May 09, 2003
Canter on Maya portages, Shaw intro


Here, with formatting that I will have to work on (apologies to Ron and Chris) is Ron Canter's study of possible Maya portage routes on the Usumacinta River. We will create a PDF version with maps at some point, but the report is important enough to our current efforts that making it available now seems to me to be a priority. (Full text with footnotes below - click MORE)

The Usumacinta River Portages from the Maya Classical Period to the Present
By Ronald L. Canter, cartographer, Federal Aviation Administration

Introduction

Ronald Canter and the Geography of Mesoamerican Canoe Culture
By Christopher Shaw

Ronald L. Canter, a cartographer for the Federal Aviation Administration, in
Washington DC, belongs to the tradition of so-called amateurs who have made
significant contributions to Maya studies, through their immersion in the
latest advances and the rich lore of the field, as well as their knowledge
of the geography and personal experience of the living Maya. Canter,
self-effacing, judicious, and cautious in the extreme (though hardly timid),
is the model of a serious, self-taught scholar from an earlier time. Indeed,
some of the most celebrated Mayanists began as "amateurs."

In the U.S. Canter's twenty-year, in-depth study of the ancient and colonial
canoe geography of
the northeastern United States has already culminated in the reopening of
750-miles of traditional canoe routes through northern New York and New
England, part of a once even more extensive system of inland waterways. Now
called the Northern Forest Canoe Trail, the route runs from Old Forge, New
York, in the Adirondack Mountains, to northern Maine, all of it legally and
continuously navigable for the first time in more than a century. Canter is
also an expert and intrepid canoe voyager himself.

Canter has likewise absorbed himself in contemporary Maya studies and the
geography of ancient and modern Mesoamerica, where the canoe--the dugout,
cayuco, or chem--served as the essential and daily means of trade,
transport, travel, war, diplomacy, and cultural dispersion over the entire
region.

This paper, best read along with the paper on Panhale by Armando Anaya
(FAMSI paper on Pomona and its Hinterland, referenced below) culminates
years of documentary research, experience, map and geological study. The
portages it describes bypassed the Usumacinta River's central, difficult,
and strategically significant canyons, just downstream from the last major
city of the uplands, Piedras Negras, and just upstream from Pomona, with its
satellites of Chinikiha--itself controlling portages between the Usumacinta
and the Rio Chacamax--and its defensive fortress, Panhale, located precisely
at the mouth of the canyon, Boca del Cerro. Here two major polities of the
Classical period, one controlling the rich upper delta and water access
inland, the other upstream access to the cities of upper drainage and the
rich resources of the distant Cuchumatan mountains, fought over one of the
most strategic junctures in the extensive network of rivers, lakes, and
bajos traversing the Maya region. When the two cities engaged in their
so-called "star wars" in the 8th century, the canyons and portages must have
been as heavily disputed as those separating lakes George and Champlain from
the upper Hudson River in North America's French and Indian War.

Until recently, however, river navigation, the properties of ancient
cayucos, and the practice of portaging, have remained obscure to many
archaeologists, obscuring in turn the geography of the Usumacinta and the
importance of the canoe. (Thompson, Schele, Freidel, Hopkins, Houston,
Aliphat, and Hammond are among those who have seen its importance.) The
portage itself, for instance, a poorly understood phenomenon to
non-canoeists, is a mere nuisance rather than the blockage to navigation it
is often assumed to be, especially in terms of long-distance travel and
trade.

In the time frame of voyages such as those undertaken by coastal and inland
traders from the Olmec to the post-Classic, portaging or even constructing
new boats amounted to no great loss of time. Those navigators surely
encountered dangers and uncertainties that held them up much longer. Like
whalers, however, they were unconstrained by the time-limits of a vacation
or a sabbatical. Gone for months at a time, or with no homes at all, they
had the time wait out sudden floods from unseasonable rains, to use any
number of strategies to get around difficult passages. They could portage
their cayucos over short distances using logs for rollers, for instance, as
cayuco builders still do. Over longer distances, they could carry their
goods and equipment in stages from one set of boats to another.

