weakly columnar 

jointed. All are heavily oxidized, and most are amygdaloidal. Their source is a small 

plug northwest of the intersection of Highway 93 and 58th Street. 

Lava Flow 2

Lava flow 2 appears only at site A (fig. 3), where it has a thick eastern part and 

a thin western one, about 15 and 9 m thick, respectively. The change in thickness 

takes place abruptly along a steep contact that is not a fault; rather, it appears to be 

Field Observations 5
6 Table Mountain Lava Flows, Colorado

Figure 2. Geologic map of area around 

Ralston intrusive bodies,

5–8 km north of Golden, Colo. Sites 1–15 

provided samples that were point counted. 

Base from U.S. Geological Survey, 1:24,000, 

Golden, 1965 (photorevision as of 1980).

Qa Qaf Qc Qls

Kpa

Kfh

Qao

CORRELATION OF MAP UNITS IN AREA

AROUND RALSTON INTRUSIVE BODIES

Qb

Ql

Qs

Qv

Qrf

Kl

Kn

Kb

Kd Kds

Kdl

TKd

Tsc

Tsr

Tss

Ka

Kf

Kp

Younger alluvium (Quaternary)

Artificial fill (Quaternary)

Colluvium (Quaternary)

Landslide deposits (Quaternary)

Older alluvium (Quaternary)

Terrace deposits (Quaternary)

Broadway Alluvium

Louviers Alluvium

Slocum Alluvium

Verdos Alluvium—Queried where scraped off for 

construction purposes

Rocky Flats Alluvium

Denver Formation (Paleocene and Upper Cretaceous)

Shoshonite porphyry (Paleocene)

Core phase

Rim phase

Southeastern group

Arapahoe Formation (Upper Cretaceous)

Laramie Formation (Upper Cretaceous)

Fox Hills Formation (Upper Cretaceous)

Pierre Shale (Upper Cretaceous)

Altered zone

Niobrara Formation (Upper Cretaceous)

Fort Hays Limestone Member

Benton Shale (Upper Cretaceous)

Dakota Group (Lower Cretaceous)

South Platte Formation

Lytle Formation

Morrison Formation (Upper Jurassic)

Older rocks (Triassic to Precambrian)

LIST OF MAP UNITS IN AREA

AROUND RALSTON INTRUSIVE BODIES

QUATERNARY

TERTIARY

CRETACEOUS

JURASSIC

TRIASSIC TO

PRECAMBRIAN

Paleocene

Upper

Cretaceous

Lower

Cretaceous

Upper

Jurassic

pe-do

Jm

Qa

Qc

Qb

Qs

Qaf

Qlo

TKd

Qls

Qao

Qv

Qrf

Ka

Kl

Kf

Kp

Kpa

Kn

Kfh

Kb

Kd

Kds

Kdl

Tss

Tsr

Tsc

10 85 

Strike and dip of beds

Inclined

Vertical

Overturned

Sample locality—

Showing sample 

number

12

10

65

Jm

pe-do

Contact—Showing dip. Dotted where concealed

Fault—Showing dip and lineation (half arrow);

bar and ball on downthrown side where known. 

Dotted where concealed

Concealed thrust fault—Sawteeth on upper plate

Quarry wall—Showing levels. Hachures point 

toward lower level

Trail

N 39°

50’

105°15’

39°

47’

30”

0

0

1 MILE

0.5 1 KILOMETER

1/2

75 

75 

45 

35 

75 

80 

63 

61 65

59 

58 

58 

10 

85 

60 

65 

85 

15 

30 

5 

45 

60 

60 

35 90 

80

70

70

70

80

85

35

70 60

60 72

42

80

20

18

80

70

70

60

60

60

62

50

60

65

45

1

5

4

10

3

6

7

9

11

12

13

14

15

8

2

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qls

Qls

Qls

Qls

Qao

Qao

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

pe-do

Qrf Qrf

Qrf

Qrf Qrf

Qrf

TKd

Tsc

Tsc

Tsc

Tsc

Tsc

Tsc

Tsc

Tsc

Tsc

Tsr

Tsr

Tss

Tss Tss

Tss Tss

Tss

Tss

Tss

Tss

Tss

Ka

Ka

Ka

Kl Kl

Kf

Kf

Kp

Kp

Kp Kp

Kp

Kp

Kp

Kp

Kp

Kp

Kp

Kp

Kp

Kp

Kp

Kp

Kp

Kpa

Kpa

Kpa

Kpa

Kfh

Kfh

Kfh

Kb

Kb

Kb

Kds

Kds

Kdl

Kdl

Jm

Jm

Qb

Qb

Qb

Qb

Qb

Qb

Qb

Qb

Qs

Qs

Qs?

Qs

Qv

Qv

Qv

Qv Qv Qv

Qv

Qv

Qv

Qv

Qv?

Qlo

QUARRY

PLANT

Tsr
Field Observations 7

Figure 3. Geologic map of North Table Mountain, 1–4 km 

northeast of Golden, Colo. Mapped at scale of 1:12,000 and here 

shown slightly reduced. Sites 16–23 and 25–27 provided samples 

that were point counted. Sites A–H are referred to in text: A, 

flow 2; B and C, flow 1 lens mapped by Van Horn (1972) and now 

extended to the southwest; D, previously interpreted flow 1 lens 

now seen as a patch of slump material derived from overlying 

flows; E, communications building; F–H, line of tumuli on flows 

3 and 4; F, possible southern extension of subsurface channel 

containing flow 2; G, channel flow (Van Horn, 1972); H, low dome 

in flow foliation. See figure 4 for correlation and description of 

map units. Base from U.S. Geological Survey, 1:24,000, Golden, 

1965 (photorevision as of 1980).

B

C

D

F E

G

H

Qa

Qa

Qa

Qa

Qc

Qc

Qc

Qc Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qc

Qr

Qly

Qly

Qly

Qly

Qly

Qly

Qly

Qly

Qly

Qly

Qly

Qly

Qly

Qly

Qly

Qaf

Qaf

Qaf

Qlo

Qlo

Qlo

Tf1

Tf1

Tf1a

Tf1

Tf1

Tf1

Tf1

Tf1

Tf2

Tf4

Tf4

Tf4

Tf4

Tf4

Tf4

Tf4

Tf4

Tf4

Tf3

Tf3

TKd?

TKd

TKd

TKd

TKd

TKd

TKd

TKd

TKd

TKd

TKd

Qr

Tf3 Tf3

TKd

Tf3

Tf3

Tf3

Quarry

Quarry

Quarry

Develope

Developed

d

Developed

Developed

TKd

P

P

2

2

2

2

2

2

3

3

5

3

3

7

3

2

2

2

2

8

10

2

2

8

25

15

12

3

3

3

2

2

2

3

3 2

2

2

2

3

4

4

3

3

3

3

3

5

5 2

3

5

2

6

6

6

2

3

3

3

3

3

3

2

10

10 10

5

5

10

3

2

5 5

3

3 3

5

7

2

3

10

15

10

5

A

Dam

0

0

1 MILE

0.5 1 KILOMETER

1/2

Map unit—Qa, younger alluvium; Qc, colluvium; Qr, rock-fall

deposits; Qly, younger landslide deposits; Qaf, artificial fill; 

Qao, older alluvium; Qlo, older landslide deposits; Tf1 – 4 and 

Tf1a, lava flows 1–4 and 1a; TKd, Denver Formation.

See figure 4 for correlation and description of map units

Contact—Dashed where inferred or gradational; dotted where

concealed

Supplemental horizon—Base of amygdaloidal zone of lava flow 3. 

Dotted where concealed

Fold—Showing dip of limbs. Dotted where concealed

Anticline

Syncline

Trace of inferred stream channel beneath flow 3 (unit Tf3)

Landslide scarp headwall

Fault—Bar and ball on downthrown side where known.

Dashed where approximately located; dotted where

concealed

Trail or jeep track, supplements that on base map

Strike and dip of inclined beds

Strike and dip of inclined flow foliation

Quarry—Hachures point toward quarried area

Sample locality—Showing sample number as referenced in text

Site described in text

Public parking area

Building not shown on base map

17

10

Tf1

EXPLANATION

A

P

2

25

21

16

20

22

26

18

27 19

23

17

N

105°12’30”

39°

47’

