United States 

Agriculture Lithologic Composition and 

Forest Service 

Northeastern Forest 

Rock Weathering Potential o 

Experiment Station Forested, Glacial-Till Soils 

Research Paper NE-662 

Scott W. Bailey 

James W. Hornbeck 
Abstract 

Proportions of bedrock lithologies in a wedge-shaped source area were a 

reasonable model for predicting lithologies present in soils developed on glacial 

till. Examination of weathering characteristics of rocks collected to verify the 

model showed substantial staining and pitting of interior surfaces. This information, 

even though qualitative, can further the understanding of nutrient cycling in 

forest ecosystems. 

The Authors 

SCOTT W. BAILEY, Research Geologist, and JAMES W. HORNBECK, Research 

Forester, are with the Northeastern Forest Experiment Station, Forestry Sciences 

Laboratory, Durham, New Hampshire. 

Acknowledgment 

Leon Bailey of tongmeadow, Massachusetts, assisted with the laborious task 

of sawing rock specimens. Ralph Perron, USDA Forest Service, assisted with 

sample processing. Kenneth Dudzik, USDA Forest Service, photographed rock 

specimens. Donald Buso, Institute of Ecosystem Studies, provided valuable 

suggestions. 

Manuscript received for publication 5 August 1991 

Cover Photo 

A 250x enlargement of a thin section from the Rangeley bedrock formation in 

central New Hampshire. Thin sections, prepared by slicing rock samples to 30 

um thickness, are viewed microscopically to identify minerals. This section 

shows quartz, hornblende, plagioclase, garnet, and pyrite. Plagioclase (striped 

mineral left of center) and hornblende (large, darker area in lower right quadrant) 

are major sources of base cation weathering. . 

Northeastern Forest Experiment Station 

5 Radnor Corporate Center, Suite 200 

100 Matsonford Road 

P.O. Box 6775 

Radnor, Pennsylvania 19087-4585 

March 1992 
Introduction 

Readily available supplies of soil cations such as calcium, 

magnesium, and potassium are critical for maintaining 

productive forests. Federer and others (1 989) have 

suggested that some forests in Eastern United States 

may be experiencing accelerated depletion of calcium due 

to acidic deposition and intensive harvesting. One effect 

could be the development of an unfavorable ratio of calcium 

to aluminum in forest soils, resulting in a decline in tree 

growth rates (Shortle and Smith 1988). The extent to 

which this may happen depends on whether calcium 

contributions from weathering of rocks in geologic parent 

material and soils will offset depletion rates. Rock 

weathering is highly variable depending upon lithology, 

but is the major source of base cations, exceeding 

atmospheric inputs by several orders of magnitude (Likens 

and others 1977; Paces 1986). 

Unfortunately not much is known about the contributions 

of rock weathering to nutrient cycles of forest ecosystems. 

Usually the only geologic information readily available for 

forested sites is listed on state bedrock maps that give 

names and accompanying descriptive material of bedrock 

lithologies. In glaciated terranes, additional lithologic types 

deposited in till but derived from off-site sources are 

seldom considered. The geologic materials in the surficial 

deposits are probably more important than bedrock in 

terms of contributing nutrients for forest growth, and 

buffering forest soils and streams against acidification by 

atmospheric deposition. 

This paper describes methods for predicting lithologies 

present in soils developed on glacial till, and the potential 

weathering contributions from rock particles >2 mrn in 

diameter. The methods are not quantitative in terms of 

providing weathering rates, but they provide information 

that can further the understanding of forest nutrient cycles, 

and possibly assist with decisions about forest harvesting. 
The Study Area to represent elevations, soil depths, and forest cover types 

Our methods were developed and tested on a 34-ha 

watershed that includes the main inlet to Cone Pond in 

the White Mountains of central New Hampshire. Cone Pond 

and its watershed are being used to study natural and 

human-caused acidification of forested and aquatic 

ecosystems (Buso and others 1985). Cone Pond watershed 

is underlain by a high-grade pelitic schist assigned to the 

Silurian Perry Mountain formation. The area was glaciated 

during the Pleistocene epoch. Soils are primarily Spodosols 

that have developed on Wisconsin-age glacial till that 

averaged <2 m deep. The watershed is forested with red 

spruce/balsam fir (78 percent) interspersed with patches 

of northern hardwoods (19 percent) and exposed bedrock 

(3 percent). 