In Classical times, they would have portaged when possible, purchased or
hired new boats on the opposite side, or even built new ones. They might
carry only their cargo and run the boats down navigable rapids, or, in a
pinch, launch their empty boats down a rapid and pick them up on the
downstream side, a technique used by Wolfgang Cordan's vogas on the Jatate
in the 1960s, and a probable technique on the Usumacinta canyons. They might
"line" their craft from shore, using ropes, construct canals between low
lying waterways, or build dams with runnable sluiceways, such as Alfred
Siemens has described on the Rio Candelaria. All these methods are in the
documentary or archaeological record. When viewed this way marginal
landscapes and waters that once appeared land-bound now appear within the
navigation capabilities of canoe people The landings at the ends of portages
naturally became, as they did all over North America, places where travelers
congregated, where trade and cultural exchange happened, where cities grew.
Inevitably they also became points of
strategic control.

San Jose Canyon is the only extended stretch of questionable navigability on
the Usumacinta corridor, especially at certain water levels. For craft of
sufficient volume, with experienced crews, it is also navigable much of the
year. In sporting terms, it rates a Class III at average winter levels,
advanced-intermediate difficulty for canoes by the International Standard of
Whitewater Difficulty. It seems certain therefore that local and far-flung
teams of Maya in large canoes, such as those depicted in the art record,
sometimes ran it. (We also know monteros ran the canyons, often illegally,
in the nineteenth century.) But the Usumacinta is subject to enormous volume
differentials, and for long periods of the year the canyons are unnavigable.
At such times the only recourse, even for the most intrepid river runners,
is to portage.

Canter's research and experience uses colonial records and those of more
recent travelers, such as Maler and Morley, contemporary reports (including
this writer's), current scholarship, and his professional expertise with GIS
technology, to paint a detailed picture of the canyon geography from a canoe
navigator's point of view. It also uses one of the intangibles unavailable
to the desk-bound professional, a deep knowledge of "canoe behavior" as it
has been practiced all over the western hemisphere for a hundred centuries.
We know from expedition reports, ethnography, art, and our own experience,
how canoe people meet the challenges of specific geographies. It is a
remarkably consistent record over time and distance, and there is no reason
to suspect it was any different for the ancient canoe travelers of the
Usumacinta.

It is also essential to understanding the meaning of the Maya region's "deep
landscape," the subtle and not immediately visible interplay of its shapes,
features, idiosyncrasies. To ancient minds, at least those who lived near
water, the geography would have been a seamless continuity, as the Aegean
was to the children of Piraeus, or coastal New England to the wives of New
Bedford. To Classical travelers and citizens along the rivers, the region
would have been imprinted on mental maps by story and anecdote from Pomona
to Rio Azul, and from Cancuen and Tonina to Yaxchilan: place names,
"portals," battles, stories, hazards, available women, mythic overtones. A
young canoe trader from the delta may already have known to hug the left
shore when running San Jose Canyon before he even laid eyes on it.

Within this essential understanding lies the meaning of the relationships
connecting Pomona, Panhale, Piedras Negras and all the upstream cities;
ultimately of the entire Classical era. It is also this understanding, this
irreplaceable piece of the Maya puzzle, that would be obliterated forever by
an unnecessary and useless dam at Boca del Cerro.

Canter is currently preparing a detailed map of the portage routes. And
while the Usumacinta portages are only one small fragment of the region's
ancient and modern water routes, this description, likewise, is only the
first installment of a life-work that will describe the navigable waters of
the entire Maya region. It is already the only such guide in existence. When
complete it will be an invaluable multipurpose resource.

Christopher Shaw is author of Sacred Monkey River (Norton, 2000), and
co-founder of Rios Mayas.

***************************************************

The Usumacinta River Portages in the Maya Classical Period
By Ronald L. Canter

Distance: 47 km from El Porvenir to Boca del Cerro, with options ranging
from 26 to 65 km.

Portage Option #1: El Porvenir to Tenosique or Boca del Cerro, 46 or 47 km.

Many ancient portage routes in Mesoamerica can be tentatively reconstructed
by extrapolating from the ends of navigation upstream and down, examining
and understanding the controlling terrain features, and considering the
distribution of known Maya sites. [fn1] When this approach is applied to
finding the most likely carry past the Usumacinta canyons, between Piedras
Negras (Y'okib) and Boca Del Cerro, at the head of upstream navigation, the
logical route follows a linear karst valley in Guatemala and Mexico running
from El Porvenir, just downstream from Piedras Negras, to Corrigedora Ortiz
(Tres Champas), then over a low divide to Francisco Madero. The second half
of the route would work north through the limestone hills to reach the
lowlands, where the river emerges from the canyons into the broad estuary of
the delta.