30”
8 Table Mountain Lava Flows, Colorado

Qao

Qa Qc Qr Qly Qaf

Qlo

Qlo

TKd

TKd

TKd

Ka

Tf4

Tf2

Tf1 Tf1a

Qao

CORRELATION OF MAP UNITS

QUATERNARY

TERTIARY

CRETACEOUS

Paleocene

Upper Cretaceous

Unconformity

Tf3

Artificial fill (Quaternary)—Typically road fill and quarry dumps

Older alluvium (Quaternary)—Caps a small knoll southeast of 

 South Table Mountain; may be Rocky Flats Alluvium

Older landslide deposits (Quaternary)—Probably inactive

Denver Formation (Paleocene and Upper Cretaceous)—Mainly 

 pale-yellowish-brown volcaniclastic siltstone, sandstone, grit, 

 and conglomerate; some cobble conglomerate and mudflow 

 deposits. Poorly sorted and crossbedded; mainly derived from 

 andesites, locally also of shoshonite of Tf1–4 and 

 rarely of older rocks; contains chips of petrified wood and 

 rarely of charcoal. Lava flows intercalated at four levels

Lava flow 4 (Paleocene)—Shoshonite porphyry, augite, plagioclase, 

 and some olivine and orthoclase phenocrysts. Has an upper 

 amygdaloidal zone, largely eroded, columnar joints, and flow 

 foliation

Lava flow 3 (Paleocene)—Shoshonite porphyry; columnar jointed 

 and flow foliated. Zone at top of flow contains zeolite amygdules; 

 weak weathering; uppermost red, oxidized scoria zone 1–2 m 

 thick; locally, scoria blocks are pressed into base of unit Tf4; 

 elsewhere, thin conglomerate lenses of unit TKd are present 

 between units Tf3 and Tf4

Lava flow 2 (Paleocene)—Shoshonite porphyry containing abundant

 zeolite amygdules, pods, and veins; 10 m beneath lava flow 3; 

 weakly columnar jointed

Lava flow 1 (Paleocene)—Shoshonite porphyry; mainly felty texture; 

 several lenses on flanks of North Table Mountain about 35 m 

 beneath lava flow 3; weakly columnar jointed

Arapahoe Formation (Upper Cretaceous)—Nearly white quartzose

 sandstone and pebble conglomerate

Tf4

Tf3

Tf2

Tf1

Tf1a

Ka

DESCRIPTION OF MAP UNITS FOR

NORTH AND SOUTH TABLE MOUNTAINS 

Younger alluvium (Quaternary)—Of undesignated terraces

Colluvium (Quaternary)—Mainly 1–2 m thick on mesas; 

 may be thicker on mesa flanks

Rock-fall deposits (Quaternary)

Younger landslide deposits (Quaternary)

Qa

Qc

Qr

Qly

Qaf

Lava flow 1a—Low-level lava flow having mainly blocky texture

Figure 4. Correlation and description of map units for North (fig. 3) and South (fig. 6) Table Mountains, Golden, Colo. Radiometric ages: 

Unit Tf4, 63.5±1.2 Ma; unit Tf3, 64.2±1.1 Ma; unit Tf1, 63.0±1.7 Ma. These ages are as reported by Obradovich (2002); they are not in normal 

stratigraphic sequence but are correct within the stated margin of error.
GOLDEN FAULT

GOLDEN FAULT

GOLDEN FAULT

H G

FLOWS 3 AND 4

FLOW 1

Road

FLOW 4

FLOW 3

FLOW 2

FLOW 1

FLOW 3 FLOW 4

E

F

NORTH TABLE MOUNTAIN

SOUTH TABLE MOUNTAIN

Figure 5. North Table Mountain, Golden, Colo., looking south. E, communications building; F–H, line of tumuli. Photograph by Shawn Steigner.

Field Observations 9
10 Table Mountain Lava Flows, Colorado

the edge of a deep channel or gully, with a terrace on its west side. The flow filled 

the channel to a level above that of the terrace. Flow 2 shows strong zeolitization; the 

zeolites and calcite not only filled vesicles but also larger cavities and veinlets. This 

strong zeolitization may have been facilitated by water in the channel that the lava 

flow followed.

The top of lava flow 2 underlies a small bench between the cliffs formed by the 

lower parts of flows 2 and 3. Stronger alteration and greater vesiculation than found 

in the lower parts of flows 2 and 3 have made this bench-forming zone weak. The 

zeolitization and alteration end abruptly upward at the base of flow 3, thereby marking 

a disconformity. The colluvial cover on this bench contains some rounded fragments of 

shoshonite; these may be pebbles, but the rounded fragments could alternatively have 

been weathered round from the underlying rock. In figure 3 a thin sheet of sedimentary 

rocks (unit TKd) is shown to lie between flows 2 and 3 to highlight the presence of a 

disconformity there.

At the cliff outcrop (fig. 3, site A), the channel that contains lava flow 2 is seen 

to head only southward, but as described subsequently I suggest that it trends southwestward toward the communications building (figs. 3 and 5, site E) and onward to 

site F (figs. 3 and 5), where capping flows 3 and 4 provide some features that support 

the presence of the channel and its contained lava flow 2.

Lava Flow 3

Lava flow 3 is the most extensive one on North Table Mountain. It is about 24 m 

thick to the northeast and thins slightly to the south. The flow base is mostly covered 

and, where exposed, directly overlies the typical sandstone or siltstone beds of the 

Denver Formation, altering them but little. The main part of flow 3 is strongly columnar jointed. Fresh rock is dark gray, and weathered surfaces are brownish gray. The 

top 6–9 m of the flow is strongly oxidized and amygdaloidal and underlies a narrow 

bench between two cliffs. This amygdaloidal zone is absent from the southeastern 

part of South Table Mountain, again owing to erosion. However, in this rather thin 

area of flow 3, the entire flow is slightly amygdaloidal.

Lava Flow 4

Lava flow 4 closely resembles the lower part of flow 3. Flow 4 has no upper 

amygdaloidal or vesicular zone, but one was probably originally present and has 

been removed through more extensive erosion than the underlying, protected flows 

have been subject to. Some of the original lateral extent of flow 4 has also been 

reduced by erosion, as described at the beginning of this section. Flow 4 is strongly 

columnar jointed, and on the top of the mesa the many bare rock surfaces are laced 

with the hexagonal fracture system of these columns. The attitude of these bare rock 

surfaces of the mesa top is controlled by a flow foliation, which provides useful 

structural information in several places, as described next. Also seen in places on the 

mesa surface is a knobby weathering product on a centimeter scale; the 

weathered-out spherules, like so many oversized rabbit pellets, may be spherulites, 

which represent an incipient crystallization of glassy (or near-glassy?) material.

At site F (figs. 3 and 5) is the north end of a 1-km-long, low, arcuate (concave 

to the east) ridge in flow 4. The flow foliation along this ridge shows it to be an anticline whose flanks dip mostly about 10° but locally as much as 25°. Half a kilometer 

southeast of the end of the ridge (figs. 3 and 5, site H) is a low dome whose flanks 

dip 5°.

The ridge and dome are tumuli formed in the lava when it was already marked 

by the flow foliation but not quite cooled and hardened. The process was driven 

by pressure from steam that was generated when the flows heated the water in an 

underlying channel. This pressure was sufficient to bow up the covering flows, yet 

not enough to explode, as at a maar. As the steam pressure decreased, lava filled in 

any voids. The incorporated water may have allowed the development of the flow’s 

exceptionally abundant zeolites. The arcuate trend and narrowness of the ridge 

suggest that the water was present in a drainage, and the ridge’s position suggests 

that the drainage was the same one used by flow 2. Flow 2 probably ended where 

the north end of the arcuate ridge begins; the water along the lower reaches of the 

drainage remained to be turned into steam, except perhaps along the gap between the 

tumulus ridge and the tumulus dome.

Human Use of the Lava Rocks

The ridge surface is marked by scores of topographic depressions, half circles in 

plan view and surrounded by cusplike edges. The depressions are about 10 m across 

on the downhill side and are rounded uphill; they have a cut depth of 1–1.5 m. I suggest that these features are miniature quarries from which rock slabs were obtained, 

though to date no historical records have been found to support this interpretation. 

The up-arched flow foliation of the arcuate ridge would have made it possible for 

workers to pry up slabs for use in house or foundation construction, probably in 

Golden during its pioneer days. The size restraints may have been determined by 

a lease unit, reflecting the common need at that time for, say, the walls of a single 

foundation. The uphill rounding and depth control were determined by how deeply 

one could pry up slabs by using muscle power and pry bar. Because prying up the 

slabs would have been difficult in corners, a rounded headwall inevitably remained. 

Support for the inference that these depressions are not natural features comes 

from additional field observations. For one thing, a few stockpiles of the quarried 

blocks survive; however, most of the material was removed from the small quarries 
and nothing was constructed on the mountain. Furthermore, lichen abundance, size, 

and variety on the walls of the miniature quarries are reduced compared to those 

lichen characteristics on nearby unquarried rock surfaces. This contrast is greater 

on the south-facing quarry walls than on the north-facing ones, likely reflecting a 

difference in growth rate due to a variation in moisture. Also, small (10–40 cm long) 

debris fragments on the ground surfaces away from the miniature quarries are commonly subrounded to subangular and carry lichens. The quarry floors and the ground 

surfaces near the quarry edges carry about equal amounts of such lichen-covered, 

subrounded to subangular debris fragments and angular blocks and slabs (coming 

from the quarries, as waste) with few or no lichens. Finally, should an alternative 

explanation such as periglacial processes be considered, it is difficult to indicate why 

periglacial features are absent elsewhere on the mesas, as well as in the nearby Front 

Range.

At another quarry on the south side of North Table Mountain in flow 3, cobblestones were produced, probably for use in downtown Denver early in the twentieth 

century, perhaps at a time when trolley tracks were laid. The lava columns were first 

split into slabs along the flow foliation and then cut into 10-cm cubes or blocks of 

10×10×20 cm. Likely, such skill required European stonecutters already practiced in 

this craft.

South Table Mountain Lava Flows

Only lava flows 3 and 4 reached the area of South Table Mountain. The volume 

of flow 1 and flow 1a was insufficient to bring them south of the present Clear Creek 

gap. However, a lens of channel gravel carrying shoshonite porphyry clasts crops out 

beneath flow 3 at site I (fig. 6). The level of this outcrop is appropriate for an interpretation that the lens lies just down-channel from the distal end of a lobe of flow 1 

now eroded away by Clear Creek.