Methods 

Predicting Lithologies 

Our approach to predicting lithologic composition of soils 

at Cone Pond watershed is based on research by 

Goldthwaite and others (1951). They studied dispersal of 

nepheline syenite in glacial-till soils of New Hampshire and 

Vermont. Nepheline syenite is a distinctive and rare bedrock 

type in northeastern United States found only at Mount 

Ascutney, near Windsor, Vermont, and at Red Hill, near 

Moultonborough, New Hampshire. They found that 

fragments of nepheline syenite from these two discrete 

sources were dispersed in a 60" arc about the mean 

direction of glacial movement. Fragments of nepheline 

syenite were plentiful close to the source, but 32 km away, 

and still within the dispersal envelope, <0.1 percent of the 

rock fragments in the soil were nepheline syenite. This 

suggests that 32 km was about the maximum distance that 

rocks were transported by the glacier before being deposited 

in till, or disintegrating into sand-/silt-size fractions. 

Based on these data for rock dispersal in till deposits, we 

determined a source area for the Cone Pond watershed 

by reversing such an envelope about the axis of glacial 

movement (Fig. 1). This area includes all bedrock types 

<32 km distant and within 530" of the direction of glacial 

movement. The direction was based on glacial grooves and 

striations on exposed bedrock at the Cone Pond watershed. 

These markings were substantial in number and all were 

on an azimuth qf about 15l0, or close to the mean of 

regional values of 140 to 158" reported by Goldthwaite and 

others (1951). The area underlain by each lithology in the 

till source area was determined by plotting the envelope 

on the bedrock geologic map for New Hampshire (Fig. I), 

then planimetering. 

Field Sampling 

As a first approximation, we hypothesized that each lithology 

present in soils on Cone Pond watershed was proportional 

to its area in the source envelope (Fig. 1). To test this 

model, we excavated five, 20- x 20-cm soil pits chosen 

on the Cone Pond watershed. All soil and rocks from the 

forest floor to the C horizon of each pit were removed and 

transported to the laboratory. After air drying and weighing, 

all rocks greater than 7 g (about 8 to 10 mm in diameter) 

were sawed in half to expose a cross section, and were 

then identified by lithologic unit. The minimum of 7 g was 

necessary because smaller fragments were rarely 

identifiable due to either extreme weathering or having an 

inadequate surface to show mineral and textural 

characteristics used to distinguish lithologic units. 

Characterization of Weathering 

To obtain estimates of the potential for rocks to contribute 

nutrients, weathering of rock fragments was characterized 

by measuring the thickness of the weathefing rind on the 

cross section, and noting the extent of weathering features 

on a semi-quantitative basis. Iron staining and pitting were 

noted on a scale of 0 to 2, with 0 denoting not present, 

2 denoting extensive staining or pitting throughout the 

specimen, and 1 denoting some staining or pitting in 

portions of the specimen. Other features rated as 0 (absent) 

or 1 (present) include the presence of fresh sulfides, 

alteration of feldspars to chalky material, and extensive 

overall weathering to the point where the sample was 

physically unstable under moderate finger pressure. 

Results 

Lithologic Composition 

Field sampling from the five soil pits yielded a total of 302 

rocks, each weighing >7 g. Rocks accounted for 

approximately 50 percent of the total weight of material 

from each sample pit, illustrating the rocky character of 

these forest. soils. Characteristics of most rocks were 

distinctive enough to allow classification by lithologic unit. 

Only 1 percent of the fragments were completely 

unrecognizable, primarily due to extreme weathering 

alteration. 