Not only is this route the obvious choice topographically, it is indeed the
well-documented 19th-century portage trail. From Desempeño, the historical
trail at first worked through karst ridges (the expression of a transverse
arch crossing the La Linea Syncline), on which sit Piedras Negras and El
Porvenir. It then followed a karst valley the rest of the way. The valley,
which parallels the La Linea Syncline, is the only route that avoids high,
dry, and rugged karst ridges. It would have been the best route in any
period, from Preclassic to Postclassic. In the Classic, however, downstream
through-paddlers probably shortened it by starting from El Porvenir.

The ejido Francisco Madero lies on a calm pool 20 km downstream from El
Porvenir (33 km from Desempeño), one of the few places in the canyons where
a ferry crossing by canoe is practical. But it is not the end of the
portage. [fn2] The 19th-century trail continued north from there for 26 km,
first following the river, then winding between knobs and over ridges a few
kilometers east of San Jose Canyon. From Los Rieles the trail used the
valley of Arroyo Tepesquintla to drop down to the coastal plain, passing
Adolfo Lopez Mateos on the way. From the edge of the hills, the old trail
bee-lined through Rancho Grande, to Tenosique (Tanoche), 46 km from El
Porvenir and 59 km from Desempeño. It represented a good compromise between
a reasonable grade and directness, and probably reproduces a Postclassic
trail.

Using a digital terrain model, however, Armando Anaya calculated the line of
least effort between Piedras Negras and Pomona. The resulting trace closely
follows the known19th-century portage as far as Adolfo Lopez Mateos, in the
foothills. It then veers west-northwest along the front of the mountain,
past the Rojo Gomez site, to the Usumacinta near Panhale, 47 km downstream
from El Porvenir. Dr. Anaya's computer-generated trace is a very logical
compromise between effort and directness, and a very good candidate for a
well-used Classic Period portage trail.

Anaya refers to unconfirmed reports of a gravel causeway heading west from
Rancho de Herradura, 7 km downriver from Panhale, on the west shore of the
river at Arroyo Tacalate a mere 6 km east of Pomona [Pia], which may have
been Pomona's port on the Usumacinta. It would be interesting to see if a
corresponding port exists somewhere on the Rio Chacamax, north of Arroyo
Negro and 8 km west of Pomona. If so, then Pomona would have occupied the
height of land near the midpoint of a 14 km portage between the two rivers,
and controlled a port on a river approach to Palenque.

Option #2, El Porvenir to Chuncheje or Lindavista, 34 to 40 km.

The shortest valley route between definitely navigable sections of the
Usumacinta required ferrying across the river between canyons. By ferrying
the river at either Francisco Madero or San Jose Usumacinta, porters could
then follow a broad valley (the northern extension of the "Intermontane
Valley" along the La Linea Syncline) west to the Santa Margarita site, and
then northwest through Victorico Grajales. From the Las Delicias Maya site
and village, old trails once ran north between broken hills to the river
opposite Chuncheje, then northwest to Lindavista, 3.5 km upstream from the
Boca bridge. A portage from El Porvenir to the river between Chuncheje and
Lindavista would have passed directly to the head of navigation below San
Jose Canyon. This portage option would have cut 7 to 13 km from the 47-km
trail carry from El Porvenir to Panhale. (Chuncheje is 34 km from El
Porvenir and Lindavista is 40 km from El Porvenir.)

On his 1953 "La Selva Lacandona" map, Frans Blom marked the river as again
becoming navigable, at a point labeled Chuncheje, about 10 km upstream of
Boca del Cerro. In low water, Chuncheje appears to have been the head of
navigation. Lindavista may have been as high up as dugouts could reach in
high water. The rapids of Iguanas (Boca Del Cerro) Canyon are minor. Neither
variation was used in the 19th century because vogas and boatmen for the
monterias were simply forbidden to run any part of the canyons. (They
sometimes did anyway.) A chain of Maya sites marked on Blom's map between
Santo Tomas and Lindavista fits the most likely route nicely, and supports
the probability of an actual Classic period portage from El Porvenir to
Chuncheje and/or Lindavista. Between Las Delicias and Lindavista three Maya
sites are strung along a terrace 30 meters above the river: Ojo de Agua,
Camino a Las Delicias, and Chinikiha, where a bat mural, fine sculpture
[fn3], unusual Preclassic ceramics, and a ball court have been unearthed,
suggesting it was a site of some importance. The Lindavista (Boca de
Chiniquija) site sits beside the river and seems a likely port site. All
four would be inundated by a 40 meter dam at Boca del Cerro.