Lava flow 3 spread widely across the area of South Table Mountain (fig. 6), 

but flow 4 was more restricted; indeed, flow 4 covered much less area than mapped 

by Scott (1976). At the north end of South Table Mountain, flow 3 developed a low, 

broad dome such that the lava is 18–21 m thick on the dome’s top and only 9–13 m 

thick on its flanks to the east or west. Flow 3 to the south and southeast of this dome 

gradually thins to 2–5 m. This thin part of flow 3 is slightly amygdaloidal throughout 

its thickness, rather than only in its upper third. Flow foliation delineates the dome; 

off-dome dips to the west (fig. 6, site J) exceed 30° in places, and those to the northeast (fig. 6, site K) are as much as 17°, but mostly somewhat less. The northern part 

of the dome has been eroded away by Clear Creek.

The south and southeast flanks of the dome dip more gently (3°–5°) but are 

modified by six subordinate, short pressure ridges (sites marked L, M, and N in 

fig. 6). Some of these features are shown in figure 7. The ridges are about 100 

m broad and a few hundred meters long with east- to south-trending axes. Flow 

foliation dips 5°–10° off the flanks and ends of the ridges but grades northward or 

westward toward the main dome (fig. 6). These features suggest that the dome represents a local thickening (“inflation”) of lava flow 3, rather than a normal flow cover 

over a raised subflow terrain. Although the flow crust cooled and stiffened, its core 

lava retained fluidity, and pressure in the dome built up as more lava accumulated. 

The ridge structures provided a local release of this pressure. Perhaps a final surge 

southeastward permitted the thin amygdaloidal part of flow 3 to extend farther down 

the broad plain.

At its maximum southward extent, lava flow 3 reached the north flank of the 

present Green Mountain (Scott, 1976; Drewes and Townrow, 1999) on the first rise 

south of the Jefferson County Fairgrounds (fig. 1). The outcrop there was small and 

consisted of heavily oxidized, amygdaloidal shoshonite porphyry. In about 1990, the 

outcrop was scraped off and covered by soil around new houses. Although I did get 

to see the outcrop in time, I lacked the foresight to sample it. Other lava outcrops 

have been reported on this part of Green Mountain (Reichert, 1952) but found wanting in the present study. Some reported sites of lava outcrops might be boulder concentrations in channels or boulder piles pushed together by builders; others remain 

enigmatic.

Lava flow 4 is found only on the west and northeast flanks of the dome formed 

by the underlying flow 3 of South Table Mountain. Flow 4 was less voluminous than 

flow 3, so when it reached the dome it divided and followed lower ground on flow 3. 

Streams seem not to have run along this low ground; no sedimentary lenses separate 

the flows. The base of flow 4 immediately overlies the blocks of the oxidized upper 

crust of flow 3. The western prong of flow 4 is fairly straight and about 1 km long 

(fig. 7). The northeastern prong is sinuous and at least 1.5 km long. Both extend onto 

the mesa to the zones of steepened flow foliation on the flanks of the dome on flow 

3. Although an upper oxidized and amygdaloidal zone is seen at the top of flow 3 

along cliff faces that cut the dome, such a zone has not been seen within flow 3 on 

the dome flanks. Perhaps the dome rock cooled more slowly than the thinner parts of 

flow 3 on the flanks of the dome, which were covered by flow 4 before oxidation and 

vesiculation of the underlying thin part of flow 3 were far advanced. Another possibility that would explain the difference in lava character is that the volatile elements 

could have been concentrated upward into the dome. With fewer volatile elements 

in the flank lavas, oxidation and amygdule formation in those lavas would have been 

impaired. 

As described, flows 3 and 4 on both North Table Mountain and South Table 

Mountain usually consist of a lower columnar type of structure overlain by an amygdaloidal, noncolumnar type. The lava flows on South Table Mountain are slightly 

deformed in the manner of those on North Table Mountain, but in places they are 

Field Observations 11
12 Table Mountain Lava Flows, Colorado

P

Dam

Developed Developed

I

K Q

O

P

L M

J N

R

28 24

3

3

3

7

4

3

10

10 15

3

3

3 3

2

2

2

2

2

3 3

3

3

3

3 3

2

3

3

3

3

3

3

3 3

4

4

3

3

3

3

3

8

8

2

2

2

2

2

2

2

2

2

2

2

2

2 2 2

2

2 2

2

2

2

2

2

2

2

2

2

5

2

2

2

2

2

2

2

4

4

6

7

5

5

5

5

5

5

5

5

5

5

5

15

25

25

15

5

3

3

5

5

5

32 32

10

10

10

12

12

15

20

20

20

5

5

5

5

5

5

10

13

15

10

25

35

35

30

45

15

3 45

3

3

6

2

2

45

77

30

35

45

75

Qr Ka?

Tf4

Tf4

Tf4

Tf4

Tf4

Tf4

Tf4

Tf3

Tf3

Tf3

Tf3

Tf3

Tf3

Tf3

Tf3

Tf3

Qc

Qc

Qc

Qc Qly

Qly

Qly

Qly

Qly

Qly

Qaf

Qaf

Qly

Qly

Qly

Qc

Qly

Qly

Qly

Qao

Qly

Qly

TKd

TKd

Qc

Qc

Qc

Qc

Qc

Qc

Qa

Qr

Qr

Qr

TKd

TKd

TKd

TKd TKd

TKd

TKd

TKd

TKd?

TKd

TKd

TKd TKd

TKd?

TKd

TKd

TKd

Quarry

Quarry

Recent movement

Water

Tank

0

0

1 MILE

0.5 1 KILOMETER

1/2

N

39°

45’

105°12’30” 105°10’

Figure 6. Geologic map of South Table Mountain, 1–6 km southeast of Golden, Colo. Mapped at scale of 1:6,000 and here shown reduced. Sites 24 and 28 

provided samples that were point counted. Sites I–R are referred to in text: I, shoshonite porphyry clast–bearing channel gravel; J, zone of steeply dipping 

flow foliation on west side of dome in flow 3; K, zone of moderately dipping flow foliation on northeast side of dome; L–N, areas of pressure ridges; O, Castle 

Rock; P, charcoal-bearing mudflow deposit within Denver Formation (unit TKd) 3 m below flow 3; Q and R, small weathered patches of silt and clay, probably 

remnants of Denver Formation overlying the lava flows. See figure 4 for correlation and description of map units. Base from U.S. Geological Survey, 

1:24,000, Golden and Morrison, 1965 (photorevision as of 1980).
also internally deformed by flow turbulence. In addition, at Castle Rock (site O 

in figs. 6 and 7) both the northern ends of the channelized flow 4 and the adjacent 

flow 3 show structural irregularities. The flows are broken into several small sheets, 

and south of Castle Rock, the bold prominence overlooking Golden, flow breccia is 

found in place of the usual columns. Likely, the dome is linked to these irregularities 

in some way not yet clear.

About 20 small faults cut the lava flows of South Table Mountain, and near the 

southeastern end of South Table Mountain another open, south-plunging syncline is 

developed in the flow foliation. Displacement on the faults is normal, and dips are 

commonly steep. Southeast of Castle Rock (site O in fig. 6) several faults dip gently 

to moderately. This fold and these faults are also viewed as responses to the settling 

of deposits in the Denver Basin.

A few local sedimentary features near the flows on South Table Mountain are 

of interest. The charcoal-bearing mudflow bed in the underlying Denver Formation 

lies about 3 m below flow 3 at site P in figure 6. At sites Q and R are small bodies of 

siltstone, likely of the Denver Formation, over the lava flows.

Problems in Interpreting the Lava Flows

The field study of the shoshonite porphyry lava flows and intrusive bodies not 

only provides a more complete picture of their origin and emplacement history but also 

raises some questions. From which vent did the flows come? How far did the flows 

once spread? Why and when did the southeast-flowing Paleocene streams shift to the 

present east-flowing ones? And why are the broadest gaps between segments of the 

igneous system located where today’s crosscutting streams are smallest (fig. 8)? 

The question of the source vents of the lava flows has been raised before (Allan, 

1922; Ahmad, 1971). The conventional wisdom of the recent decades would place 

the source of the flows as the “Ralston dike” because their rocks are much alike and 

the flows thin away from that proposed source.

The field study shows that the southeastern group of intrusive bodies is probably 

linked to channel flows 1 and 2; the low volume of these flows is compatible with a 

vent or vents lacking the heat to alter its host rocks. In contrast, the field study shows 

that the Ralston plug may have been the source of flows 3 and 4. Their larger volume 

is compatible with a vent having a wide altered zone. One objective of the petrographic study described later in this report was to determine whether the rocks might 

verify this field-based interpretation.

The question of the past extent of the lava flows remains speculative, but one 

observation is useful. The outcrops of flow 3 that now are found on North and South 

Table Mountains are parts of an originally much larger flow area. These remnants 

were once connected across the canyon at Clear Creek but are now separated by a 

Field Observations 13

Map unit—Qa, younger alluvium; Qc, colluvium; Qr, rock-fall deposits;

Qly, younger landslide deposits; Qaf, artificial fill; Qao, older alluvium;

Tf3–4, lava flows 3–; TKd, Denver Formation; Ka, Arapahoe Formation. 

Contact—Dashed where inferred or gradational; dotted where concealed

Fold—Showing dip of limbs. Dashed where approximately located;

dotted where concealed

Anticline

Syncline

Pressure ridge—Showing fold axis and plunge of nose

Landslide scarp headwall

24

Tf1

EXPLANATION

K

P

2

Fault—Showing dip where known. Bar and ball on downthrown side where known

Gravel road

Trail or jeep track

Strike and dip of inclined beds

Strike and dip of inclined flow foliation

Quarry—Hachures point toward quarried area

Sample locality—Showing sample number as referenced in text

Site described in text

Public parking area

Building not shown on base map

77

Supplemental horizon—Base of amydaloidal zone of lava flow 3.