Table 1 shows the percentage of rock fragments by 

lithologic unit as predicted by their occurrence in the source 

envelope (Fig. 1) versus that observed by identifying field 

samples. Fragments of diabase dike rocks and pegmatite 

were found in small quantitites (7 percent or less) but were 

not predicted by the model. Neither of these rocks were 

mapped in the source area due to the small nature of 

igneous bodies relative to existing mapping scales. The 

Oliverian Granite probably includes some Concord Granite 

(also called Binary Granite). These two rock types are 

difficult to distinguish when present in small fragments. 

Concord Granite was not mapped in the source envelope, 

but small plutons have been identified closer to Cone Pond 

watershed than have Oliverian plutons (C. Barton, U.S. 

Geological Survey, personal communication). 

The formations of the Devonian-Silurian metasedimentary 

sequence were grouped since it was impossible to 
Moosilauke I Franconia 

Glacial 

Movement 

Jqs 

DSm 

Scale 1 :250,000 

Jqs Garfield Quartz Syenite 

Jcg Conway Granite 

Jmo Mount Osceola Granite 

Jg Lafayette Granite Porphyry 

Dkqm Kinsman Quartz Monzonite 

DSm 

-Dkqm 

. DSm 

Plymouth 

lkqm 

Cone Pond Watershed 

Dbgn Bethlehem Granite Gneiss 

DSm metasedimentary 

Sc Clough Formation 

Oam Ammonoosuc Volcanics 

Oog Oliverian Granite 

Figure 1 .-Source envelope for predicting lithologic composition of soils on 

Cone Pond watershed. The envelope was drawn on the map of bedrock geology 

for New Hampshire revised in 1986. The main axis for the wedge is an azimuth 

of 15l0, corresponding to the direction of glacial movement. Outlines of 15-minute 

U.S. Geological Survey quadrangles show location. 
Table 1.-Rock fragments in soils of Cone Pond watershed 

Predicted Observed 

Age Lithologic unit occurrence occurrence 

by weight by weight 

...................... Percent ..................... 

Jurassic Diabase 0 1 

Garfield Quartz Syenite 1 1 

Conway Granite 8 4 

Mount Osceola Granite 4 1 

Lafayette Granite Porphyry 2 1 

Devonian Pegmatite 

Kinsman Quartz Monzonite 

Bethlehem Granite Gneiss 

Devonian Littleton Formation 1 

Silurian Madrid Formation 

Smalls Falls Formation L 

Perry Mountain Formation 

Rangeley Formation I 

Silurian Clough Quartzite 1 .. 2 

Ordovician Ammonoosuc Volcanics 

Oliverian Granite 

Unknown 

Total 100 100 

Table 2.-Weathering characteristics of rock fragments 

Mean mass of Number of 

Characteristics Weight individual rocks specimens 

(n = 302) 

Staining: 

None 

Moderate 

Intensive 

Pitting: 

None 

Moderate 

Intensive 

Percent 

of total 

Chalky Feldspars 7. 27.1 31 

Sulfides 3 44.8 9 

Extreme weathering 5 10.7 49 

Total sample mean 38.2 
distinguish between these units in some fragments. All of 

these rocks have been mapped in the White Mountains 

as the Littleton Formation (Billings 1956). However, in the 

last 10 years, much of the Littleton has been reinterpreted 

as belonging to the various Silurian units first mapped in 

the adjacent region of western Maine by Moench (1971). 

Some of these units contain distinctive lithologies, such as 

calc-silicate granulite and sulfidic schist, which can be 

identified in small fragments. However, the majority of the 

rocks are quartz and mica schists which are only 

distinguishable by larger scale outcrop features or by 

stratigraphic position, or both. 

For both the predicted and observed percentages, only 

Kinsman Quartz Monzonite and the Devonian-Silurian 

metasedimentary sequence contribute greater than 10 

percent of the rocks found in the soil profile. The 

metasediments contributed 43 percent compared to 29 

percent predicted whereas Kinsman contributed 28 percent 

compared to 39 percent predicted. The greater amount of 

observed metasediments and lesser amount of observed 

Kinsman may reflect the fact that bedrock underlying the 

watershed and areas immediately to the north is 

metasedimentary and was underpredicted due to its 

proximity (Fig. 1). 