During the Classic Period, Pomona established a fortress at Panhale,
directly in the Boca Del Cerro gap, where the Usumacinta breaks out of the
mountains south of Tenosique. The site controlled river traffic through Boca
del Cerro and the most likely portage around all the canyons. According to
Anaya, steep slopes and "massive platforms and observation points" protect
its hilltop ruins. Panhale Acropolis 2, an eyrie perched 320 meters above
the coastal plain, caps the highest peak overlooking the north shore of the
Usu', making the site the Maya equivalent of a Rhine castle. [fn4]

Panhale was in the thick of the moves and countermoves of two great Maya
cities throughout the Classic. It may be a key to understanding the regional
conflicts and trade routes of the Classic period. Pomona and its outliers
were well sited to dominate the best land and water routes between the
lowlands and the upper Usumacinta basin. Most of Panhale would be torn apart
by construction of any dam placed in the mountain gap. Its destruction would
be an irreplaceable loss. In fact, the site's Group B has already been badly
damaged by quarrying and CFE exploratory work.

A broad pass at La Estrella, just 7 km south of Pomona, separates the
lowlands and the valley of the Rio Chiniquija (See discussion of Portage
Option #4), controlling traffic on and between the rios Usumacinta and
Chacamax. The valley of the Chacamax also provides a natural approach to
Palenque from the east. Therefore, Pomona and Panhale would have controlled
every reasonable route from Piedras Negras to the coastal plain. According
to Stephen Houston, "Pomona was the natural enemy of Piedras Negras: it
controlled a different ecological zone to the north and formed a bottleneck
through which Piedras Negras would naturally choke." Pomona's one weakness
was a lack of natural defenses, which Panhale may have partly alleviated.

Piedras Negras launched two "star wars" against Pomona, first in 792 and
then in 794 CE. The wars ended in a crushing defeat for Pomona. There are
three possible scenarios for Panhale's involvement. It may have turned a
blind eye to the second attack, i.e. double-crossed Pomona, since no army
could pass without notice. It may have been thinly garrisoned and overrun
before Pomona could send reinforcements. Or, the nobility and forces of
Pomona may have taken refuge in Panhale and eventually fallen to a
determined siege, Tolkein's "Helms Deep" scenario played out in real life to
a grimmer end. Dr. Anaya's continued research could solve the riddle, but
dam construction on the site would forever close the book.

Option #3: El Porvenir to Chuncheje via San Jose los Rieles ? 36 km.

Another short route from El Porvenir to navigable water at Chuncheje may
have followed the 19th century portage almost to San Marcos, then swung left
through the San Jose Los Rieles site, ferried across the river, and followed
a narrow linear valley west 4.5 km to the pool at Chuncheje. Such a route is
only 36 km long, vs 47 km from El Porvenir to Panhale. Only one small site
lies along it, so this route remains hypothetical.

Option #4: La Linea to Lindavista ? 26 km.

A theoretical portage, that may have passed from Rapidos La Linea west
through Morelos [Jose Maria Morelos y Pavo], then north through Vista
Hermosa to Lindavista, would have avoided all the canyon's rapids. At 26 km,
it would have been short. However, to get out of the canyon porters would
have had a steep climb of more than 200 meters in two km from La Linea to
Netzahualcoyotl. From there, a rough modern road through Nuevo Retiro and
Morelos crosses a high, rugged karst plateau. Between Morelos and Vista
Hermosa the road drops into the valley, descending 300 meters in five km.
The last leg to Lindavista would have been flat and easy. There are no sites
reported along most of the projected route. The climb and descent are both
greater and steeper than on any other possible portage, offsetting the
advantage of shorter distance. It is the only projected route that climbs up
and over the highest range of mountains. Overall, this portage option seems
unlikely to have been used much, if at all.

Option #5: El Porvenir to Rio Chacamax ? 65 km.