 Dotted where concealed

See figure 4 for correlation and description of map units

10
14 Table Mountain Lava Flows, Colorado

FLOW 4

NORTH TABLE MOUNTAIN

NORTH TABLE MOUNTAIN

FLOWS 3 AND 4

FLOW 3

FLOWS 3 AND 4

FLOWS 3 AND 4 FLOWS 3 AND 4

EDGE OF DOME ON FLOW 3

FLOW 3

O in

figure 6

DOME (SUMMIT)

DOME (NORTHEAST FLANK)

Golden 

SOUTH TABLE MOUNTAIN

Castle 

Rock

Figure 7. South Table Mountain, Golden, Colo., looking northeast, with North Table Mountain in distance. Arrows show direction of dip of flow foliation. Photograph by

Shawn Steigner. 
0.5- to 0.7-km-wide gap. The same is true of flow 4. Since canyon incision began, 

therefore, each mesa edge has retreated about 0.3 km. One might surmise that the 

degree of erosion was similar east and west of the mesas, where flow thicknesses 

were the same as on the nearby mesas. Should the east side of the broad valley plain 

on which flows 3 and 4 poured have been higher laterally (because of the presence of 

an erosional remnant), thinner flows might have spread there, permitting more rapid 

erosion, other factors remaining constant. However, other factors did not remain 

constant. Since the present drainage system was established, the tributary drainages 

east and west of the mesas were much smaller than Clear Creek and so had a much 

lower capability for erosion These considerations suggest that the eastern and western edges of the mesas were eroded less than the amount removed at Clear Creek 

Canyon.

This question on lava flow erosion intrigued Waldschmidt (1939), who proposed 

that lava flows 3 and 4 extended west of the mesas to the position of the Dakota 

Ridge, a distance of 0.8–1.5 km, although the Dakota Sandstone (now Dakota Group) 

is largely faulted out. Because he offered no basis for his proposal, I here favor the 

more conservative distance of lateral erosion of about 0.2–0.3 km, somewhat less 

than half of the width of Clear Creek Canyon; a larger stream erodes more than a 

smaller one.

Lava flow thickness is an important factor in judging the likely erosion history 

of the flows. To the north near their source at the narrow part of the plain, flows 3 

and 4 together may have been 20–30 m thick (as seen in the cliff heights in fig. 7). To 

the south, flow 3, the only flow there, was only 2–5 m thick and was spread across a 

wide plain. Again, given transverse drainages of the same size, the southern part of 

the lava field should erode more rapidly.

With these considerations of the past extent of flows in mind (fig. 8), their combined total volume is estimated to have been about 0.7–0.8 km3

.

The general south-southeastward flow direction implies that higher ground lay 

to the east-northeast. This high ground need only have been slightly higher than the 

valley in which the lava flowed, not a major upland. Most likely, this minor upland 

was underlain by an older part of the Denver Formation. The surface of the valley 

beneath flow 3 would then have become a local disconformity within the Denver Formation. It is easier to envision the larger Clear Creek cutting the gap in flow continuity between North Table Mountain and South Table Mountain than it is to picture the 

cutting of the broader gap between North Table Mountain and the intrusive bodies 

(Van Bibber Creek gap) and that between South Table Mountain and Green Mountain (Lena Gulch gap). The Van Bibber Creek gap is probably wide because all flows 

that the creek crossed were channelized this close to the vents, thus offering less 

resistance to erosion. The Lena Gulch gap is probably wide because the distal end of 

flow 3 was thin and amygdaloidal. Furthermore, projecting flow 3 southward from 

South Table Mountain using the prevalent dip, or even half that dip, leads to a misfit 

with the level of the Green Mountain outcrop site (the projection using the flow 3 dip 

is lower than the Green Mountain outcrop), suggesting that some faulting in this gap 

has further weakened the lava and facilitated erosion.

The fault that is proposed to run through the Lena Gulch gap area would have 

its south block raised a few tens of meters. In figure 8 the proposed fault is given a 

northwest strike to mimic another such structure mapped a short distance south of 

the Green Mountain lava outcrop (Drewes and Townrow, 1999); the position of the 

proposed fault is only broadly constrained.

Finally, how and when did the local river systems change their courses and what 

was needed geologically to bring about this change? The basic answer, of course, is 

textbook geomorphology. Streams are superposed from a covering layer during uplift 

and erosion. The shift in flow direction of streams would be enhanced by a slight 

northward tilt of the region before incision began. Such tilting would less likely have 

occurred during the Paleocene, when east-west compression was the main regional 

stress, than during the mid-Tertiary, when the Rocky Mountains were worn low and 

unconsolidated formations, such as the White River Formation of the ColoradoKansas border area, were deposited (fig. 9).

Figure 9 shows the White River Formation as it may have been distributed along 

an east-west profile during mid-Tertiary time. It is Miocene in Kansas and Oligocene farther north, and its proposed extension to the Golden area is also likely to 

have been Oligocene. The proposed extension of the White River may have lapped 

onto the residual uplands to the west. This unit is likely to have been fine grained, 

moderately well sorted, and unconsolidated. Although a few small patches of Tertiary 

rocks are known to exist west of Golden, the deposits are undated; those that consist 

of coarse gravels may be too old—possibly Green Mountain Conglomerate (Paleocene) equivalents. The proposed mid-Tertiary formation X of figure 9 might easily 

have been mistakenly assigned to older or younger piedmont deposits, if indeed any 

formation X remnants still exist.

Geophysical Data

Several geophysical studies have been made in the area of the intrusive bodies; 

one of them provides only raw data, but the others provide data and diverse interpretations without reaching a clear consensus. Although these studies deserve attention, 

they must be approached with circumspection.

Of greatest interest in these studies is the recurring idea that the Ralston plug 

narrows downward. The ground magnetic data of Hasbrouck and others (1975) are 

herein interpreted to show that the body narrows downward to the form of a dike 

approximately aligned along the western shore of Upper Long Lake. The gravity 

Geophysical Data 15
16 Table Mountain Lava Flows, Colorado

6

93

(Eocene?)

A

B C Surficial deposits

Unknown unit

Green

Mountain

Conglomerate

(Tgm)

63.9 (Obradovich, 2002)

*

*

*

*

Member

Member

Member

Member

Ash bed

4

3

2

1

Denver Formation (TKd)

63.0

64.2

63.5

Flow

Flow

Flow

Flow

4

3

2

1

Tertiary

Cretaceous

Arapahoe Sandstone

Laramie Formation

Fox Hills Formation RALSTON RESERVOIR

BEND IN SECTIONBEND IN SECTIONBEND IN SECTION

BEND IN SECTIONBEND IN SECTIONBEND IN SECTION

BEND IN SECTION

RALSTON PLUG SOUTHEASTERN PLUG GOLDEN FAULT PROBABLE VENT FOR LAVA FLOWS 1–4 Van Bibber Creek HIGHWAY 93

Thick 

colluvial 

cover FLOW 3

FLOW 2

LEVEL OF

FLOW 1 (Projected into 

plane of section)

NORTH TABLE MOUNTAIN

FLOW 4

FLOW 4

FLOW 4

FLOW 3 FLOW 2

FLOW 3

FLOW 3 FLOW 3

FLOW 1

RR

Possible fault

Road

SOUTH TABLE MOUNTAIN

(Possible Tgm cover)

CRETACEOUS CRETACEOUS TERTIARY TERTIARY

GOLDEN QUAD. MORRISON

QUAD.

TKd

Ka Kl

*

Clear Creek

FEET

9,000

8,000

7,000

?

? ?

Thrust flap (Van Horn, 1957, 1972)

(Development probaby linked to

magma emplacement)

Probable vent

for lava flows 1-2,

for flows 3 and 4 SOUTHEASTERN 

PLUG

A RALSTON PLUG

Approximate

trace of Fox Hills

Sandstone

Offset 1.9 km

Plugs may merge in the

subsurface and follow

the Golden fault

Flows

1, 3, 

and 4

Paleocene stream

(probably from lower

reaches of Coal Creek

Canyon)

Flow 1

Outcrop

controls

Flow 2 (new)

End of

 flow

Tumulus ridge

on flow 4

NORTH TABLE MOUNTAIN

Channel-fill

lava tongues

Inferred extent of lava flows 3 and 4

Erosional remnants

of flows 3 and 4

spread on a plain

Fault ?

 Flows

3 and 4

Clear

Creek

SOUTH TABLE MOUNTAIN

Area of dome

on flow 3

GOLDEN

MORRISON

QUADRANGLE

QUADRANGLE

Approximate extent

of dome

GOLDEN TOWN CENTER

Inferred extent of lava flows 3 and 4

Inferred extent of lava flow 3

Paleocene stream

(probably from middle

reaches of Clear Creek

Canyon)

JEFFERSON COUNTY

GOVERNMENT 

CENTER

Fault ?