For the other lithologies, differences between predicted and 

observed weight may reflect variations in susceptibility to 

weathering, or distance from the source area to Cone 

Mountain. For example, Conway Granite and Bethlehem 

Granite Gneiss were overpredicted by the model. These 

are found only at distal parts of the source envelope (Fig. 

1). Clough Quartzite, also found only at extreme distal areas 

of the source envelope, was slightly in excess of predicted 

amounts, possibly reflecting the resistance of quartzite to 

weathering and disintegration during transport. 

The above results suggest that predictions could be 

improved by weighting lithologies by "up-glacier" distance 

from the Cone Pond watershed. We tested one simple 

weighting scheme by dividing the envelope into three 

sections based on equal distances along the 32-km radius, 

obtaining area of lithologies in each section, and then 

averaging to determine the overall prediction. The section 

of the envelope from 0 to 10.7 km had the smallest area 

but received the same weight as larger sections from 10.7 

to 21.3 km and from 21.3 to 32 km. This approach improved 

the prediction for Devonian-Silurian metasedimentary units 

considerably, but still overestimated the proportion of 

Kinsman Quartz Monzonite (Table 1). 

Descriptions of Weathering 

Based on qualitative measures, weathering of small rock 

fragments is extensive (Table 2). Red to brown staining 

was evident on interiors of the majority of specimens 

sampled (Fig. 2), and 24 percent, by weight, were 

extensively stained throughout. The degree of staining was 

greater in smaller fragments. 

A total of 148 specimens, or about half the sample, 

exhibited moderate to extreme interior pitting (Table 2, Fig. 

3). As with staining, smaller rocks exhibited a greater degree 

of pitting than larger rocks. 

By weight, 3 percent of the sample showed fresh sulfides 

in interior portions (Table 2), indicating that some reserve 

of easily weathered minerals is still available. Thirty-one 

specimens, representing 7 percent by weight, had feldspar 

grains extremely altered (Table 2). These feldspars were 

quite soft (2 to 3 on the Mohs scale) and had a dull, chalky 

appearance, in contrast to an unweathered hardness of 5 

to 6 on the Mohs scale and a vitreous appearance. The 

chemical consequences of this degree of alteration is 

unknown, but in granitic terranes, feldspar weathering may 

be the major source of base cations (Clayton 1986; Drever 

1 988). 

Forty-nine specimens were so altered that they had lost 

their physical coherence. These were highly discolored and 

extensively pitted. The average size of these specimens 

was small, (Table 2), indicating enhanced weathering of 

small particles. 

Only 34 specimens exhibited the surficial rinds classically 

thought of as indicating rock weathering (Fig. 3). Rocks 

exhibiting rinds ranged from 7 to 527 g, and their rinds 

ranged up to 10 mm in thickness. Four rock types, Kinsman 

Quartz Monzonite, Bethlehem Granite Gneiss, Silurian 

metasedimentary, and Oliverian Granite, accounted for 93 

percent of the rocks with rinds. 

Discussion 

A source envelope can easily be used to approximate 

lithologic composition of soils developed on glacial till. The 

information obtained can be used in conjunction with state 

bedrock maps to more fully understand geologic 

contributions to forest nutrient cycles. 

Comparison of source areas between watersheds may help 

explain differences in weathering or base cation supply. 

For example, the annual net calcium loss (watershed inputs 

minus outputs) for Cone Pond watershed is 3 kg ha' yr-' 

or 70 percent less than that for watersheds at the Hubbard 

Brook Experimental Forest located 12 km to the west. Since 

atmospheric loading and other factors are similar between 

the two sites, this difference in net loss has been 

hypothesized as being due to greater calcium contributions 

from rock weathering at Hubbard Brook. 