A last possible route could have made an end run around the Boca del Cerro
Ridge. By continuing west 11 km from Chinikiha 5 up the valley of Rio
Chiniquija to the Old Tenosique Road at La Estrella, travelers could reach
the lowlands at Coronel Gregorio Mendez Magana. [fn5] (Penjamo, the old
name, was more compact.) From the wide, low pass at La Estrella, Pomona is
only 7 km north, and the rios Usumacinta and Chacamax are both equally
accessible. In fact, travelers headed upstream from the coast in the wet
season would have found a route up the Chacamax, then overland for 65 km via
Chinikiha and Santa Margarita to El Porvenir, faster and less work than
other routes. The Rio Chacamax has less current and is 140 km shorter than
the comparable section of the Usumacinta. The Tierra Blanca site, on the
Usu' at the mouth of the Chacamax, shares an unusual daubed volcanic-glass
beaded Preclassic ceramic type with Chinikiha, suggesting long and direct
contact between the two via the Rio Chacamax. There are also suggestions of
Olmec-Chinikiha contacts, per Dr. Rands The directness and ease of upriver
travel on the Chacamax would more than offset a longer portage to El
Porvenir. The Rio Chacamax may have been a key part of long distance trade
networks in the region, and its shores merit more attention.

To summarize:
In the 19th Century the Usumacinta Portage ran north from Desempeño for 59
km to Tenosique on the Usumacinta below the canyons. A Classic Period
portage would have been constrained by topography to follow much the same
route, but probably started at El Porvenir and ran to Panhale, as Armando
Anaya has demonstrated. This probable Classic Period portage would have been
47 km long, 12 km shorter than the 19th century trail. A number of other
options were possible. Most involved ferrying across the Usumacinta and then
following the south shore, either to continue downriver by boat, or to
travel west through the valley of the Rio Chinikija to Pomona and the Rio
Chacamax. Coming up the Rio Chacamax, and then carrying 65 km south to El
Porvenir, would have avoided 140 km of current and meanders on the
Usumacinta. Since the region was not at all wild in the Classic, it is
possible that all portage options were used, as occasion demanded. A
proposed 40-meter dam at Boca del Cerro threatens to destroy at least five
known Maya sites, and possibly others not yet located. Of those, Panhale
and Chinikiha may hold keys to the puzzle of past trade and conquest routes.
Chinikiha is at the juncture of several portage options. The destruction of
Panhale, a Maya mountain fortress in the Boca del Cerro gap, could retard
understanding of the long running feud between the major cities of Piedras
Negras and Pomona.

Footnotes

1. Every portage discussed here would have been far too long to haul dugouts
across. The cargo would have been carried from one set of canoes to another.
2. Christopher Shaw, on his 1989 descent, saw a Chol cayuco workshop in
operation there.
3. Per rubbings done by Merle Greene Robertson of pieces looted from
Chinikiha.
4. A major function of castles along the Rhine River in Germany was to
"control" traffic, ie. extort tolls to enrich local barons. Tolls were
cumulatively so high that some overland routes, though arduous, were
competitive with upriver Rhine traffic, and not radically more expensive
than downriver. Today, regulated tolls levied by the communities along the
Usumacinta might be a reasonable alternative to violent and
counterproductive banditry.
5. Chiniquija means "Mouth of the Disappearing Water". The river sinks east
of Reforma Agraria, flows under a karst ridge, and reappears 2.5 km north.
The Old Tenosique Road connects Reforma Agraria to the lowlands, first
through a narrow pass in a karst ridge separating the upper and lower
valleys of Rio Chiniquija, and then via the pass at La Estrella.

Bibloigraphy

"Research takes you places you absolutely don't want to go, and rattles all
your preconceptions." Nicholas Clooney, 2002.