Lena Gulch

GOLDEN FAULT

Van Bibber Creek

Flow 1

Flow 1

Flow 3

Flow 4

Flow 3

Flow 4

Flow 3

Flow 3

Flow 3

Flow 4

Flow 4

Flow 4

Crest

Channel

beneath

flow 3

Flow 1

Flow 1

Inferred extent of lava flow 3

Ralston Reservoi r

Approximate western extent of coeval high ground

Approximate trace of Fox Hills Sandstone

Inferred extent

of lava flow 4

Inferred

extent

of lava 

flow 4

? ? See inset for details

6,000

5,000
Figure 8. Map and profile of lava flows from source vents to North and South Table 

Mountains and to Green Mountain, near Golden, Colo. A, North-northwest to southsoutheast profile. Vertical scale exaggerated; some contacts near intrusive bodies 

projected to their probable positions beneath colluvial cover. B, Stratigraphic chart, 

abbreviated to show only units germane to interpretation of lava flows. C, Map of area 

from Ralston plug to Green Mountain, showing units key to study of lava flows, in part from 

Drewes and Townrow (2005).

Geophysical Data 17

TKd Map unit—Tgm1–4, members 1–4 of Green Mountain Conglomerate; 

Tfl–4, lava flows 1–4; Tkd, Denver Formation; Ka, Arapahoe 

Formation; K1, Laramie Formation

Outcrop of lava flow 1—Mapped for control

Contact

Approximate trace of Fox Hills Sandstone—Concealed in most places

Inferred extent of lava flow

Crest of tumulus ridge

Approximate extent of dome on South Table Mountain

Approximate western extent of coeval high ground

Paleocene stream—Arrow shows direction of flow

Fault—Dashed where approximately located; dotted where concealed; 

queried where location uncertain

Inferred thrust fault—Sawteeth on upper plate

Lava flow contact—Projected above present surface

EXPLANATION 

FLOW 4

FLOW 3 FLOW 4

FLOW 3

FLOW 1

CRETACEOUS TERTIARY

? ?

40

70

26

6

C-470

BEND IN SECTION

FLOW 3 

(Outcrop 

now covered)

MORRISON QUAD. Lena Gulch Colfax Ave–US 40 Interstate 70 6th Ave–US 6

JeffersonCountyFairgrounds

GREEN MOUNTAIN

Tgm4 Tgm3 Tgm2

Tgm1

ALAMEDA PKWY

Summit 6,855 ft

TKd

Ka Kl

JEFFERSON COUNTY

FAIRGROUNDS

Outcrop now

buried

Fault ?

GREEN MOUNTAIN

Summit *

 6855

TKd

TKd

TKd

Tgm1

Tgm2

Tgm3

Tgm4

Tgm1

A’

ALAMEDA PKWY

Approximate trace of Fox Hills Sandstone

(Inferred movement 60–52 Ma)

VISITOR CENTER

Dakota Hogback = Dinosaur Ridge

RED

ROCKS

PARK

Mount Vernon Creek

Inferred movement 70–55 Ma

SPLAY OFF GOLDEN FAULT

GOLDEN FAULT

0 1 MILE

0 1 KILOMETER

N
18 Table Mountain Lava Flows, Colorado

Figure 9. Diagrammatic cross sections across the Front Range, North and South Table Mountains, Green Mountain, and the Denver Basin to the ColoradoKansas border.

West East Front Range Continental Divide

INDEX

NORTH TABLE MOUNTAIN

SOUTH TABLE MOUNTAIN

GREEN MOUNTAIN

Golden

A

B B’

C’

A’

C

COLORADO

KANSAS

GREAT

PLAINS

GOLDEN FAULT

GOLDEN FAULT

GOLDEN FAULT

Lava

conduit

N Soffset 3 km

1

1

1

2

J-h

J-h

J-h

pe

pe

pe

Tf4

Tf3

Tf1 Tf2

NORTH TABLE MOUNTAIN

Ka-Kb

Kd

TKdl

TKdu

Tx

J-h

Ka-Kb

Kd

TKdl

TKdu

Tx

J-h

Ka-Kb

Kd

TKdl

TKdu

Tx

J-h

pe

pe

GREEN MOUNTAIN

4

Tgm 3 Tf3

SOUTH TABLE MOUNTAIN

3

Tf4 Tf4

Tf3

South Platte River

DENVER BASIN

A

Tw

KEY FEATURES

1. Late Cretaceous andesite volcanic field, source of 

volcanic clasts in unit TKd; streamflow to southsoutheast.

2. Early Paleocene shoshonite porphyry vent along 

fault in Precambrian rocks at depth (?) and along 

Golden fault near surface.

3. Four shoshonite porphyry lava flows on North 

Table Mountain and only two lava flows on South 

Table Mountain.

4. Green Mountain conglomerate cap, derived from 

andesite volcanic field plus clasts of Pennsylvanian 

to Jurassic provenance and, in the upper 

members, increasing volumes of Precambrian 

clasts; stream flow to south-southeast.

5. Inferred formation: Eocene (?) Tx, enabling shift to 

eastward streamflow with renewed uplift.

3

5

5

5

B B‘

C C‘

Not to scale

Vertical exaggeration about 50×

A’
Tf1 Map unit—Tw, Tertiary White River Formation; Tx, unknown Tertiary (Eocene (?) unit x; 

Tgm, Tertiary Green Mountain Conglomerate; Tf1–4, Tertiary lava flows 1–4; TKdu, 

inferred upper part of Tertiary and Cretaceous Denver Formation; TKdl, lower part of 

Tertiary and Cretaceous Denver Formation; Ka–Kb, Upper Cretaceous sequence (Arapahoe 

Formation to Benton Shale); Kd, Lower Cretaceous sedimentary rocks; J–h, Jurassic to 

Pennsylvanian sedimentary rocks; pe, Precambrian granite and metamorphic rocks

Lava flow—Showing columnar jointing

Topographic profile showing present-day surface

Contact—Dashed where projected above present-day surface

Fault—Arrow shows direction of movement. Dashed where projected above present-day 

surface

EXPLANATION

study of Stanlav and Morrison (1952) led them to infer that most of the plug’s mass 

lies above a level of about 60 m below the lake level and that the bottom of the plug 

gradually deepens below that level to the west. The seismic (vibroseis) study of 

Shuck (1976) along an east-west trace across the southern part of the plug led him to 

infer that the intrusive body is a rootless and faulted sill dipping gently eastward.

The magnetic field study of Levings (1932), covering the area of the Ralston 

plug and the southeastern group of intrusive bodies, was set up to dispel the idea 

that the anomalies are solely related to the impressing of the Earth’s magnetic field 

on cooling igneous rocks. His results show magnetic highs and lows in and near the 

intrusive bodies. Although variously explained by Levings, the subsurface shape of 

the intrusive bodies was not considered.

Additional structural interpretations have also been offered by the Colorado 

School of Mines students mentioned previously (Levings, 1932; Shuck, 1976; Stanlav and Morrison, 1952; and Hasbrouck and others, 1975). However, these interpretations are based on minimal geophysics, no field mapping, and much speculation 

on issues of particular interest during their school years. Where is the vent that fed 

the lava flows? Does the Golden fault dip gently or steeply? Is the structural flap 

of Pierre Shale east of Highway 93 a thrust slice or a gravity-impelled slide? The 

issue of the vent sites has already been addressed in this report. The structural issues 

remain beyond the scope of this report. Clearly, a comprehensive, detailed geophysical study tied to mapping and petrography is still to be made.

Petrographic Observations

The igneous rocks of the Ralston intrusive bodies and the Table Mountain lava 

flows are generally accepted to be of one family, if not identical. Once the issue of 

an appropriate petrologic name was settled, however, the rocks attracted little further 

attention in the literature. Past petrographic studies, for instance, provide little more 

than a rock description, a modal analysis, and a chemical analysis. Lacking are information on sample localities, whether the sample is single or composite, and which 

flow or intrusive body is represented. Only the zeolite minerals have been given 

further attention, and this interest is for their mineralogy rather than for their information about lava flow history (Johnson and Waldschmidt, 1925; Kile and Modreski, 

1988). The reports of Allan (1922), Waldschmidt (1939), Ahmad (1971), and Kile 

(2004) best summarize the past work on these igneous rocks; they reflect the needs 

and working styles of their times.

The present study was undertaken with two objectives in mind. First, it was 

desirable to describe the rocks more systematically and tie them to field maps. They 

might tell a bit more of their history than already known. Second, with such a start it 

might be possible to test the links between vents and lava flows inferred from field 

observations. As much like one another as the rocks seem to be, it was clear that this 

second objective would require meticulous observations, systematic procedures, and 

large counts, not only of mineral kinds and abundance, but of textures and alteration 

as well. In order to make the modal counts of minerals as objective as possible, then, 

2,000-point counts were taken, following identical grid patterns in each thin section. 

Thin sections for which modes had been determined at the beginning of the petrographic study were recounted once specific identification skills became routine. Thin 

sections stained for potassium feldspar were point counted for modal analysis first, 

unstained and unaltered rocks next, and slightly altered rocks last.

Textures

All the rocks are porphyritic; phenocrysts are 1–4 mm long and make up 

l5–20 percent of the rocks. The phenocrysts include plagioclase, pyroxene, olivine, 

and magnetite. The groundmass is mainly blocky but has some felty texture. The 

groundmass grain size is in the range of 0.1–0.5 mm, except along the few sampled 

chilled margins, where it was even finer, and thus impractical to determine the mode. 

Sample 15 (fig. 2), from the southeasternmost point of the small plug of the Ralston 

intrusive bodies, has an entirely felty groundmass. In some rocks, crystals having a 

felty texture are faintly flow aligned or ophitic.