Construction of a source envelope for Hubbard Brook shows 

that Ammonoosuc Volcanics account for 23 percent of the 

source area, compared to only 6 percent of the source 

area for the Cone Pond watershed. Ammonoosuc Volcanics 

is the only lithology in the region with a large amount of 

hornblende, a relatively unstable mineral in the weathering 

environment, and a prime source of calcium. Another major 

mineral in the ~mmonoosuc Volcanics is plagioclase 

feldspar, also an important source of calcium. Three percent 

of the Hubbard Brook source area is underlain by the Fitch 
Figure 2.-Cross section of a granite fragment (347 g, 65 mm long) showing 

iron staining (arrows). The staining on the left side goes deeper for some 

unknown reason. Except for staining, the minerals in this fragment appear to 

be relatively fresh in that original luster and hardness are preserved. 

Figure 3.-Cross section of a diabase fragment (346 g, 67 mm long) showing 

weathering rind, interior dissolution pits (arrows), and weathered fractures. 
formation, the only carbonate bearing lithology in the region. 

Carbonates are extremely reactive and disproportionately 

contribute base cations compared to silicates. None of these 

minerals are prevalent in other major lithologies found in 

till on the Cone Pond watershed. This scarcity of calcium 

sources may explain the lesser calcium outputs for Cone 

Pond watershed, and the low pH and acid neutralizing 

capacity of streamwater (Bailey and others 1987). 

Predictions of lithologies in till soils may be of value when 

planning forest harvests. Soils developed on tills with rocks 

that are less resistant to weathering and that have high 

cation contents are apt to be more suitable for intensive 

harvests such as block clearcutting or whole-tree harvesting. 

Cations are likely to be in greater supply and losses due 

to harvest may be replenished more quickly by rock 

weathering. Lithologies like those found in till and soils on 

the Cone Pond watershed indicate a need for less intensive 

harvests and longer rotations. 

A.lthough the source envelope worked well for the Cone 

Pond watershed, there are limitations for other terranes. 

The method could not be expected to work where surficial 

materials have been deposited or extensively reworked by 

fluvial processes. Application also would be complicated 

where more than one episode of till deposition is 

preserved. 

Current studies of forest nutrient cycling usually rely on 

analyses of soil particles <2 mm in diameter to obtain 

estimates of site nutrient capitals and readily available plant 

nutrients. However, rocks from the Cone Pond watershed 

showed extensive alteration (Figs. 2 and 3), suggesting 

that rock weathering could be providing significant 

contributions of nutrients beyond that indicated in the <2 

mm fraction. The extensive staining found on the interior 

of the rocks represents dissolution, oxidation, and migration 

of iron in these rocks. The iron could be from soil water 

percolating through the rocks, or from weathering of ironbearing minerals such as biotite commonly found in the 

rocks. In either situation, the observed staining indicates 

that the interiors of many rock fragments are vulnerable 

to weathering and are interacting with the forest nutrient 

cycle. 

Interior pitting of rocks from our study site is caused by 

preferential dissolution of minerals that are more easily 

weathered than the overall rock mass. Sulfides and 

amphiboles are likely candidates for preferential dissolution 

in the rocks found in soil on the Cone Pond watershed. 

Dissolution pits were found in rocks as large as 209 g, 

which gives testimony to the potential of large, seemingly 

impermeable rocks to contribute to nutrient cycling. 

When viewed under a dissecting microscope, interior pits, 

fractures, and exterior rinds appear sponge-like and highly 

porous. Thus, the active weathering surface is far greater 

than would be calculated for smooth rocks. The absence 

of classical weathering rinds on most rocks was a surprise. 

However, although rinds were lacking, most specimens had 

ragged and pitted surfaces. This may indicate that 

weathering rinds are physically unstable, and that 

mechanical disintegration is concurrent with chemical 

alteration. 