Books and Articles:

1. Routes of Communication in Mesoamerica: the Northern Guatemala Highlands
and the Peten, Richard E. W. Adams, 1978. In "Mesoamerican Communication
Routes and Cultural Contacts", New World Archaeological Foundation
2.Classic Maya Landscape in the Upper Usumacinta River Valley, Mario M.
Aliphat, 1994. University of Calgary, Calgary, Alberta, Canada.
3. Site Interaction and Political Geography in the Upper Usumacinta Region
During the Late Classic: A GIS Approach, Ph.d. Dissertation by Armando Anaya
Hernandez, 1999
4. The Pomona Kingdom and its Hinterland, Armando Anaya Hernandez, 2002.
FAMSI Report
5. Letters from Mexico, The Fifth Letter, Hernan Cortez, 1525.
6. Long Distance Transport Costs in Pre-Hispanic Mesoamerica, Robert D
.Drennan, American Anthropologist Research Reports, 1984
7. River of Ruins, Louis Halle, 1941. Henry Holt & Co., New York, NY.
8. Classic Maya Canoes, Norman Hammond, 1981. International Journal of
Nautical Archaeology and Underwater Exploration, (10-3): 173-185
9. In the Land of the Turtle Lords, Stephen Houston, 2000. FAMSI Report
10 Among the River Kings, Stephen Houston, 1999. FAMSI Report
11. Between the Mountains and the Sea, Stephen Houston, 1998. FAMSI Report
12. The Piedras Negras Project, Stephen Houston and Hector Escobedo, 1997.
FAMSI Report
13. Commerce and Trade Routes of the Maya, Christopher Jones, 1990,
University Museum, UPA.
14. River of the Sacred Monkey, Dimitar Krustev, 1970.
15. Chronicle of the Maya Kings and Queens, Simon Martin & Nikolai Grube,
2000. Thames & Hudson, Ltd. London, UK.
16. Palenque and Selected Survey Sites in Chiapas and Tabasco, Robert L.
Rands, 2002. FAMSI Report
17. The Maya Chontal Indians of Acalan-Tixchel, France V. Scholes & Ralph L.
Roys, 1948.
18. Sacred Monkey River, Christopher Shaw, 2000. W.W Norton & Co.
19. Incidents in the Life of a Maya Archaeologist, Edwin M. Shook & Winifred
Veronda, 1998. Southwestern Academy Press, San Marino, CA
20. Incidents of Travel in Yucatan, John Lloyd Stephens, 1843. Republished
1963, Dover Publications, New York, NY.
21. Incidents of Travel in Central America, Chiapas, and Yucatan, John Lloyd
Stephens, 1841. Harper & Bros, New York, NY. Republished 1969, Dover
Publications, New York, NY.
22. Classic Maya Place Names, David Stuart & Stephen Houston, 1994. Studies
in Pre-Columbian Art & Archaeology, Dumbarton Oaks Research Library, Wash,
DC.
23. Canoes and Navigation of the Maya and their Neighbors, J. Eric S.
Thompson, 1951, Journal of the Royal Anthropological Society
24. Voices from the Rapids, Wheeler, Kenyon, Woolworth, & Birk, 1975.
Minnesota Historical Society
25. The Flowing Road, Caspar Whitney, 1912. J. B. Lippincott & Co,
Philadelphia, PA.>

Maps

1. The Ancient Maya World, Cartographic Division, NGS, 1989
2.Carta Fotogeologica Del Peten, series of reconnaissance geological maps at
1:100,000
3. La Selva Lacandona, y Tierras Colindantes, Frans Blom, 1953. Superb map.
4. Map of El Peten, Guatemala, and Bounding Regions of British Honduras and
Mexico, Carl Hubbs & Henry van der Schalie, 1937
5. Map of Tabasco, 1579, circle map attributed to Melchior de Alfaro Santa
Cruz.
6. Mapa Base de las Cuencas de los Rios, Mexico y Guatemala, 1:500,000,
1980.
7. Mapa de la Republica de Guatemala, Escala 1:1,000,000, Teodoro Paschke,
1889. Shows colonial trails, Peten, Escala 1:800,000, 1900. Rough schematic
of trails.
8. Sistema Fluvial Tabasqueno, 1946, map showing limits of navigability for
rivers of Tabasco, and over a dozen ruins.
9. Topographic Maps of Mexico, Guatemala, and Belize, 1:250,000 series and
partial coverage 1:50,000 series

Web Sites
http://www.famsi.org/reports/00082/index.html The Pomona Kingdom and its
Hinterland, Armando Anaya Hernandez, FAMSI Report, Dec 2002
http://www.archaeology.org/magazine.php?page=online/news/usumacinta
"Maya Sites Face Flooding", Jason McGahan, Archaeology Magazine, Feb 19,
2003

Posted by Dave at May 09, 2003 09:46 PM