Most samples have a blocky groundmass that contains 5–10 percent interstitial crystals, chiefly anhedral sanidine but also some biotite, both of which are 

Petrographic Observations 19
20 Table Mountain Lava Flows, Colorado

remarkably unaltered. Some specimens from the core of the Ralston plug have large 

interstitial sanidine anhedra; in one rock (sample 6) they even reach the size of the 

phenocrysts. In this specimen, biotite forms beautiful subhedral crystals (half of hexagonal platelets) attached to some groundmass blocky crystals. These biotite crystals 

resemble crystals lining cavity walls. Although not implying that groundmass cavities existed in the intrusive rock, the subhedral biotite crystals nevertheless suggest a 

period of late crystallization from a highly fluid residual liquid. In contrast, the phenocrysts probably crystallized earliest, as suggested by the cores of some plagioclase 

phenocrysts that are strongly altered to clay minerals and by other phenocrysts that 

are bent or crystallographically distorted.

All transparent crystals are laced with trace amounts of acicular microlites. 

Some of these are apatite; others are probably plagioclase and many are undetermined. Their presence in both early-formed phenocrysts and late-formed interstitial 

crystals needs further explanation.

Several noteworthy textural features were observed sparingly in most specimens, and one of them was seen only in the intrusive rocks; they bear describing 

because they indicate a bit about the development of the magma. Most noticeable 

are scattered rounded grains, chiefly of magnetite, but also of pyroxene, olivine, and 

rarely of apatite. They range in abundance from about 10 to 50 per square centimeter 

of thin section, and they were seen among phenocrysts and groundmass alike. Some 

small rounded grains are included in phenocrysts. Among the intrusive rocks they 

seem to be most abundant in the core phase and least common in the southeastern 

group of the small plug, dikes, and sills.

Even less common are strongly fragmented pyroxene phenocrysts—crystals 

that literally look chewed up. They have been found in only a few intrusive and lava 

flow specimens and then in only a few phenocrysts of a specimen. Chewed-up grains 

are broken up into small subrounded segments that are slightly rotated relative to 

their neighbors. Crystals were found showing various stages of development of this 

texture. Some have only incipient curved fractures; others are completely fractured, 

but the segments are not rotated. Next in sequence are grains having the rotated segments. In all these stages the original pyroxene crystal outline is retained. Perhaps 

the ultimate stage of development is patches of clustered small pyroxene crystals no 

longer retaining a phenocryst outline and admixed with a few small crystals of other 

minerals. Resorption as a result of a change in magma chemistry is not indicated, 

because resorbed crystals typically show embayments along fractures and cleavage 

planes. No such embayments were observed in the igneous rocks studied here.

In a single case, a small ring of such subrounded small pyroxene crystals, each 

having its own orientation, was seen. The usual groundmass mixture both fills and 

surrounds the ring.

Taken together, these special textural features suggest that conditions of magma 

and lava flows were turbulent, a not-surprising situation. What remains surprising, 

however, is the scarcity of these features. Furthermore, finding signs of limited turbulence encapsulated in phenocrysts attributed to deep-level crystallization along with 

similar signs in a groundmass that probably crystallized at a higher level is interesting; mildly turbulent conditions persisted.

The groundmass material provides a different message. These crystals are undeformed and unaltered. Magma movement at this late stage of crystallization surely 

was not turbulent. The magma may even have stopped moving. The last fluid, with 

its higher alkali content giving it a lower temperature of crystallization, may simply 

have moved through a mush of phenocrysts and groundmass crystals.

Some of the intrusive rocks show narrow reaction rims or partial rims of biotite 

around pyroxene or magnetite; less common are rims of pyroxene around olivine. 

Rare as they are, reaction rims are most common in rocks of the Ralston plug. Fewer 

than half the samples of the southeastern group intrusive rocks have reaction rims.

Reaction rims are signs of a chemical disequilibrium, a condition of these rocks 

long known through the association of olivine with sanidine and biotite. The scarcity of these reaction rims in the intrusive rocks and their apparent absence in the 

lava flows need explanation. Perhaps the reaction rims formed late in the vents after 

most of the magma had surfaced. They may also be so scarce because very little 

potassium-bearing material was incorporated by the magma.

Minerals

The major minerals of the shoshonite porphyry are andesine plagioclase, augite 

pyroxene, and olivine. Accessory minerals are magnetite and apatite, and in a few 

samples sphene was suspected but not proved. Additionally, all rocks contain interstitial sanidine and biotite. Secondary minerals include alteration products and zeolites. 

Alteration products are mainly clay minerals, sericite, iron oxides and hydroxides, 

iddingsite, and antigorite. The zeolite minerals commonly seen in thin section are 

analcime, thomsonite, and chabazite; many more zeolite minerals are reported present by Kile and Modreski (1988). Calcite and pyrite veinlets cut through the rocks 

in a few places. Because most samples were taken from cliff faces, quarry walls, and 

excavation spoil piles, surface weathering was minimal; only a few samples could 

not be point counted because of such alteration, and these were mostly from South 

Table Mountain.

Plagioclase phenocrysts are white, as much as 3.5 mm long, euhedral to subhedral, and compositionally zoned. Most phenocryst cores and some zones are altered 

to clay minerals, and others are bleary (having indistinct extinction-angle effects, 

possibly stressed). Nevertheless, compositions were usually obtainable by using the 

mp (that is, crystallographic and cleavage planes 010 and 001) section method, which 

is more precise than the usually applied Michel-Levy method. Both phenocryst and 
groundmass plagioclase are calcic andesine; in some samples groundmass crystals 

are sodic andesine, and rarely phenocrysts from the southeastern group intrusive 

rocks are sodic labradorite. Groundmass plagioclase is subhedral to anhedral and 

0.1–0.5 mm long.

Pyroxene crystals appear black in the rock but in thin section are pale green. 

They show a faint pleochroism from a grayish green to a pale brownish green. 

Phenocrysts are as much as 4 mm long and largely euhedral; groundmass grains are 

anhedral. Extinction angles are 44°–45°. Cleavage is orthogonal. These characteristics are indicative of augite. Although most rocks contain unaltered augite, many 

from South Table Mountain samples are altered to FeO-bearing secondary minerals.

Some of the dark crystals are olivine. Olivine phenocrysts are rarely more than 

2 mm long and form euhedral crystals; groundmass olivine is subhedral or anhedral. 

The mineral has its distinctive high relief and birefringence as well as alteration to 

iddingsite or antigorite along crystal margins and fractures. Generally, groundmass 

olivine is completely altered. The intensity of the alteration increases toward the 

distal ends of lava flows 3 and 4.

Magnetite is present as small phenocrysts but mainly as groundmass grains. It 

reflects the lavender-gray color of titaniferous magnetite. Grain shapes range widely 

from euhedral to anhedral. Some magnetite is clustered with pyroxene and apatite.

Apatite is clear in thin section and mostly euhedral, but some grains are rounded. 

The larger crystals are delicately striated. It has a parallel extinction, a feature of about 

one-fourth of the microlites. Therefore, these are interpreted to be apatite.

Sanidine is interstitial and hence anhedral. Although it typically is a pale 

pinkish-orange in the rock, it is clear in thin section. Usually these crystals are small, 

but in sample 6 they reach 1–2 mm long. They have a diagnostic low relief, an 

absence of polysynthetic twins, and a negative 2V. Sanidine is fresh in the intrusive 

rocks, slightly altered to kaolinite on North Table Mountain, and strongly altered to 

kaolinite on South Table Mountain.

Biotite is typically small and subhedral or anhedral; in sample 6, however, 

biotite crystals reach 1 mm in diameter and appear as incomplete hexagonal platelets. 

Biotite is pleochroic from pale brown to moderate brown and has a very low to zero 

extinction angle. Toward the distal ends of lava flows it is altered to sericite.

Mineral Modes

Having now examined the shoshonite porphyry in the field and under the 

microscope, I turn to the second objective. Might the rocks themselves show a connection between particular lava flows and certain intrusive bodies? If any connection 

were found, would it support the correlation inferred from the field study? Already 

mentioned was the concern that, because anticipated differences in modes (mineral 

counts) would be subtle, counting procedures would require special care.

In brief, more points were counted (2,000) per thin section than is commonly 

done. The eyes were first “fine-tuned” through counting several thin sections purely 

for practice and discarding those results; then counting, in order, thin sections stained 

for potassium feldspar, unstained thin sections of unaltered samples, and finally thin 

sections of slightly altered samples. Except for lava flow 2, modes were made of four 

or five thin sections of each group, and the results averaged. Procedural systematics 

is recorded but is too lengthy to warrant description here. A tabulation of 28 modes 

appears in tables 1 and 2.

Once modal averages were obtained, typically of 8,000–10,000 points per rock 

unit, 5 minerals (pyroxene, olivine, magnetite, apatite, biotite) and feldspars as a 

group were selected from the 11 tallied in order to avoid those that still gave some 

concern during the counting. For example, despite the care taken, distinguishing the 

two feldspars from one another remained more uncertain than other routine decisions, not only in comparing the feldspar counts of one half-slide with those of the 

other, but also in comparing the counts within a group. Overlapping wedges of small 

groundmass grains might lead to uncertainties, even among the stained thin sections. Accordingly, combined feldspar was tabulated. Likewise, phenocryst counts 

remained suspect because of problems in distinguishing small cross sections through 

tips or keels of phenocrysts from cross sections through middles of groundmass 

grains.