Literature Cited 

Bailey, S. W.; Hornbeck, J. W.; Martin, C. W.; Buso, 

D. C. 1987. Watershed factors affecting stream 

acidification in the White Mountains of New 

Hampshire, USA. Environmental Management. 

11: 53-60. 

Billings, M. P. 1956. The geology of New Hampshire. 

II. Bedrock geology. Concord, NH: New Hampshire 

Department of Resources and Economic Development. 

200 p. 

Buso, D. C.; Bailey, S. W.; Baird, F.; Hornbeck, J. W.; 

Martin, C. W. 1985. Watershed interactions affecting 

pond acidification. Durham, NH: Water Resources 

Research Center, University of New Hampshire; Res. 

Rep. No. 62. 61 p. 

Clayton, J. L. 1986. An estimate of plagioclase 

weathering rate in the Idaho batholith based upon 

geochemical transport rates. In: Colman, S. Mi.; 

Dethier, D. P., eds. Rates of chemical weathering of 

rocks and minerals. Orlando, FL: Academic Press: 

453-466. 

Drever, J. 1. 1988. The geochemistry of natural waters. 

Englewood Cliffs, NJ: Prentice Hall. 437 p. 

Federer, C.A.; Hornbeck, J. W.; Tritton, L. M.; Martin, 

C. W.; Pierce, R. S.; Smith, C. T. 1989. Long-term 

depletion of calcium and other nutrients in eastern 

US forests. Environmental Management. 13: 593-601. 

Goldthwaite, J. W.; Goldthwaite, L.; Goldthwaite, R. P. 

1951. The geology of New Hampshire. Part I. Surficial 

geology. Concord, NH: New Hampshire Department of 

Resources and Economic Development. 83 p. 

Likens, G. E.; Bormann, F. H.; Pierce, R. S.; Eaton, J. S.; 

Johnson, N. M. 1977. Biogeochemistry of a forested ecosystem. New York: Springer-Verlag. 146 p. 

Moench, R. H. 1971. Geologic map of the Rangeley 

and Phillips quadrangles, Franklin and Oxford 

Counties, Maine. Washington, DC: US. Geological 

Survey; Misc. Geol. Invest. Map 1-605. 

Paces, T. 1986. Weathering rates of gneiss and depletion 

of exchangeable cations in soils under environmental 

acidification. Journal of Geological Sciences. 143: 

673-677. 

Shortle, W. C.; Smith, K. T. 1988. Aluminum-induced 

calcium deficiency syndrome in declining red spruce. 

Science. 240: 101 7-1 01 8. 
Bailey, Scott W.; Hornbeck, James W. 1992. Lithologic composition 

and rock weathering potential of forested, glacial-till soils. Res. 

Pap. NE-662. Radnor, PA: US. Department of Agriculture, Forest 

Service, Northeastern Forest Experiment Station. 7 p. 

Describes methods for predicting lithologies present in soils developed on 

glacial till, and the potential weathering contributions from rock particles 

>2 mm in diameter. The methods are not quantitative in terms of providing 

weathering rates, but provide information that can further the understanding 

of forest nutrient cycles, and possibly assist with decisions about forest 

harvesting. 

Keywords: weathering rate, soil formation, nutrient cycling 
Headquarters of the Northeastern Forest Experiment Station is in Radnor, Pennsylvania. Field laboratories are maintained at: 

Amherst, Massachusetts, In cooperation with the University of Massachusetts 

Burlington, Vermont, in cooperation with the University of Vermont 

Delaware, Ohio 

Durham, New Hampshire, in cooperation with the Unlversity of New Hampshire 

Hamden, Connecticut, in cooperation with Yale University 

Morgantown, West Virginia, in cooperation with West Virglnia University 

Orono, Maine, in cooperation with the University of Maine 

Parsons, West Virginia 

Princeton, West Virginia 

Syracuse, New York, in cooperation with the State University of New York, 

College of Environmental Sciences and Forestry at Syracuse University 

University Park, Pennsylvania, in cooperation with The Pennsylvania State University 

Warren, Pennsylvania 

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