From averages obtained for each of the 6 parameters (5 minerals and the feldspars as a group), then, comparisons were made between each of the 4 lava flows 

and each of the 3 intrusive units, a total of 12 comparisons. The percentage deviation 

between, say, modal pyroxene in the rim phase of the Ralston plug and modal pyroxene in flow 1 was totaled with the deviations of the other five parameters, and that 

total deviation was compared with the total deviation of the other flows. The lowest 

total deviation registers the best fit, or likeliest correlation. The deviation comparisons are illustrated in figure 10. 

The results of the mineral counts provide only partial support for the inferences made through field and petrographic observation. The reason for this lack of 

full confirmation remains unclear. Perhaps chemical analyses remain more reliable 

than modes (mineral counts), no matter the effort put into the counting. Overall, the 

potassium content increases from 0.4 to 3.1 percent for biotite and from 0.0 to 14.8 

percent for sanidine (fig. 11).

The petrographic study also suggests that the southeastern plug is multiphased, 

but field evidence to support this inference has not yet turned up. Apparently, samples 15 (fig. 2) and 16 (fig. 3) are related; the former was taken from the southeast 

corner of the southeastern group plug, whereas the latter is from channel lava flow 

1 nearest to that plug. These two samples alone are entirely felty textured, and they 

Petrographic Observations 21
22 Table Mountain Lava Flows, Colorado

have had little or no potassium-rich material added (that is, they contain little or no 

biotite or sanidine). The channel lava flow at site 16 is provisionally viewed as flow 

1a; the other channel flows at this level, not all of which have been closely examined, 

are accepted as flow 1. Probably little time elapsed between the effusion of flows 1a 

and 1, and much more time separated the effusion of those flows and flow 2.

Petrogenesis of the Lava Flows

The development of the magma is inferred from both its mineral assemblage 

and textures. The magma originated probably as basalt from a deep source (the 

mantle), possibly including a scattering of labradorite, augite, olivine, and magnetite 

phenocrysts formed early and felty-textured plagioclase following soon thereafter. On moving upward, minor turbulence is indicated by the presence of rounded 

phenocrysts. At a higher crustal level, perhaps 5–8 km deep, the magma paused in its 

progress, forming a chamber in granitic host rock. The heat of the magma was sufficient to melt a small amount of its host rock, thereby acquiring an alkali component. 

A blocky-textured groundmass began to crystallize before the magma resumed its 

upward journey. A chemical equilibrium between early crystals and alkalicontaminated melt had not yet been reached. Some of the early crystals were 

stressed, altered, and rounded, because minor turbulence was again part of the 

upward movement. Near the surface, magma egress was facilitated by a steep segment of the Golden fault and ultimately also by a thick zone of nearly upended, weak 

Table 1. Mineral modes of intrusive rocks of the Golden, Colo., area.

Phenocrysts

Sample no.Field no.Plagioclase(andesine)Pyroxene(augite)OlivineMagnetiteApatiteBiotiteSanidineTotalPyroxenePlagioclaseFeldspar(composite)

Rim phase of Ralston plug

1 23D03 56.1 15.4 7.0 4.1 0.9 3.8 12.7 100.0 8.6 6.7 68.8

2 22D03 65.5 12.4 8.2 4.7 1.0 2.1 6.1 100.0 2.1 7.9 71.6 1

3 17D03 37.8 15.2 7.7 4.2 0.7 2.1 32.3 100.0 11.9 7.1 70.1

4 2D03 57.5 18.4 5.8 5.9 0.7 3.5 8.3 100.1 9.2 10.7 65.8

5 15D03 49.2 18.8 9.1 3.8 0.4 2.5 16.2 100.0 8.1 3.3 65.4

1–5 Average 53.2 16.0 7.6 4.5 0.7 2.8 15.1 100.0 10.0 7.1 68.3

Core phase of Ralston plug

2

6 16D03 33.7 18.4 8.1 3.0 0.2 3.7 32.9 100.0 4.3 4.7 66.6

7 10D03 63.5 17.0 5.7 4.0 0.8 2.3 6.7 100.0 6.5 9.2 70.2 1

8 13D03 40.5 22.2 10.1 4.1 0.8 4.2 20.1 100.0 7.5 9.2 60.6

9 8D03 62.7 16.2 5.6 4.5 0.9 3.1 7.0 100.0 3.8 3.8 69.7

10 19D03 62.0 18.0 6.2 4.8 1.3 2.2 7.5 100.0 8.5 9.5 69.5

7–10 Average 57.2 18.0 6.9 4.4 0.9 3.0 10.3 100.0 6.6 7.9 67.5

Southeastern group of intrusive rocks

11 24D03 42.8 19.6 7.4 5.8 1.0 2.4 21.0 100.0 9.5 9.0 63.8

12 33D03 55.2 15.7 9.1 4.6 0.7 2.0 12.7 100.0 10.8 9.0 67.9

13 29D03 66.6 12.8 8.9 4.6 1.1 1.3 4.7 100.0 10.7 9.2 71.3 1

14 30D03 23.8 16.1 6.4 5.8 1.1 2.4 44.4 100.0 10.8 9.5 68.2 3

15 35D03 70.1 14.5 9.4 4.1 0.4 0.4 1.1 100.0 4.8 7.4 71.2

11–14 Average 47.4 16.1 8.0 5.2 1.0 2.0 20.7 100.1 10.7 9.2 67.8

1

Stained for potassium feldspar.

2

Not in average; probably laden with residual fluid; thus atypical.

3

Not in average; atypical felty textured, low potassium-mineral content, early phase; probably associated with sample 16.
Phenocrysts

Sample No.Field No.Plagioclase(andesine)Pyroxene(augite)OlivineMagnetiteApatiteBiotiteSanidineTotalPyroxenePlagioclaseFeldspar(composite)

Flow 1 1

16 45D03 60.7 14.9 10.2 7.6 0.8 0.0 5.8 100.0 9.5 10.0 66.5

17 01D13 55.3 16.8 8.0 6.4 0.7 (2) 12.8 100.0 7.5 10.7 68.1

18 01D21 55.4 20.6 7.8 3.3 0.8 0.4 11.7 100.0 14.1 13.3 67.1

19 01D5 56.2 17.7 6.4 4.9 0.8 1.0 13.0 100.0 9.7 10.3 69.2

17–19 Average 56.9 18.4 7.4 4.9 0.8 0.7 10.8 100.0 10.2 11.1 68.1

Flow 2

20 01D18 65.1 14.3 6.2 5.2 0.7 0.9 7.6 100.0 11.3 10.3 72.7

Flow 3

21 01D14 61.4 13.6 6.9 8.5 0.6 1.1 7.9 100.0 11.4 9.3 69.3

22 01D17 51.7 19.7 7.3 5.8 0.9 1.0 13.6 100.0 9.4 10.6 65.3

23 01D1 60.0 17.2 4.0 5.0 1.0 0.6 12.2 100.0 18.8 11.1 72.2

24 02D9 55.2 13.6 8.5 5.7 1.0 0.5 15.5 100.0 11.7 8.0 70.7

21–24 Average 57.1 16.0 6.8 6.3 0.9 0.8 12.3 100.0 12.8 9.8 69.4

Flow 4

25 01D15 63.9 15.5 3.8 4.8 0.1 1.0 10.4 100.0 14.4 9.2 74.3

26 01D19 57.7 13.6 6.6 6.0 0.9 0.4 14.8 100.0 9.5 8.3 72.5

27 01D12 59.0 14.8 4.3 5.2 0.6 0.9 15.2 100.0 11.0 8.6 74.2

28 02D8 60.7 12.6 6.1 5.1 0.9 0.5 14.1 100.0 12.6 6.7 74.8

25–28 Average 60.3 14.1 5.2 5.3 0.7 0.7 13.6 100.0 11.9 8.2 74.0

1

Not in average; atypical felty textured; low potassium-mineral content, early phase, probably associated with sample 15. Stained for 

potassium feldspar.

2

Not in average; probably laden with residual fluid; thus atypical.

Pierre Shale. At first the magma pressure was barely enough to generate a few small 

channel flows. No explosive activity is recorded by scoriaceous ejecta in contemporaneous beds of the Denver Formation; a low dome over the small southeastern plug 

and possibly a fissure vent over the small western dikes supplied lava flows 1 and 1a.

A pause in volcanic activity of perhaps most of the million-year age span permitted 

magma pressure to build up again and gave the magma a chance to melt more granitic 

material around the high magma chamber, slightly changing the magmatic composition 

again. The lava flows are dated at 64.2±1.1 to 63±1.7 Ma, as reported by Obradovich 

(2002). The statistical errors ±1.1 and ±1.7 indicate that the ages could range from as 

old as 65.3 Ma to as young as 61.3 Ma. Nevertheless, other factors suggest that a 1-m.y. 

range is the most reasonable. A few tens of meters below flow 3 on the southeast side 

of South Table Mountain, an ash-bearing bed in the Denver Formation was dated at 

65.8±1.4 Ma (Evernden and others, 1964; Obradovich and Cobban, 1975). Furthermore, 

near the top of member 4 (the uppermost member) of the Green Mountain Conglomerate, 

another ash-rich bed provided an age of 63.94±0.28 Ma (Obradovich, 2002). During this 

short span of time, much happened—deposition of much of the Denver Formation and 

most of the Green Mountain Conglomerate. Therefore, interpreting a narrow age span of 

64–63 Ma for the Table Mountain lava flows is reasonable. 

Although the first magma of the renewed upward surge followed the old passageway, the increased pressure soon forced a new passageway a bit farther northwest but along the same structural controls. Perhaps the structure involving the Pierre 

Shale was changing at that time because of movement on the Golden fault. Abundant 

shoshonite porphyry lava now poured forth at the Ralston plug, skirting the earlier 

vent mounds in a large channel at first and then surging across the broad reaches of a 

Table 2. Mineral modes of lava flows of North and South Table Mountains, near Golden, Colo.

Petrogenesis of the Lava Flows 23
24 Table Mountain Lava Flows, Colorado

valley sloping down to the south-southeast. Only a brief pause separated the 

emplacement of flows 3 and 4. On their way south they covered the lower part 

of flow 2 and the gully it followed. Steam generated in the lower, un-lavafilled part of this channel bowed up the overlying parts of flows 3 and 4. With 

the dissipation of this local steam pocket, other lava filled in the space (which 

at any one time need never have been large) in a matter of hours, if not just 

minutes.

Upon reaching an area 2–3 km south of the elongate tumulus, lava flow 

3 formed a broad, low dome for unknown reasons. During the brief interval 

between the outpouring of flows 3 and 4, stream deposits covered a bit of the 

northern end of the broad plain, but none of the southern part near the dome. 

Flow 4 was diverted into two prongs by this dome. Soon thereafter lava extrusion ceased. The Pierre Shale around the Ralston plug had been hot enough 

long enough to bake the shale. The last fluids in the flows and vents deposited 

the interstitial crystals, and those fluids in the vent even interacted with some 

of the older crystals to make the sparse reaction rims.

Summary of Conclusions

Some conclusions reached through this study confirm ideas already in 

print, whereas others are newly offered herein. Inevitably, new questions are 

raised; such is the nature of our science.

1. During its upward movement the shoshonite porphyry magma had a 

complex history.

2. Magma extrusion at 64–63 Ma was nonexplosive; low domed or fissure 

vents formed, rather than a large volcano.

3. One or several of the southeastern group of vents supplied the earlier channel lava flows, of small volume. Provisionally, the composite channel flow 

1 is seen as comprising two phases, a slightly older, felty-textured flow 1a 

and several slightly younger, mainly blocky-textured flows termed flow 1.

Figure 10. Summary histograms of six parameters (mineral modes) used in 

correlating lava flows with intrusive rocks, using averages of several samples 

each. Deviations between average percentages of a mineral in a given flow and 

average percentages of the same mineral in the samples from each possible 

source vent were compared, as described in the text. The lowest total deviation 

for each flow (circled numbers) registers the best fit, or likeliest correlation, with a 

source vent. For data on which these calculations are based, see tables 1 and 2.

PyroxeneOlivineMagnetiteApatiteBiotiteFeldsparsPyroxeneOlivineMagnetiteApatiteBiotiteFeldsparsPyroxeneOlivineMagnetiteApatiteBiotiteFeldspars

Youngest

Oldest

FLOW

1

FLOW

2

FLOW

3

FLOW

4

6

4

2

6

4

2

6

4

2

4

2

5.0

9.9 9.9 12.5

12.8 12.9 11.6

6.9 5.9 8.2

5.4 4.4

INTRUSIVE BODIES

SOUTHEASTERN

GROUP

RALSTON PLUG

RIM PHASE CORE PHASE

MINERALS

COUNTED
4. A fourth lava flow, number 2 in rising sequence, is herein recognized.

5. Lava flows 3 and 4 came from the Ralston plug (not a dike at the present 

surface). Flow 3 is correlated with the rim phase of that plug and flow 4 with its 

core phase.

6. A linear tumulus was formed over the channel used by flow 2 below its distal 

end, at what is now the western part of North Table Mountain.

7. At the position that later became the northern part of South Table Mountain, 

a broad, low dome was formed on flow 3. Flow 4 parted around the dome and 

continued in channels now appearing as benches upon flow 3 along the mesa 

flanks.

8. General settling of deposits into the Denver Basin produced open synclinal sags 

and many small normal faults affecting flows 3 and 4.

9. Sedimentation continued, at least intermittently, through Paleogene time. A 

proposed Oligocene or Miocene deposit, possibly an extended part of the White 

River Formation, covered the Golden area. Likely, at that time a slight northward regional tilting shifted the local streams from a south-southeast–flowing 

direction to an east-flowing direction.

10. Renewed uplift through the Neogene led to stream incision that now erodes 

vents from flows and segments of flows from each other.

11. This study also supports a proposal that the source of the andesite detritus 

making up the Denver Formation and the contaminated basalt of the Table 

Mountain lava flows may be genetically linked. Consider that in the underlying 

asthenosphere the leading edge of a Pacific plate descended eastward. As the 

plate descended, it was heated up enough to melt the less refractory part of the 

plate and andesite magma rose and fed the volcanic field lying west or northwest 

of Golden. Andesite detritus was shed into a basin farther east. As the plate 

remnant descended farther, temperature rose some more and the more refactory basaltic remnant of the plate also melted. This slightly younger and more 

easterly magma source then fed the lava flows intercalated in the upper part of 

the Denver Formation.

Intrusive bodies

Biotite Sanidine

Composite:

Core of Ralston plug ------------------------- 3.1 14.8

Rim of southeastern plug and nearby dikes 0.4 1.1

Ralston plug:

Core--------------------------------------------- 3.1 14.8

Rim --------------------------------------------- 2.8 15.1

Southeastern plug and nearby dikes:

Core and nearby dikes ----------------------- 2.0 20.7

Rim --------------------------------------------- 0.4 1.1

 Extrusive bodies

Flow Biotite Sanidine

4 0.7 13.6

1a 0.0 5.8

4 0.7 13.6

3 0.8 12.3

1 0.7 12.5

1a 0.0 5.8

Younger

Older

Figure 11. Amounts, in percent, of the potassium-rich minerals, biotite and sanidine, in the intrusive and extrusive rocks of the 

shoshonite porphyry. The composite shows an overall increase in the younger rocks. Subset of the two plugs and flows shows a 

general increase in potassium-rich minerals from older to younger rocks.

Acknowledgments 25

Acknowledgments

This study prospered greatly through the help, patience, and encouragement of 

family, friends, and colleagues. Although some help was direct and obvious, other 

important help was indirect or inconspicuous. I owe sincere thanks to everyone in 

both categories. Serving as a formal tie with the U.S. Geological Survey and as a 

frequent consultant was Peter Modreski. To my ever-patient wife I owe special 

gratitude for letting me pursue my rocks instead of her projects. The support of the 

Asphalt Paving Company of Golden, Colo., in providing thin sections is gratefully 

acknowledged. Finally, I am indebted to Jeff Keller of the Asphalt Paving Company 

(and the Ralston Quarry) for his help as field assistant.
26 Table Mountain Lava Flows, Colorado

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Glossary

A

acicular Needlelike; commonly refers to 

tiny crystals, highly elongate, that are found 

in larger ones.

amygdaloidal An igneous-rock texture in 

which gas bubble holes are filled with latedeposited minerals, commonly zeolites or 

calcite.

andesite An igneous rock having a composition intermediate to that of rhyolite (high silica 

and alkali minerals) and basalt (low silica and 

alkali minerals).

anhedral Under the microscope, a mineral 

not showing crystal form.

B

backthrust A local or subordinate thrust 

fault on which the upper plate of rock moved 

toward, rather than away from, the source of 

compressional stress.

basalt An igneous rock low in silica and 

alkali minerals.

C

columnar joints In sheets of igneous rock, 

sets of cooling fractures typically forming 

columns with hexagonal cross sections 1–2 

m across.

cuesta A ridge with a gentle side and a 

steep side formed by the differential weathering of a gently dipping, erosionally resistant rock layer and adjacent weak layers.

D

dike A tabular-shaped (long and thin) 

intrusive body.

M

Michel-Levy method A procedure using 

one crystallographic plane to obtain the 

general composition of plagioclase feldspar; 

the commonly used method.

microlite A tiny crystal included in a large 

one; may be acicular.

mp method A procedure using two crystallographic planes to obtain the precise composition of plagioclase feldspar; the rarely 

used method.

O

orogeny The complex processes that 

structurally deform rocks on a large scale, 

including folding, faulting, igneous intrusion, metamorphism, and volcanism.

P

periglacial Features formed near a glacial 

margin.

phenocryst Crystals larger than those of 

the groundmass.

pleochroic Under the microscope, the 

effect of changing colors of certain minerals 

upon rotating the platform on which the thin 

section lies; useful in mineral identification.

plug A cylindrically shaped (round or oval 

in section), middle-sized (about 0.1–10 km) 

pipelike intrusive body.

Glossary 27
28 Table Mountain Lava Flows, Colorado

porphyry An igneous-rock texture of scattered large crystals among many small ones.

pumice A finely porous volcanic rock, 

commonly less dense than water so it floats 

until it becomes water logged.

R

reaction rim A ring of one mineral around 

another, usually seen in thin section but 

sometimes visible to the naked eye.

S

scoria A coarsely porous volcanic rock 

that is denser than pumice.

shoshonite A petrographer’s name for an 

alkali-rich basalt.

T

thrust flap A local wedgelike thrust plate.

tumulus On a lava flow, an upward-bulged 

structure formed by moderate steam pressure.

V

vesicular Having small gas bubble holes, 

for example, in a lava flow.

vibroseis A seismic method that puts vibrational energy into the ground; the returning 

energy is analyzed to determine the structure 

of layers below the ground surface

W

wall rock The rock immediately adjacent 

to an intrusive body, as distinct from “host 

rock,” the rock generally intruded.

Z

zeolite A family of minerals, mainly alkaline, that are associated with the latest stage 

of lava cooling or with the early stage after 

lava has cooled.